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Ancient History Sourcebook



                                     350 BC

                                  by Aristotle

                   translated by R. P. Hardie and R. K. Gaye

                              Book I

  WHEN the objects of an inquiry, in any department, have
principles, conditions, or elements, it is through acquaintance with
these that knowledge, that is to say scientific knowledge, is
attained. For we do not think that we know a thing until we are
acquainted with its primary conditions or first principles, and have
carried our analysis as far as its simplest elements. Plainly
therefore in the science of Nature, as in other branches of study, our
first task will be to try to determine what relates to its principles.
  The natural way of doing this is to start from the things which
are more knowable and obvious to us and proceed towards those which
are clearer and more knowable by nature; for the same things are not
'knowable relatively to us' and 'knowable' without qualification. So
in the present inquiry we must follow this method and advance from
what is more obscure by nature, but clearer to us, towards what is
more clear and more knowable by nature.
  Now what is to us plain and obvious at first is rather confused
masses, the elements and principles of which become known to us
later by analysis. Thus we must advance from generalities to
particulars; for it is a whole that is best known to sense-perception,
and a generality is a kind of whole, comprehending many things
within it, like parts. Much the same thing happens in the relation
of the name to the formula. A name, e.g. 'round', means vaguely a sort
of whole: its definition analyses this into its particular senses.
Similarly a child begins by calling all men 'father', and all women
'mother', but later on distinguishes each of them.

  The principles in question must be either (a) one or (b) more than
one. If (a) one, it must be either (i) motionless, as Parmenides and
Melissus assert, or (ii) in motion, as the physicists hold, some
declaring air to be the first principle, others water. If (b) more
than one, then either (i) a finite or (ii) an infinite plurality. If
(i) finite (but more than one), then either two or three or four or
some other number. If (ii) infinite, then either as Democritus
believed one in kind, but differing in shape or form; or different
in kind and even contrary.
  A similar inquiry is made by those who inquire into the number of
existents: for they inquire whether the ultimate constituents of
existing things are one or many, and if many, whether a finite or an
infinite plurality. So they too are inquiring whether the principle or
element is one or many.
  Now to investigate whether Being is one and motionless is not a
contribution to the science of Nature. For just as the geometer has
nothing more to say to one who denies the principles of his
science-this being a question for a different science or for or common
to all-so a man investigating principles cannot argue with one who
denies their existence. For if Being is just one, and one in the way
mentioned, there is a principle no longer, since a principle must be
the principle of some thing or things.
  To inquire therefore whether Being is one in this sense would be
like arguing against any other position maintained for the sake of
argument (such as the Heraclitean thesis, or such a thesis as that
Being is one man) or like refuting a merely contentious argument-a
description which applies to the arguments both of Melissus and of
Parmenides: their premisses are false and their conclusions do not
follow. Or rather the argument of Melissus is gross and palpable and
offers no difficulty at all: accept one ridiculous proposition and the
rest follows-a simple enough proceeding.
  We physicists, on the other hand, must take for granted that the
things that exist by nature are, either all or some of them, in motion
which is indeed made plain by induction. Moreover, no man of science
is bound to solve every kind of difficulty that may be raised, but
only as many as are drawn falsely from the principles of the
science: it is not our business to refute those that do not arise in
this way: just as it is the duty of the geometer to refute the
squaring of the circle by means of segments, but it is not his duty to
refute Antiphon's proof. At the same time the holders of the theory of
which we are speaking do incidentally raise physical questions, though
Nature is not their subject: so it will perhaps be as well to spend
a few words on them, especially as the inquiry is not without
scientific interest.
  The most pertinent question with which to begin will be this: In
what sense is it asserted that all things are one? For 'is' is used in
many senses. Do they mean that all things 'are' substance or
quantities or qualities? And, further, are all things one
substance-one man, one horse, or one soul-or quality and that one
and the same-white or hot or something of the kind? These are all very
different doctrines and all impossible to maintain.
  For if both substance and quantity and quality are, then, whether
these exist independently of each other or not, Being will be many.
  If on the other hand it is asserted that all things are quality or
quantity, then, whether substance exists or not, an absurdity results,
if the impossible can properly be called absurd. For none of the
others can exist independently: substance alone is independent: for
everything is predicated of substance as subject. Now Melissus says
that Being is infinite. It is then a quantity. For the infinite is
in the category of quantity, whereas substance or quality or affection
cannot be infinite except through a concomitant attribute, that is, if
at the same time they are also quantities. For to define the
infinite you must use quantity in your formula, but not substance or
quality. If then Being is both substance and quantity, it is two,
not one: if only substance, it is not infinite and has no magnitude;
for to have that it will have to be a quantity.
  Again, 'one' itself, no less than 'being', is used in many senses,
so we must consider in what sense the word is used when it is said
that the All is one.
  Now we say that (a) the continuous is one or that (b) the
indivisible is one, or (c) things are said to be 'one', when their
essence is one and the same, as 'liquor' and 'drink'.
  If (a) their One is one in the sense of continuous, it is many,
for the continuous is divisible ad infinitum.
  There is, indeed, a difficulty about part and whole, perhaps not
relevant to the present argument, yet deserving consideration on its
own account-namely, whether the part and the whole are one or more
than one, and how they can be one or many, and, if they are more
than one, in what sense they are more than one. (Similarly with the
parts of wholes which are not continuous.) Further, if each of the two
parts is indivisibly one with the whole, the difficulty arises that
they will be indivisibly one with each other also.
  But to proceed: If (b) their One is one as indivisible, nothing will
have quantity or quality, and so the one will not be infinite, as
Melissus says-nor, indeed, limited, as Parmenides says, for though the
limit is indivisible, the limited is not.
  But if (c) all things are one in the sense of having the same
definition, like 'raiment' and 'dress', then it turns out that they
are maintaining the Heraclitean doctrine, for it will be the same
thing 'to be good' and 'to be bad', and 'to be good' and 'to be not
good', and so the same thing will be 'good' and 'not good', and man
and horse; in fact, their view will be, not that all things are one,
but that they are nothing; and that 'to be of such-and-such a quality'
is the same as 'to be of such-and-such a size'.
  Even the more recent of the ancient thinkers were in a pother lest
the same thing should turn out in their hands both one and many. So
some, like Lycophron, were led to omit 'is', others to change the mode
of expression and say 'the man has been whitened' instead of 'is
white', and 'walks' instead of 'is walking', for fear that if they
added the word 'is' they should be making the one to be many-as if
'one' and 'being' were always used in one and the same sense. What
'is' may be many either in definition (for example 'to be white' is
one thing, 'to be musical' another, yet the same thing be both, so the
one is many) or by division, as the whole and its parts. On this
point, indeed, they were already getting into difficulties and
admitted that the one was many-as if there was any difficulty about
the same thing being both one and many, provided that these are not
opposites; for 'one' may mean either 'potentially one' or 'actually

  If, then, we approach the thesis in this way it seems impossible for
all things to be one. Further, the arguments they use to prove their
position are not difficult to expose. For both of them reason
contentiously-I mean both Melissus and Parmenides. [Their premisses
are false and their conclusions do not follow. Or rather the
argument of Melissus is gross and palpable and offers no difficulty at
all: admit one ridiculous proposition and the rest follows-a simple
enough proceeding.] The fallacy of Melissus is obvious. For he
supposes that the assumption 'what has come into being always has a
beginning' justifies the assumption 'what has not come into being
has no beginning'. Then this also is absurd, that in every case
there should be a beginning of the thing-not of the time and not
only in the case of coming to be in the full sense but also in the
case of coming to have a quality-as if change never took place
suddenly. Again, does it follow that Being, if one, is motionless? Why
should it not move, the whole of it within itself, as parts of it do
which are unities, e.g. this water? Again, why is qualitative change
impossible? But, further, Being cannot be one in form, though it may
be in what it is made of. (Even some of the physicists hold it to be
one in the latter way, though not in the former.) Man obviously
differs from horse in form, and contraries from each other.
  The same kind of argument holds good against Parmenides also,
besides any that may apply specially to his view: the answer to him
being that 'this is not true' and 'that does not follow'. His
assumption that one is used in a single sense only is false, because
it is used in several. His conclusion does not follow, because if we
take only white things, and if 'white' has a single meaning, none
the less what is white will be many and not one. For what is white
will not be one either in the sense that it is continuous or in the
sense that it must be defined in only one way. 'Whiteness' will be
different from 'what has whiteness'. Nor does this mean that there
is anything that can exist separately, over and above what is white.
For 'whiteness' and 'that which is white' differ in definition, not in
the sense that they are things which can exist apart from each
other. But Parmenides had not come in sight of this distinction.
  It is necessary for him, then, to assume not only that 'being' has
the same meaning, of whatever it is predicated, but further that it
means (1) what just is and (2) what is just one.
  It must be so, for (1) an attribute is predicated of some subject,
so that the subject to which 'being' is attributed will not be, as
it is something different from 'being'. Something, therefore, which is
not will be. Hence 'substance' will not be a predicate of anything
else. For the subject cannot be a being, unless 'being' means
several things, in such a way that each is something. But ex hypothesi
'being' means only one thing.
  If, then, 'substance' is not attributed to anything, but other
things are attributed to it, how does 'substance' mean what is
rather than what is not? For suppose that 'substance' is also 'white'.
Since the definition of the latter is different (for being cannot even
be attributed to white, as nothing is which is not 'substance'), it
follows that 'white' is not-being--and that not in the sense of a
particular not-being, but in the sense that it is not at all. Hence
'substance' is not; for it is true to say that it is white, which we
found to mean not-being. If to avoid this we say that even 'white'
means substance, it follows that 'being' has more than one meaning.
  In particular, then, Being will not have magnitude, if it is
substance. For each of the two parts must he in a different sense.
  (2) Substance is plainly divisible into other substances, if we
consider the mere nature of a definition. For instance, if 'man' is
a substance, 'animal' and 'biped' must also be substances. For if
not substances, they must be attributes-and if attributes,
attributes either of (a) man or of (b) some other subject. But neither
is possible.
  (a) An attribute is either that which may or may not belong to the
subject or that in whose definition the subject of which it is an
attribute is involved. Thus 'sitting' is an example of a separable
attribute, while 'snubness' contains the definition of 'nose', to
which we attribute snubness. Further, the definition of the whole is
not contained in the definitions of the contents or elements of the
definitory formula; that of 'man' for instance in 'biped', or that
of 'white man' in 'white'. If then this is so, and if 'biped' is
supposed to be an attribute of 'man', it must be either separable,
so that 'man' might possibly not be 'biped', or the definition of
'man' must come into the definition of 'biped'-which is impossible, as
the converse is the case.
  (b) If, on the other hand, we suppose that 'biped' and 'animal'
are attributes not of man but of something else, and are not each of
them a substance, then 'man' too will be an attribute of something
else. But we must assume that substance is not the attribute of
anything, that the subject of which both 'biped' and 'animal' and each
separately are predicated is the subject also of the complex 'biped
  Are we then to say that the All is composed of indivisible
substances? Some thinkers did, in point of fact, give way to both
arguments. To the argument that all things are one if being means
one thing, they conceded that not-being is; to that from bisection,
they yielded by positing atomic magnitudes. But obviously it is not
true that if being means one thing, and cannot at the same time mean
the contradictory of this, there will be nothing which is not, for
even if what is not cannot be without qualification, there is no
reason why it should not be a particular not-being. To say that all
things will be one, if there is nothing besides Being itself, is
absurd. For who understands 'being itself' to be anything but a
particular substance? But if this is so, there is nothing to prevent
there being many beings, as has been said.
  It is, then, clearly impossible for Being to be one in this sense.

  The physicists on the other hand have two modes of explanation.
  The first set make the underlying body one either one of the three
or something else which is denser than fire and rarer than air then
generate everything else from this, and obtain multiplicity by
condensation and rarefaction. Now these are contraries, which may be
generalized into 'excess and defect'. (Compare Plato's 'Great and
Small'-except that he make these his matter, the one his form, while
the others treat the one which underlies as matter and the
contraries as differentiae, i.e. forms).
  The second set assert that the contrarieties are contained in the
one and emerge from it by segregation, for example Anaximander and
also all those who assert that 'what is' is one and many, like
Empedocles and Anaxagoras; for they too produce other things from
their mixture by segregation. These differ, however, from each other
in that the former imagines a cycle of such changes, the latter a
single series. Anaxagoras again made both his 'homceomerous'
substances and his contraries infinite in multitude, whereas
Empedocles posits only the so-called elements.
  The theory of Anaxagoras that the principles are infinite in
multitude was probably due to his acceptance of the common opinion
of the physicists that nothing comes into being from not-being. For
this is the reason why they use the phrase 'all things were
together' and the coming into being of such and such a kind of thing
is reduced to change of quality, while some spoke of combination and
separation. Moreover, the fact that the contraries proceed from each
other led them to the conclusion. The one, they reasoned, must have
already existed in the other; for since everything that comes into
being must arise either from what is or from what is not, and it is
impossible for it to arise from what is not (on this point all the
physicists agree), they thought that the truth of the alternative
necessarily followed, namely that things come into being out of
existent things, i.e. out of things already present, but imperceptible
to our senses because of the smallness of their bulk. So they assert
that everything has been mixed in every. thing, because they saw
everything arising out of everything. But things, as they say,
appear different from one another and receive different names
according to the nature of the particles which are numerically
predominant among the innumerable constituents of the mixture. For
nothing, they say, is purely and entirely white or black or sweet,
bone or flesh, but the nature of a thing is held to be that of which
it contains the most.
  Now (1) the infinite qua infinite is unknowable, so that what is
infinite in multitude or size is unknowable in quantity, and what is
infinite in variety of kind is unknowable in quality. But the
principles in question are infinite both in multitude and in kind.
Therefore it is impossible to know things which are composed of
them; for it is when we know the nature and quantity of its components
that we suppose we know a complex.
  Further (2) if the parts of a whole may be of any size in the
direction either of greatness or of smallness (by 'parts' I mean
components into which a whole can be divided and which are actually
present in it), it is necessary that the whole thing itself may be
of any size. Clearly, therefore, since it is impossible for an
animal or plant to be indefinitely big or small, neither can its parts
be such, or the whole will be the same. But flesh, bone, and the
like are the parts of animals, and the fruits are the parts of plants.
Hence it is obvious that neither flesh, bone, nor any such thing can
be of indefinite size in the direction either of the greater or of the
  Again (3) according to the theory all such things are already
present in one another and do not come into being but are constituents
which are separated out, and a thing receives its designation from its
chief constituent. Further, anything may come out of anything-water by
segregation from flesh and flesh from water. Hence, since every finite
body is exhausted by the repeated abstraction of a finite body, it
seems obviously to follow that everything cannot subsist in everything
else. For let flesh be extracted from water and again more flesh be
produced from the remainder by repeating the process of separation:
then, even though the quantity separated out will continually
decrease, still it will not fall below a certain magnitude. If,
therefore, the process comes to an end, everything will not be in
everything else (for there will be no flesh in the remaining water);
if on the other hand it does not, and further extraction is always
possible, there will be an infinite multitude of finite equal
particles in a finite quantity-which is impossible. Another proof
may be added: Since every body must diminish in size when something is
taken from it, and flesh is quantitatively definite in respect both of
greatness and smallness, it is clear that from the minimum quantity of
flesh no body can be separated out; for the flesh left would be less
than the minimum of flesh.
  Lastly (4) in each of his infinite bodies there would be already
present infinite flesh and blood and brain- having a distinct
existence, however, from one another, and no less real than the
infinite bodies, and each infinite: which is contrary to reason.
  The statement that complete separation never will take place is
correct enough, though Anaxagoras is not fully aware of what it means.
For affections are indeed inseparable. If then colours and states
had entered into the mixture, and if separation took place, there
would be a 'white' or a 'healthy' which was nothing but white or
healthy, i.e. was not the predicate of a subject. So his 'Mind' is
an absurd person aiming at the impossible, if he is supposed to wish
to separate them, and it is impossible to do so, both in respect of
quantity and of quality- of quantity, because there is no minimum
magnitude, and of quality, because affections are inseparable.
  Nor is Anaxagoras right about the coming to be of homogeneous
bodies. It is true there is a sense in which clay is divided into
pieces of clay, but there is another in which it is not. Water and air
are, and are generated 'from' each other, but not in the way in
which bricks come 'from' a house and again a house 'from' bricks;
and it is better to assume a smaller and finite number of
principles, as Empedocles does.

  All thinkers then agree in making the contraries principles, both
those who describe the All as one and unmoved (for even Parmenides
treats hot and cold as principles under the names of fire and earth)
and those too who use the rare and the dense. The same is true of
Democritus also, with his plenum and void, both of which exist, be
says, the one as being, the other as not-being. Again he speaks of
differences in position, shape, and order, and these are genera of
which the species are contraries, namely, of position, above and
below, before and behind; of shape, angular and angle-less, straight
and round.
  It is plain then that they all in one way or another identify the
contraries with the principles. And with good reason. For first
principles must not be derived from one another nor from anything
else, while everything has to be derived from them. But these
conditions are fulfilled by the primary contraries, which are not
derived from anything else because they are primary, nor from each
other because they are contraries.
  But we must see how this can be arrived at as a reasoned result,
as well as in the way just indicated.
  Our first presupposition must be that in nature nothing acts on,
or is acted on by, any other thing at random, nor may anything come
from anything else, unless we mean that it does so in virtue of a
concomitant attribute. For how could 'white' come from 'musical',
unless 'musical' happened to be an attribute of the not-white or of
the black? No, 'white' comes from 'not-white'-and not from any
'not-white', but from black or some intermediate colour. Similarly,
'musical' comes to be from 'not-musical', but not from any thing other
than musical, but from 'unmusical' or any intermediate state there may
  Nor again do things pass into the first chance thing; 'white' does
not pass into 'musical' (except, it may be, in virtue of a concomitant
attribute), but into 'not-white'-and not into any chance thing which
is not white, but into black or an intermediate colour; 'musical'
passes into 'not-musical'-and not into any chance thing other than
musical, but into 'unmusical' or any intermediate state there may be.
  The same holds of other things also: even things which are not
simple but complex follow the same principle, but the opposite state
has not received a name, so we fail to notice the fact. What is in
tune must come from what is not in tune, and vice versa; the tuned
passes into untunedness-and not into any untunedness, but into the
corresponding opposite. It does not matter whether we take attunement,
order, or composition for our illustration; the principle is obviously
the same in all, and in fact applies equally to the production of a
house, a statue, or any other complex. A house comes from certain
things in a certain state of separation instead of conjunction, a
statue (or any other thing that has been shaped) from
shapelessness-each of these objects being partly order and partly
  If then this is true, everything that comes to be or passes away
from, or passes into, its contrary or an intermediate state. But the
intermediates are derived from the contraries-colours, for instance,
from black and white. Everything, therefore, that comes to be by a
natural process is either a contrary or a product of contraries.
  Up to this point we have practically had most of the other writers
on the subject with us, as I have said already: for all of them
identify their elements, and what they call their principles, with the
contraries, giving no reason indeed for the theory, but contrained
as it were by the truth itself. They differ, however, from one another
in that some assume contraries which are more primary, others
contraries which are less so: some those more knowable in the order of
explanation, others those more familiar to sense. For some make hot
and cold, or again moist and dry, the conditions of becoming; while
others make odd and even, or again Love and Strife; and these differ
from each other in the way mentioned.
  Hence their principles are in one sense the same, in another
different; different certainly, as indeed most people think, but the
same inasmuch as they are analogous; for all are taken from the same
table of columns, some of the pairs being wider, others narrower in
extent. In this way then their theories are both the same and
different, some better, some worse; some, as I have said, take as
their contraries what is more knowable in the order of explanation,
others what is more familiar to sense. (The universal is more knowable
in the order of explanation, the particular in the order of sense: for
explanation has to do with the universal, sense with the
particular.) 'The great and the small', for example, belong to the
former class, 'the dense and the rare' to the latter.
  It is clear then that our principles must be contraries.

  The next question is whether the principles are two or three or more
in number.
  One they cannot be, for there cannot be one contrary. Nor can they
be innumerable, because, if so, Being will not be knowable: and in any
one genus there is only one contrariety, and substance is one genus:
also a finite number is sufficient, and a finite number, such as the
principles of Empedocles, is better than an infinite multitude; for
Empedocles professes to obtain from his principles all that Anaxagoras
obtains from his innumerable principles. Lastly, some contraries are
more primary than others, and some arise from others-for example sweet
and bitter, white and black-whereas the principles must always
remain principles.
  This will suffice to show that the principles are neither one nor
  Granted, then, that they are a limited number, it is plausible to
suppose them more than two. For it is difficult to see how either
density should be of such a nature as to act in any way on rarity or
rarity on density. The same is true of any other pair of contraries;
for Love does not gather Strife together and make things out of it,
nor does Strife make anything out of Love, but both act on a third
thing different from both. Some indeed assume more than one such thing
from which they construct the world of nature.
  Other objections to the view that it is not necessary to assume a
third principle as a substratum may be added. (1) We do not find
that the contraries constitute the substance of any thing. But what is
a first principle ought not to be the predicate of any subject. If
it were, there would be a principle of the supposed principle: for the
subject is a principle, and prior presumably to what is predicated
of it. Again (2) we hold that a substance is not contrary to another
substance. How then can substance be derived from what are not
substances? Or how can non-substances be prior to substance?
  If then we accept both the former argument and this one, we must, to
preserve both, assume a third somewhat as the substratum of the
contraries, such as is spoken of by those who describe the All as
one nature-water or fire or what is intermediate between them. What is
intermediate seems preferable; for fire, earth, air, and water are
already involved with pairs of contraries. There is, therefore, much
to be said for those who make the underlying substance different
from these four; of the rest, the next best choice is air, as
presenting sensible differences in a less degree than the others;
and after air, water. All, however, agree in this, that they
differentiate their One by means of the contraries, such as density
and rarity and more and less, which may of course be generalized, as
has already been said into excess and defect. Indeed this doctrine too
(that the One and excess and defect are the principles of things)
would appear to be of old standing, though in different forms; for the
early thinkers made the two the active and the one the passive
principle, whereas some of the more recent maintain the reverse.
  To suppose then that the elements are three in number would seem,
from these and similar considerations, a plausible view, as I said
before. On the other hand, the view that they are more than three in
number would seem to be untenable.
  For the one substratum is sufficient to be acted on; but if we
have four contraries, there will be two contrarieties, and we shall
have to suppose an intermediate nature for each pair separately. If,
on the other hand, the contrarieties, being two, can generate from
each other, the second contrariety will be superfluous. Moreover, it
is impossible that there should be more than one primary
contrariety. For substance is a single genus of being, so that the
principles can differ only as prior and posterior, not in genus; in
a single genus there is always a single contrariety, all the other
contrarieties in it being held to be reducible to one.
  It is clear then that the number of elements is neither one nor more
than two or three; but whether two or three is, as I said, a
question of considerable difficulty.

  We will now give our own account, approaching the question first
with reference to becoming in its widest sense: for we shall be
following the natural order of inquiry if we speak first of common
characteristics, and then investigate the characteristics of special
  We say that one thing comes to be from another thing, and one sort
of thing from another sort of thing, both in the case of simple and of
complex things. I mean the following. We can say (1) 'man becomes
musical', (2) what is 'not-musical becomes musical', or (3), the
'not-musical man becomes a musical man'. Now what becomes in (1) and
(2)-'man' and 'not musical'-I call simple, and what each
becomes-'musical'-simple also. But when (3) we say the 'not-musical
man becomes a musical man', both what becomes and what it becomes
are complex.
  As regards one of these simple 'things that become' we say not
only 'this becomes so-and-so', but also 'from being this, comes to
be so-and-so', as 'from being not-musical comes to be musical'; as
regards the other we do not say this in all cases, as we do not say
(1) 'from being a man he came to be musical' but only 'the man
became musical'.
  When a 'simple' thing is said to become something, in one case (1)
it survives through the process, in the other (2) it does not. For man
remains a man and is such even when he becomes musical, whereas what
is not musical or is unmusical does not continue to exist, either
simply or combined with the subject.
  These distinctions drawn, one can gather from surveying the
various cases of becoming in the way we are describing that, as we
say, there must always be an underlying something, namely that which
becomes, and that this, though always one numerically, in form at
least is not one. (By that I mean that it can be described in
different ways.) For 'to be man' is not the same as 'to be unmusical'.
One part survives, the other does not: what is not an opposite
survives (for 'man' survives), but 'not-musical' or 'unmusical' does
not survive, nor does the compound of the two, namely 'unmusical man'.
  We speak of 'becoming that from this' instead of 'this becoming
that' more in the case of what does not survive the change-'becoming
musical from unmusical', not 'from man'-but there are exceptions, as
we sometimes use the latter form of expression even of what
survives; we speak of 'a statue coming to be from bronze', not of
the 'bronze becoming a statue'. The change, however, from an
opposite which does not survive is described indifferently in both
ways, 'becoming that from this' or 'this becoming that'. We say both
that 'the unmusical becomes musical', and that 'from unmusical he
becomes musical'. And so both forms are used of the complex, 'becoming
a musical man from an unmusical man', and unmusical man becoming a
musical man'.
  But there are different senses of 'coming to be'. In some cases we
do not use the expression 'come to be', but 'come to be so-and-so'.
Only substances are said to 'come to be' in the unqualified sense.
  Now in all cases other than substance it is plain that there must be
some subject, namely, that which becomes. For we know that when a
thing comes to be of such a quantity or quality or in such a relation,
time, or place, a subject is always presupposed, since substance alone
is not predicated of another subject, but everything else of
  But that substances too, and anything else that can be said 'to
be' without qualification, come to be from some substratum, will
appear on examination. For we find in every case something that
underlies from which proceeds that which comes to be; for instance,
animals and plants from seed.
  Generally things which come to be, come to be in different ways: (1)
by change of shape, as a statue; (2) by addition, as things which
grow; (3) by taking away, as the Hermes from the stone; (4) by putting
together, as a house; (5) by alteration, as things which 'turn' in
respect of their material substance.
  It is plain that these are all cases of coming to be from a
  Thus, clearly, from what has been said, whatever comes to be is
always complex. There is, on the one hand, (a) something which comes
into existence, and again (b) something which becomes that-the
latter (b) in two senses, either the subject or the opposite. By the
'opposite' I mean the 'unmusical', by the 'subject' 'man', and
similarly I call the absence of shape or form or order the 'opposite',
and the bronze or stone or gold the 'subject'.
  Plainly then, if there are conditions and principles which
constitute natural objects and from which they primarily are or have
come to be-have come to be, I mean, what each is said to be in its
essential nature, not what each is in respect of a concomitant
attribute-plainly, I say, everything comes to be from both subject and
form. For 'musical man' is composed (in a way) of 'man' and 'musical':
you can analyse it into the definitions of its elements. It is clear
then that what comes to be will come to be from these elements.
  Now the subject is one numerically, though it is two in form. (For
it is the man, the gold-the 'matter' generally-that is counted, for it
is more of the nature of a 'this', and what comes to be does not
come from it in virtue of a concomitant attribute; the privation, on
the other hand, and the contrary are incidental in the process.) And
the positive form is one-the order, the acquired art of music, or
any similar predicate.
  There is a sense, therefore, in which we must declare the principles
to be two, and a sense in which they are three; a sense in which the
contraries are the principles-say for example the musical and the
unmusical, the hot and the cold, the tuned and the untuned-and a sense
in which they are not, since it is impossible for the contraries to be
acted on by each other. But this difficulty also is solved by the fact
that the substratum is different from the contraries, for it is itself
not a contrary. The principles therefore are, in a way, not more in
number than the contraries, but as it were two, nor yet precisely two,
since there is a difference of essential nature, but three. For 'to be
man' is different from 'to be unmusical', and 'to be unformed' from
'to be bronze'.
  We have now stated the number of the principles of natural objects
which are subject to generation, and how the number is reached: and it
is clear that there must be a substratum for the contraries, and
that the contraries must be two. (Yet in another way of putting it
this is not necessary, as one of the contraries will serve to effect
the change by its successive absence and presence.)
  The underlying nature is an object of scientific knowledge, by an
analogy. For as the bronze is to the statue, the wood to the bed, or
the matter and the formless before receiving form to any thing which
has form, so is the underlying nature to substance, i.e. the 'this' or
  This then is one principle (though not one or existent in the same
sense as the 'this'), and the definition was one as we agreed; then
further there is its contrary, the privation. In what sense these
are two, and in what sense more, has been stated above. Briefly, we
explained first that only the contraries were principles, and later
that a substratum was indispensable, and that the principles were
three; our last statement has elucidated the difference between the
contraries, the mutual relation of the principles, and the nature of
the substratum. Whether the form or the substratum is the essential
nature of a physical object is not yet clear. But that the
principles are three, and in what sense, and the way in which each
is a principle, is clear.
  So much then for the question of the number and the nature of the

  We will now proceed to show that the difficulty of the early
thinkers, as well as our own, is solved in this way alone.
  The first of those who studied science were misled in their search
for truth and the nature of things by their inexperience, which as
it were thrust them into another path. So they say that none of the
things that are either comes to be or passes out of existence, because
what comes to be must do so either from what is or from what is not,
both of which are impossible. For what is cannot come to be (because
it is already), and from what is not nothing could have come to be
(because something must be present as a substratum). So too they
exaggerated the consequence of this, and went so far as to deny even
the existence of a plurality of things, maintaining that only Being
itself is. Such then was their opinion, and such the reason for its
  Our explanation on the other hand is that the phrases 'something
comes to be from what is or from what is not', 'what is not or what is
does something or has something done to it or becomes some
particular thing', are to be taken (in the first way of putting our
explanation) in the same sense as 'a doctor does something or has
something done to him', 'is or becomes something from being a doctor.'
These expressions may be taken in two senses, and so too, clearly, may
'from being', and 'being acts or is acted on'. A doctor builds a
house, not qua doctor, but qua housebuilder, and turns gray, not qua
doctor, but qua dark-haired. On the other hand he doctors or fails
to doctor qua doctor. But we are using words most appropriately when
we say that a doctor does something or undergoes something, or becomes
something from being a doctor, if he does, undergoes, or becomes qua
doctor. Clearly then also 'to come to be so-and-so from not-being'
means 'qua not-being'.
  It was through failure to make this distinction that those
thinkers gave the matter up, and through this error that they went
so much farther astray as to suppose that nothing else comes to be
or exists apart from Being itself, thus doing away with all becoming.
  We ourselves are in agreement with them in holding that nothing
can be said without qualification to come from what is not. But
nevertheless we maintain that a thing may 'come to be from what is
not'-that is, in a qualified sense. For a thing comes to be from the
privation, which in its own nature is not-being,-this not surviving as
a constituent of the result. Yet this causes surprise, and it is
thought impossible that something should come to be in the way
described from what is not.
  In the same way we maintain that nothing comes to be from being, and
that being does not come to be except in a qualified sense. In that
way, however, it does, just as animal might come to be from animal,
and an animal of a certain kind from an animal of a certain kind.
Thus, suppose a dog to come to be from a horse. The dog would then, it
is true, come to be from animal (as well as from an animal of a
certain kind) but not as animal, for that is already there. But if
anything is to become an animal, not in a qualified sense, it will not
be from animal: and if being, not from being-nor from not-being
either, for it has been explained that by 'from not being' we mean
from not-being qua not-being.
  Note further that we do not subvert the principle that everything
either is or is not.
  This then is one way of solving the difficulty. Another consists
in pointing out that the same things can be explained in terms of
potentiality and actuality. But this has been done with greater
precision elsewhere. So, as we said, the difficulties which
constrain people to deny the existence of some of the things we
mentioned are now solved. For it was this reason which also caused
some of the earlier thinkers to turn so far aside from the road
which leads to coming to be and passing away and change generally.
If they had come in sight of this nature, all their ignorance would
have been dispelled.

  Others, indeed, have apprehended the nature in question, but not
  In the first place they allow that a thing may come to be without
qualification from not being, accepting on this point the statement of
Parmenides. Secondly, they think that if the substratum is one
numerically, it must have also only a single potentiality-which is a
very different thing.
  Now we distinguish matter and privation, and hold that one of these,
namely the matter, is not-being only in virtue of an attribute which
it has, while the privation in its own nature is not-being; and that
the matter is nearly, in a sense is, substance, while the privation in
no sense is. They, on the other hand, identify their Great and Small
alike with not being, and that whether they are taken together as
one or separately. Their triad is therefore of quite a different
kind from ours. For they got so far as to see that there must be
some underlying nature, but they make it one-for even if one
philosopher makes a dyad of it, which he calls Great and Small, the
effect is the same, for he overlooked the other nature. For the one
which persists is a joint cause, with the form, of what comes to
be-a mother, as it were. But the negative part of the contrariety
may often seem, if you concentrate your attention on it as an evil
agent, not to exist at all.
  For admitting with them that there is something divine, good, and
desirable, we hold that there are two other principles, the one
contrary to it, the other such as of its own nature to desire and
yearn for it. But the consequence of their view is that the contrary
desires its wtextinction. Yet the form cannot desire itself, for it is
not defective; nor can the contrary desire it, for contraries are
mutually destructive. The truth is that what desires the form is
matter, as the female desires the male and the ugly the beautiful-only
the ugly or the female not per se but per accidens.
  The matter comes to be and ceases to be in one sense, while in
another it does not. As that which contains the privation, it ceases
to be in its own nature, for what ceases to be-the privation-is
contained within it. But as potentiality it does not cease to be in
its own nature, but is necessarily outside the sphere of becoming
and ceasing to be. For if it came to be, something must have existed
as a primary substratum from which it should come and which should
persist in it; but this is its own special nature, so that it will
be before coming to be. (For my definition of matter is just
this-the primary substratum of each thing, from which it comes to be
without qualification, and which persists in the result.) And if it
ceases to be it will pass into that at the last, so it will have
ceased to be before ceasing to be.
  The accurate determination of the first principle in respect of
form, whether it is one or many and what it is or what they are, is
the province of the primary type of science; so these questions may
stand over till then. But of the natural, i.e. perishable, forms we
shall speak in the expositions which follow.
  The above, then, may be taken as sufficient to establish that
there are principles and what they are and how many there are. Now let
us make a fresh start and proceed.
                              Book II

  Of things that exist, some exist by nature, some from other causes.
  'By nature' the animals and their parts exist, and the plants and
the simple bodies (earth, fire, air, water)-for we say that these
and the like exist 'by nature'.
  All the things mentioned present a feature in which they differ from
things which are not constituted by nature. Each of them has within
itself a principle of motion and of stationariness (in respect of
place, or of growth and decrease, or by way of alteration). On the
other hand, a bed and a coat and anything else of that sort, qua
receiving these designations i.e. in so far as they are products of
art-have no innate impulse to change. But in so far as they happen
to be composed of stone or of earth or of a mixture of the two, they
do have such an impulse, and just to that extent which seems to
indicate that nature is a source or cause of being moved and of
being at rest in that to which it belongs primarily, in virtue of
itself and not in virtue of a concomitant attribute.
  I say 'not in virtue of a concomitant attribute', because (for
instance) a man who is a doctor might cure himself. Nevertheless it is
not in so far as he is a patient that he possesses the art of
medicine: it merely has happened that the same man is doctor and
patient-and that is why these attributes are not always found
together. So it is with all other artificial products. None of them
has in itself the source of its own production. But while in some
cases (for instance houses and the other products of manual labour)
that principle is in something else external to the thing, in others
those which may cause a change in themselves in virtue of a
concomitant attribute-it lies in the things themselves (but not in
virtue of what they are).
  'Nature' then is what has been stated. Things 'have a nature'which
have a principle of this kind. Each of them is a substance; for it
is a subject, and nature always implies a subject in which it inheres.
  The term 'according to nature' is applied to all these things and
also to the attributes which belong to them in virtue of what they
are, for instance the property of fire to be carried upwards-which
is not a 'nature' nor 'has a nature' but is 'by nature' or
'according to nature'.
  What nature is, then, and the meaning of the terms 'by nature' and
'according to nature', has been stated. That nature exists, it would
be absurd to try to prove; for it is obvious that there are many
things of this kind, and to prove what is obvious by what is not is
the mark of a man who is unable to distinguish what is self-evident
from what is not. (This state of mind is clearly possible. A man blind
from birth might reason about colours. Presumably therefore such
persons must be talking about words without any thought to
  Some identify the nature or substance of a natural object with
that immediate constituent of it which taken by itself is without
arrangement, e.g. the wood is the 'nature' of the bed, and the
bronze the 'nature' of the statue.
  As an indication of this Antiphon points out that if you planted a
bed and the rotting wood acquired the power of sending up a shoot,
it would not be a bed that would come up, but wood-which shows that
the arrangement in accordance with the rules of the art is merely an
incidental attribute, whereas the real nature is the other, which,
further, persists continuously through the process of making.
  But if the material of each of these objects has itself the same
relation to something else, say bronze (or gold) to water, bones (or
wood) to earth and so on, that (they say) would be their nature and
essence. Consequently some assert earth, others fire or air or water
or some or all of these, to be the nature of the things that are.
For whatever any one of them supposed to have this character-whether
one thing or more than one thing-this or these he declared to be the
whole of substance, all else being its affections, states, or
dispositions. Every such thing they held to be eternal (for it could
not pass into anything else), but other things to come into being
and cease to be times without number.
  This then is one account of 'nature', namely that it is the
immediate material substratum of things which have in themselves a
principle of motion or change.
  Another account is that 'nature' is the shape or form which is
specified in the definition of the thing.
  For the word 'nature' is applied to what is according to nature
and the natural in the same way as 'art' is applied to what is
artistic or a work of art. We should not say in the latter case that
there is anything artistic about a thing, if it is a bed only
potentially, not yet having the form of a bed; nor should we call it a
work of art. The same is true of natural compounds. What is
potentially flesh or bone has not yet its own 'nature', and does not
exist until it receives the form specified in the definition, which we
name in defining what flesh or bone is. Thus in the second sense of
'nature' it would be the shape or form (not separable except in
statement) of things which have in themselves a source of motion. (The
combination of the two, e.g. man, is not 'nature' but 'by nature' or
  The form indeed is 'nature' rather than the matter; for a thing is
more properly said to be what it is when it has attained to fulfilment
than when it exists potentially. Again man is born from man, but not
bed from bed. That is why people say that the figure is not the nature
of a bed, but the wood is-if the bed sprouted not a bed but wood would
come up. But even if the figure is art, then on the same principle the
shape of man is his nature. For man is born from man.
  We also speak of a thing's nature as being exhibited in the
process of growth by which its nature is attained. The 'nature' in
this sense is not like 'doctoring', which leads not to the art of
doctoring but to health. Doctoring must start from the art, not lead
to it. But it is not in this way that nature (in the one sense) is
related to nature (in the other). What grows qua growing grows from
something into something. Into what then does it grow? Not into that
from which it arose but into that to which it tends. The shape then is
  'Shape' and 'nature', it should be added, are in two senses. For the
privation too is in a way form. But whether in unqualified coming to
be there is privation, i.e. a contrary to what comes to be, we must
consider later.

  We have distinguished, then, the different ways in which the term
'nature' is used.
  The next point to consider is how the mathematician differs from the
physicist. Obviously physical bodies contain surfaces and volumes,
lines and points, and these are the subject-matter of mathematics.
  Further, is astronomy different from physics or a department of
it? It seems absurd that the physicist should be supposed to know
the nature of sun or moon, but not to know any of their essential
attributes, particularly as the writers on physics obviously do
discuss their shape also and whether the earth and the world are
spherical or not.
  Now the mathematician, though he too treats of these things,
nevertheless does not treat of them as the limits of a physical
body; nor does he consider the attributes indicated as the
attributes of such bodies. That is why he separates them; for in
thought they are separable from motion, and it makes no difference,
nor does any falsity result, if they are separated. The holders of the
theory of Forms do the same, though they are not aware of it; for they
separate the objects of physics, which are less separable than those
of mathematics. This becomes plain if one tries to state in each of
the two cases the definitions of the things and of their attributes.
'Odd' and 'even', 'straight' and 'curved', and likewise 'number',
'line', and 'figure', do not involve motion; not so 'flesh' and 'bone'
and 'man'-these are defined like 'snub nose', not like 'curved'.
  Similar evidence is supplied by the more physical of the branches of
mathematics, such as optics, harmonics, and astronomy. These are in
a way the converse of geometry. While geometry investigates physical
lines but not qua physical, optics investigates mathematical lines,
but qua physical, not qua mathematical.
  Since 'nature' has two senses, the form and the matter, we must
investigate its objects as we would the essence of snubness. That
is, such things are neither independent of matter nor can be defined
in terms of matter only. Here too indeed one might raise a difficulty.
Since there are two natures, with which is the physicist concerned? Or
should he investigate the combination of the two? But if the
combination of the two, then also each severally. Does it belong
then to the same or to different sciences to know each severally?
  If we look at the ancients, physics would to be concerned with the
matter. (It was only very slightly that Empedocles and Democritus
touched on the forms and the essence.)
  But if on the other hand art imitates nature, and it is the part
of the same discipline to know the form and the matter up to a point
(e.g. the doctor has a knowledge of health and also of bile and
phlegm, in which health is realized, and the builder both of the
form of the house and of the matter, namely that it is bricks and
beams, and so forth): if this is so, it would be the part of physics
also to know nature in both its senses.
  Again, 'that for the sake of which', or the end, belongs to the same
department of knowledge as the means. But the nature is the end or
'that for the sake of which'. For if a thing undergoes a continuous
change and there is a stage which is last, this stage is the end or
'that for the sake of which'. (That is why the poet was carried away
into making an absurd statement when he said 'he has the end for the
sake of which he was born'. For not every stage that is last claims to
be an end, but only that which is best.)
  For the arts make their material (some simply 'make' it, others make
it serviceable), and we use everything as if it was there for our
sake. (We also are in a sense an end. 'That for the sake of which' has
two senses: the distinction is made in our work On Philosophy.) The
arts, therefore, which govern the matter and have knowledge are two,
namely the art which uses the product and the art which directs the
production of it. That is why the using art also is in a sense
directive; but it differs in that it knows the form, whereas the art
which is directive as being concerned with production knows the
matter. For the helmsman knows and prescribes what sort of form a helm
should have, the other from what wood it should be made and by means
of what operations. In the products of art, however, we make the
material with a view to the function, whereas in the products of
nature the matter is there all along.
  Again, matter is a relative term: to each form there corresponds a
special matter. How far then must the physicist know the form or
essence? Up to a point, perhaps, as the doctor must know sinew or
the smith bronze (i.e. until he understands the purpose of each):
and the physicist is concerned only with things whose forms are
separable indeed, but do not exist apart from matter. Man is
begotten by man and by the sun as well. The mode of existence and
essence of the separable it is the business of the primary type of
philosophy to define.

  Now that we have established these distinctions, we must proceed
to consider causes, their character and number. Knowledge is the
object of our inquiry, and men do not think they know a thing till
they have grasped the 'why' of (which is to grasp its primary
cause). So clearly we too must do this as regards both coming to be
and passing away and every kind of physical change, in order that,
knowing their principles, we may try to refer to these principles each
of our problems.
  In one sense, then, (1) that out of which a thing comes to be and
which persists, is called 'cause', e.g. the bronze of the statue,
the silver of the bowl, and the genera of which the bronze and the
silver are species.
  In another sense (2) the form or the archetype, i.e. the statement
of the essence, and its genera, are called 'causes' (e.g. of the
octave the relation of 2:1, and generally number), and the parts in
the definition.
  Again (3) the primary source of the change or coming to rest; e.g.
the man who gave advice is a cause, the father is cause of the
child, and generally what makes of what is made and what causes change
of what is changed.
  Again (4) in the sense of end or 'that for the sake of which' a
thing is done, e.g. health is the cause of walking about. ('Why is
he walking about?' we say. 'To be healthy', and, having said that,
we think we have assigned the cause.) The same is true also of all the
intermediate steps which are brought about through the action of
something else as means towards the end, e.g. reduction of flesh,
purging, drugs, or surgical instruments are means towards health.
All these things are 'for the sake of' the end, though they differ
from one another in that some are activities, others instruments.
  This then perhaps exhausts the number of ways in which the term
'cause' is used.
  As the word has several senses, it follows that there are several
causes of the same thing not merely in virtue of a concomitant
attribute), e.g. both the art of the sculptor and the bronze are
causes of the statue. These are causes of the statue qua statue, not
in virtue of anything else that it may be-only not in the same way,
the one being the material cause, the other the cause whence the
motion comes. Some things cause each other reciprocally, e.g. hard
work causes fitness and vice versa, but again not in the same way, but
the one as end, the other as the origin of change. Further the same
thing is the cause of contrary results. For that which by its presence
brings about one result is sometimes blamed for bringing about the
contrary by its absence. Thus we ascribe the wreck of a ship to the
absence of the pilot whose presence was the cause of its safety.
  All the causes now mentioned fall into four familiar divisions.
The letters are the causes of syllables, the material of artificial
products, fire, &c., of bodies, the parts of the whole, and the
premisses of the conclusion, in the sense of 'that from which'. Of
these pairs the one set are causes in the sense of substratum, e.g.
the parts, the other set in the sense of essence-the whole and the
combination and the form. But the seed and the doctor and the adviser,
and generally the maker, are all sources whence the change or
stationariness originates, while the others are causes in the sense of
the end or the good of the rest; for 'that for the sake of which'
means what is best and the end of the things that lead up to it.
(Whether we say the 'good itself or the 'apparent good' makes no
  Such then is the number and nature of the kinds of cause.
  Now the modes of causation are many, though when brought under heads
they too can be reduced in number. For 'cause' is used in many
senses and even within the same kind one may be prior to another (e.g.
the doctor and the expert are causes of health, the relation 2:1 and
number of the octave), and always what is inclusive to what is
particular. Another mode of causation is the incidental and its
genera, e.g. in one way 'Polyclitus', in another 'sculptor' is the
cause of a statue, because 'being Polyclitus' and 'sculptor' are
incidentally conjoined. Also the classes in which the incidental
attribute is included; thus 'a man' could be said to be the cause of a
statue or, generally, 'a living creature'. An incidental attribute too
may be more or less remote, e.g. suppose that 'a pale man' or 'a
musical man' were said to be the cause of the statue.
  All causes, both proper and incidental, may be spoken of either as
potential or as actual; e.g. the cause of a house being built is
either 'house-builder' or 'house-builder building'.
  Similar distinctions can be made in the things of which the causes
are causes, e.g. of 'this statue' or of 'statue' or of 'image'
generally, of 'this bronze' or of 'bronze' or of 'material' generally.
So too with the incidental attributes. Again we may use a complex
expression for either and say, e.g. neither 'Polyclitus' nor
'sculptor' but 'Polyclitus, sculptor'.
  All these various uses, however, come to six in number, under each
of which again the usage is twofold. Cause means either what is
particular or a genus, or an incidental attribute or a genus of
that, and these either as a complex or each by itself; and all six
either as actual or as potential. The difference is this much, that
causes which are actually at work and particular exist and cease to
exist simultaneously with their effect, e.g. this healing person
with this being-healed person and that house-building man with that
being-built house; but this is not always true of potential
causes--the house and the housebuilder do not pass away
  In investigating the cause of each thing it is always necessary to
seek what is most precise (as also in other things): thus man builds
because he is a builder, and a builder builds in virtue of his art
of building. This last cause then is prior: and so generally.
  Further, generic effects should be assigned to generic causes,
particular effects to particular causes, e.g. statue to sculptor, this
statue to this sculptor; and powers are relative to possible
effects, actually operating causes to things which are actually
being effected.
  This must suffice for our account of the number of causes and the
modes of causation.

  But chance also and spontaneity are reckoned among causes: many
things are said both to be and to come to be as a result of chance and
spontaneity. We must inquire therefore in what manner chance and
spontaneity are present among the causes enumerated, and whether
they are the same or different, and generally what chance and
spontaneity are.
  Some people even question whether they are real or not. They say
that nothing happens by chance, but that everything which we ascribe
to chance or spontaneity has some definite cause, e.g. coming 'by
chance' into the market and finding there a man whom one wanted but
did not expect to meet is due to one's wish to go and buy in the
market. Similarly in other cases of chance it is always possible, they
maintain, to find something which is the cause; but not chance, for if
chance were real, it would seem strange indeed, and the question might
be raised, why on earth none of the wise men of old in speaking of the
causes of generation and decay took account of chance; whence it would
seem that they too did not believe that anything is by chance. But
there is a further circumstance that is surprising. Many things both
come to be and are by chance and spontaneity, and although know that
each of them can be ascribed to some cause (as the old argument said
which denied chance), nevertheless they speak of some of these
things as happening by chance and others not. For this reason also
they ought to have at least referred to the matter in some way or
  Certainly the early physicists found no place for chance among the
causes which they recognized-love, strife, mind, fire, or the like.
This is strange, whether they supposed that there is no such thing
as chance or whether they thought there is but omitted to mention
it-and that too when they sometimes used it, as Empedocles does when
he says that the air is not always separated into the highest
region, but 'as it may chance'. At any rate he says in his cosmogony
that 'it happened to run that way at that time, but it often ran
otherwise.' He tells us also that most of the parts of animals came to
be by chance.
  There are some too who ascribe this heavenly sphere and all the
worlds to spontaneity. They say that the vortex arose spontaneously,
i.e. the motion that separated and arranged in its present order all
that exists. This statement might well cause surprise. For they are
asserting that chance is not responsible for the existence or
generation of animals and plants, nature or mind or something of the
kind being the cause of them (for it is not any chance thing that
comes from a given seed but an olive from one kind and a man from
another); and yet at the same time they assert that the heavenly
sphere and the divinest of visible things arose spontaneously,
having no such cause as is assigned to animals and plants. Yet if this
is so, it is a fact which deserves to be dwelt upon, and something
might well have been said about it. For besides the other
absurdities of the statement, it is the more absurd that people should
make it when they see nothing coming to be spontaneously in the
heavens, but much happening by chance among the things which as they
say are not due to chance; whereas we should have expected exactly the
  Others there are who, indeed, believe that chance is a cause, but
that it is inscrutable to human intelligence, as being a divine
thing and full of mystery.
  Thus we must inquire what chance and spontaneity are, whether they
are the same or different, and how they fit into our division of

  First then we observe that some things always come to pass in the
same way, and others for the most part. It is clearly of neither of
these that chance is said to be the cause, nor can the 'effect of
chance' be identified with any of the things that come to pass by
necessity and always, or for the most part. But as there is a third
class of events besides these two-events which all say are 'by
chance'-it is plain that there is such a thing as chance and
spontaneity; for we know that things of this kind are due to chance
and that things due to chance are of this kind.
  But, secondly, some events are for the sake of something, others
not. Again, some of the former class are in accordance with deliberate
intention, others not, but both are in the class of things which are
for the sake of something. Hence it is clear that even among the
things which are outside the necessary and the normal, there are
some in connexion withwhich the phrase 'for the sake of something'
is applicable. (Events that are for the sake of something include
whatever may be done as a result of thought or of nature.) Things of
this kind, then, when they come to pass incidental are said to be
'by chance'. For just as a thing is something either in virtue of
itself or incidentally, so may it be a cause. For instance, the
housebuilding faculty is in virtue of itself the cause of a house,
whereas the pale or the musical is the incidental cause. That which is
per se cause of the effect is determinate, but the incidental cause is
indeterminable, for the possible attributes of an individual are
innumerable. To resume then; when a thing of this kind comes to pass
among events which are for the sake of something, it is said to be
spontaneous or by chance. (The distinction between the two must be
made later-for the present it is sufficient if it is plain that both
are in the sphere of things done for the sake of something.)
  Example: A man is engaged in collecting subscriptions for a feast.
He would have gone to such and such a place for the purpose of getting
the money, if he had known. He actually went there for another purpose
and it was only incidentally that he got his money by going there; and
this was not due to the fact that he went there as a rule or
necessarily, nor is the end effected (getting the money) a cause
present in himself-it belongs to the class of things that are
intentional and the result of intelligent deliberation. It is when
these conditions are satisfied that the man is said to have gone 'by
chance'. If he had gone of deliberate purpose and for the sake of
this-if he always or normally went there when he was collecting
payments-he would not be said to have gone 'by chance'.
  It is clear then that chance is an incidental cause in the sphere of
those actions for the sake of something which involve purpose.
Intelligent reflection, then, and chance are in the same sphere, for
purpose implies intelligent reflection.
  It is necessary, no doubt, that the causes of what comes to pass
by chance be indefinite; and that is why chance is supposed to
belong to the class of the indefinite and to be inscrutable to man,
and why it might be thought that, in a way, nothing occurs by
chance. For all these statements are correct, because they are well
grounded. Things do, in a way, occur by chance, for they occur
incidentally and chance is an incidental cause. But strictly it is not
the cause-without qualification-of anything; for instance, a
housebuilder is the cause of a house; incidentally, a fluteplayer
may be so.
  And the causes of the man's coming and getting the money (when he
did not come for the sake of that) are innumerable. He may have wished
to see somebody or been following somebody or avoiding somebody, or
may have gone to see a spectacle. Thus to say that chance is a thing
contrary to rule is correct. For 'rule' applies to what is always true
or true for the most part, whereas chance belongs to a third type of
event. Hence, to conclude, since causes of this kind are indefinite,
chance too is indefinite. (Yet in some cases one might raise the
question whether any incidental fact might be the cause of the
chance occurrence, e.g. of health the fresh air or the sun's heat
may be the cause, but having had one's hair cut cannot; for some
incidental causes are more relevant to the effect than others.)
  Chance or fortune is called 'good' when the result is good, 'evil'
when it is evil. The terms 'good fortune' and 'ill fortune' are used
when either result is of considerable magnitude. Thus one who comes
within an ace of some great evil or great good is said to be fortunate
or unfortunate. The mind affirms the essence of the attribute,
ignoring the hair's breadth of difference. Further, it is with
reason that good fortune is regarded as unstable; for chance is
unstable, as none of the things which result from it can be invariable
or normal.
  Both are then, as I have said, incidental causes-both chance and
spontaneity-in the sphere of things which are capable of coming to
pass not necessarily, nor normally, and with reference to such of
these as might come to pass for the sake of something.

  They differ in that 'spontaneity' is the wider term. Every result of
chance is from what is spontaneous, but not everything that is from
what is spontaneous is from chance.
  Chance and what results from chance are appropriate to agents that
are capable of good fortune and of moral action generally. Therefore
necessarily chance is in the sphere of moral actions. This is
indicated by the fact that good fortune is thought to be the same,
or nearly the same, as happiness, and happiness to be a kind of
moral action, since it is well-doing. Hence what is not capable of
moral action cannot do anything by chance. Thus an inanimate thing
or a lower animal or a child cannot do anything by chance, because
it is incapable of deliberate intention; nor can 'good fortune' or
'ill fortune' be ascribed to them, except metaphorically, as
Protarchus, for example, said that the stones of which altars are made
are fortunate because they are held in honour, while their fellows are
trodden under foot. Even these things, however, can in a way be
affected by chance, when one who is dealing with them does something
to them by chance, but not otherwise.
  The spontaneous on the other hand is found both in the lower animals
and in many inanimate objects. We say, for example, that the horse
came 'spontaneously', because, though his coming saved him, he did not
come for the sake of safety. Again, the tripod fell 'of itself',
because, though when it fell it stood on its feet so as to serve for a
seat, it did not fall for the sake of that.
  Hence it is clear that events which (1) belong to the general
class of things that may come to pass for the sake of something, (2)
do not come to pass for the sake of what actually results, and (3)
have an external cause, may be described by the phrase 'from
spontaneity'. These 'spontaneous' events are said to be 'from
chance' if they have the further characteristics of being the
objects of deliberate intention and due to agents capable of that mode
of action. This is indicated by the phrase 'in vain', which is used
when A which is for the sake of B, does not result in B. For instance,
taking a walk is for the sake of evacuation of the bowels; if this
does not follow after walking, we say that we have walked 'in vain'
and that the walking was 'vain'. This implies that what is naturally
the means to an end is 'in vain', when it does not effect the end
towards which it was the natural means-for it would be absurd for a
man to say that he had bathed in vain because the sun was not
eclipsed, since the one was not done with a view to the other. Thus
the spontaneous is even according to its derivation the case in
which the thing itself happens in vain. The stone that struck the
man did not fall for the purpose of striking him; therefore it fell
spontaneously, because it might have fallen by the action of an
agent and for the purpose of striking. The difference between
spontaneity and what results by chance is greatest in things that come
to be by nature; for when anything comes to be contrary to nature,
we do not say that it came to be by chance, but by spontaneity. Yet
strictly this too is different from the spontaneous proper; for the
cause of the latter is external, that of the former internal.
  We have now explained what chance is and what spontaneity is, and in
what they differ from each other. Both belong to the mode of causation
'source of change', for either some natural or some intelligent
agent is always the cause; but in this sort of causation the number of
possible causes is infinite.
  Spontaneity and chance are causes of effects which though they might
result from intelligence or nature, have in fact been caused by
something incidentally. Now since nothing which is incidental is prior
to what is per se, it is clear that no incidental cause can be prior
to a cause per se. Spontaneity and chance, therefore, are posterior to
intelligence and nature. Hence, however true it may be that the
heavens are due to spontaneity, it will still be true that
intelligence and nature will be prior causes of this All and of many
things in it besides.

  It is clear then that there are causes, and that the number of
them is what we have stated. The number is the same as that of the
things comprehended under the question 'why'. The 'why' is referred
ultimately either (1), in things which do not involve motion, e.g.
in mathematics, to the 'what' (to the definition of 'straight line' or
'commensurable', &c.), or (2) to what initiated a motion, e.g. 'why
did they go to war?-because there had been a raid'; or (3) we are
inquiring 'for the sake of what?'-'that they may rule'; or (4), in the
case of things that come into being, we are looking for the matter.
The causes, therefore, are these and so many in number.
  Now, the causes being four, it is the business of the physicist to
know about them all, and if he refers his problems back to all of
them, he will assign the 'why' in the way proper to his science-the
matter, the form, the mover, 'that for the sake of which'. The last
three often coincide; for the 'what' and 'that for the sake of
which' are one, while the primary source of motion is the same in
species as these (for man generates man), and so too, in general,
are all things which cause movement by being themselves moved; and
such as are not of this kind are no longer inside the province of
physics, for they cause motion not by possessing motion or a source of
motion in themselves, but being themselves incapable of motion.
Hence there are three branches of study, one of things which are
incapable of motion, the second of things in motion, but
indestructible, the third of destructible things.
  The question 'why', then, is answered by reference to the matter, to
the form, and to the primary moving cause. For in respect of coming to
be it is mostly in this last way that causes are investigated-'what
comes to be after what? what was the primary agent or patient?' and so
at each step of the series.
  Now the principles which cause motion in a physical way are two,
of which one is not physical, as it has no principle of motion in
itself. Of this kind is whatever causes movement, not being itself
moved, such as (1) that which is completely unchangeable, the
primary reality, and (2) the essence of that which is coming to be,
i.e. the form; for this is the end or 'that for the sake of which'.
Hence since nature is for the sake of something, we must know this
cause also. We must explain the 'why' in all the senses of the term,
namely, (1) that from this that will necessarily result ('from this'
either without qualification or in most cases); (2) that 'this must be
so if that is to be so' (as the conclusion presupposes the premisses);
(3) that this was the essence of the thing; and (4) because it is
better thus (not without qualification, but with reference to the
essential nature in each case).

  We must explain then (1) that Nature belongs to the class of
causes which act for the sake of something; (2) about the necessary
and its place in physical problems, for all writers ascribe things
to this cause, arguing that since the hot and the cold, &c., are of
such and such a kind, therefore certain things necessarily are and
come to be-and if they mention any other cause (one his 'friendship
and strife', another his 'mind'), it is only to touch on it, and
then good-bye to it.
  A difficulty presents itself: why should not nature work, not for
the sake of something, nor because it is better so, but just as the
sky rains, not in order to make the corn grow, but of necessity?
What is drawn up must cool, and what has been cooled must become water
and descend, the result of this being that the corn grows. Similarly
if a man's crop is spoiled on the threshing-floor, the rain did not
fall for the sake of this-in order that the crop might be
spoiled-but that result just followed. Why then should it not be the
same with the parts in nature, e.g. that our teeth should come up of
necessity-the front teeth sharp, fitted for tearing, the molars
broad and useful for grinding down the food-since they did not arise
for this end, but it was merely a coincident result; and so with all
other parts in which we suppose that there is purpose? Wherever then
all the parts came about just what they would have been if they had
come be for an end, such things survived, being organized
spontaneously in a fitting way; whereas those which grew otherwise
perished and continue to perish, as Empedocles says his 'man-faced
ox-progeny' did.
  Such are the arguments (and others of the kind) which may cause
difficulty on this point. Yet it is impossible that this should be the
true view. For teeth and all other natural things either invariably or
normally come about in a given way; but of not one of the results of
chance or spontaneity is this true. We do not ascribe to chance or
mere coincidence the frequency of rain in winter, but frequent rain in
summer we do; nor heat in the dog-days, but only if we have it in
winter. If then, it is agreed that things are either the result of
coincidence or for an end, and these cannot be the result of
coincidence or spontaneity, it follows that they must be for an end;
and that such things are all due to nature even the champions of the
theory which is before us would agree. Therefore action for an end
is present in things which come to be and are by nature.
  Further, where a series has a completion, all the preceding steps
are for the sake of that. Now surely as in intelligent action, so in
nature; and as in nature, so it is in each action, if nothing
interferes. Now intelligent action is for the sake of an end;
therefore the nature of things also is so. Thus if a house, e.g. had
been a thing made by nature, it would have been made in the same way
as it is now by art; and if things made by nature were made also by
art, they would come to be in the same way as by nature. Each step
then in the series is for the sake of the next; and generally art
partly completes what nature cannot bring to a finish, and partly
imitates her. If, therefore, artificial products are for the sake of
an end, so clearly also are natural products. The relation of the
later to the earlier terms of the series is the same in both. This
is most obvious in the animals other than man: they make things
neither by art nor after inquiry or deliberation. Wherefore people
discuss whether it is by intelligence or by some other faculty that
these creatures work,spiders, ants, and the like. By gradual advance
in this direction we come to see clearly that in plants too that is
produced which is conducive to the end-leaves, e.g. grow to provide
shade for the fruit. If then it is both by nature and for an end
that the swallow makes its nest and the spider its web, and plants
grow leaves for the sake of the fruit and send their roots down (not
up) for the sake of nourishment, it is plain that this kind of cause
is operative in things which come to be and are by nature. And since
'nature' means two things, the matter and the form, of which the
latter is the end, and since all the rest is for the sake of the
end, the form must be the cause in the sense of 'that for the sake
of which'.
  Now mistakes come to pass even in the operations of art: the
grammarian makes a mistake in writing and the doctor pours out the
wrong dose. Hence clearly mistakes are possible in the operations of
nature also. If then in art there are cases in which what is rightly
produced serves a purpose, and if where mistakes occur there was a
purpose in what was attempted, only it was not attained, so must it be
also in natural products, and monstrosities will be failures in the
purposive effort. Thus in the original combinations the 'ox-progeny'
if they failed to reach a determinate end must have arisen through the
corruption of some principle corresponding to what is now the seed.
  Further, seed must have come into being first, and not straightway
the animals: the words 'whole-natured first...' must have meant seed.
  Again, in plants too we find the relation of means to end, though
the degree of organization is less. Were there then in plants also
'olive-headed vine-progeny', like the 'man-headed ox-progeny', or not?
An absurd suggestion; yet there must have been, if there were such
things among animals.
  Moreover, among the seeds anything must have come to be at random.
But the person who asserts this entirely does away with 'nature' and
what exists 'by nature'. For those things are natural which, by a
continuous movement originated from an internal principle, arrive at
some completion: the same completion is not reached from every
principle; nor any chance completion, but always the tendency in
each is towards the same end, if there is no impediment.
  The end and the means towards it may come about by chance. We say,
for instance, that a stranger has come by chance, paid the ransom, and
gone away, when he does so as if he had come for that purpose,
though it was not for that that he came. This is incidental, for
chance is an incidental cause, as I remarked before. But when an event
takes place always or for the most part, it is not incidental or by
chance. In natural products the sequence is invariable, if there is no
  It is absurd to suppose that purpose is not present because we do
not observe the agent deliberating. Art does not deliberate. If the
ship-building art were in the wood, it would produce the same
results by nature. If, therefore, purpose is present in art, it is
present also in nature. The best illustration is a doctor doctoring
himself: nature is like that.
  It is plain then that nature is a cause, a cause that operates for a

  As regards what is 'of necessity', we must ask whether the necessity
is 'hypothetical', or 'simple' as well. The current view places what
is of necessity in the process of production, just as if one were to
suppose that the wall of a house necessarily comes to be because
what is heavy is naturally carried downwards and what is light to
the top, wherefore the stones and foundations take the lowest place,
with earth above because it is lighter, and wood at the top of all
as being the lightest. Whereas, though the wall does not come to be
without these, it is not due to these, except as its material cause:
it comes to be for the sake of sheltering and guarding certain things.
Similarly in all other things which involve production for an end; the
product cannot come to be without things which have a necessary
nature, but it is not due to these (except as its material); it
comes to be for an end. For instance, why is a saw such as it is? To
effect so-and-so and for the sake of so-and-so. This end, however,
cannot be realized unless the saw is made of iron. It is, therefore,
necessary for it to be of iron, it we are to have a saw and perform
the operation of sawing. What is necessary then, is necessary on a
hypothesis; it is not a result necessarily determined by
antecedents. Necessity is in the matter, while 'that for the sake of
which' is in the definition.
  Necessity in mathematics is in a way similar to necessity in
things which come to be through the operation of nature. Since a
straight line is what it is, it is necessary that the angles of a
triangle should equal two right angles. But not conversely; though
if the angles are not equal to two right angles, then the straight
line is not what it is either. But in things which come to be for an
end, the reverse is true. If the end is to exist or does exist, that
also which precedes it will exist or does exist; otherwise just as
there, if-the conclusion is not true, the premiss will not be true, so
here the end or 'that for the sake of which' will not exist. For
this too is itself a starting-point, but of the reasoning, not of
the action; while in mathematics the starting-point is the
starting-point of the reasoning only, as there is no action. If then
there is to be a house, such-and-such things must be made or be
there already or exist, or generally the matter relative to the end,
bricks and stones if it is a house. But the end is not due to these
except as the matter, nor will it come to exist because of them. Yet
if they do not exist at all, neither will the house, or the saw-the
former in the absence of stones, the latter in the absence of
iron-just as in the other case the premisses will not be true, if
the angles of the triangle are not equal to two right angles.
  The necessary in nature, then, is plainly what we call by the name
of matter, and the changes in it. Both causes must be stated by the
physicist, but especially the end; for that is the cause of the
matter, not vice versa; and the end is 'that for the sake of which',
and the beginning starts from the definition or essence; as in
artificial products, since a house is of such-and-such a kind, certain
things must necessarily come to be or be there already, or since
health is this, these things must necessarily come to be or be there
already. Similarly if man is this, then these; if these, then those.
Perhaps the necessary is present also in the definition. For if one
defines the operation of sawing as being a certain kind of dividing,
then this cannot come about unless the saw has teeth of a certain
kind; and these cannot be unless it is of iron. For in the
definition too there are some parts that are, as it were, its matter.
                              Book III

  NATURE has been defined as a 'principle of motion and change', and
it is the subject of our inquiry. We must therefore see that we
understand the meaning of 'motion'; for if it were unknown, the
meaning of 'nature' too would be unknown.
  When we have determined the nature of motion, our next task will
be to attack in the same way the terms which are involved in it. Now
motion is supposed to belong to the class of things which are
continuous; and the infinite presents itself first in the
continuous-that is how it comes about that 'infinite' is often used in
definitions of the continuous ('what is infinitely divisible is
continuous'). Besides these, place, void, and time are thought to be
necessary conditions of motion.
  Clearly, then, for these reasons and also because the attributes
mentioned are common to, and coextensive with, all the objects of
our science, we must first take each of them in hand and discuss it.
For the investigation of special attributes comes after that of the
common attributes.
  To begin then, as we said, with motion.
  We may start by distinguishing (1) what exists in a state of
fulfilment only, (2) what exists as potential, (3) what exists as
potential and also in fulfilment-one being a 'this', another 'so
much', a third 'such', and similarly in each of the other modes of the
predication of being.
  Further, the word 'relative' is used with reference to (1) excess
and defect, (2) agent and patient and generally what can move and what
can be moved. For 'what can cause movement' is relative to 'what can
be moved', and vice versa.
  Again, there is no such thing as motion over and above the things.
It is always with respect to substance or to quantity or to quality or
to place that what changes changes. But it is impossible, as we
assert, to find anything common to these which is neither 'this' nor
quantum nor quale nor any of the other predicates. Hence neither
will motion and change have reference to something over and above
the things mentioned, for there is nothing over and above them.
  Now each of these belongs to all its subjects in either of two ways:
namely (1) substance-the one is positive form, the other privation;
(2) in quality, white and black; (3) in quantity, complete and
incomplete; (4) in respect of locomotion, upwards and downwards or
light and heavy. Hence there are as many types of motion or change
as there are meanings of the word 'is'.
  We have now before us the distinctions in the various classes of
being between what is full real and what is potential.
  Def. The fulfilment of what exists potentially, in so far as it
exists potentially, is motion-namely, of what is alterable qua
alterable, alteration: of what can be increased and its opposite
what can be decreased (there is no common name), increase and
decrease: of what can come to be and can pass away, coming to he and
passing away: of what can be carried along, locomotion.
  Examples will elucidate this definition of motion. When the
buildable, in so far as it is just that, is fully real, it is being
built, and this is building. Similarly, learning, doctoring,
rolling, leaping, ripening, ageing.
  The same thing, if it is of a certain kind, can be both potential
and fully real, not indeed at the same time or not in the same
respect, but e.g. potentially hot and actually cold. Hence at once
such things will act and be acted on by one another in many ways: each
of them will be capable at the same time of causing alteration and
of being altered. Hence, too, what effects motion as a physical
agent can be moved: when a thing of this kind causes motion, it is
itself also moved. This, indeed, has led some people to suppose that
every mover is moved. But this question depends on another set of
arguments, and the truth will be made clear later. is possible for a
thing to cause motion, though it is itself incapable of being moved.
  It is the fulfilment of what is potential when it is already fully
real and operates not as itself but as movable, that is motion. What I
mean by 'as' is this: Bronze is potentially a statue. But it is not
the fulfilment of bronze as bronze which is motion. For 'to be bronze'
and 'to be a certain potentiality' are not the same.
  If they were identical without qualification, i.e. in definition,
the fulfilment of bronze as bronze would have been motion. But they
are not the same, as has been said. (This is obvious in contraries.
'To be capable of health' and 'to be capable of illness' are not the
same, for if they were there would be no difference between being
ill and being well. Yet the subject both of health and of
sickness-whether it is humour or blood-is one and the same.)
  We can distinguish, then, between the two-just as, to give another
example, 'colour' and visible' are different-and clearly it is the
fulfilment of what is potential as potential that is motion. So
this, precisely, is motion.
  Further it is evident that motion is an attribute of a thing just
when it is fully real in this way, and neither before nor after. For
each thing of this kind is capable of being at one time actual, at
another not. Take for instance the buildable as buildable. The
actuality of the buildable as buildable is the process of building.
For the actuality of the buildable must be either this or the house.
But when there is a house, the buildable is no longer buildable. On
the other hand, it is the buildable which is being built. The
process then of being built must be the kind of actuality required But
building is a kind of motion, and the same account will apply to the
other kinds also.

  The soundness of this definition is evident both when we consider
the accounts of motion that the others have given, and also from the
difficulty of defining it otherwise.
  One could not easily put motion and change in another genus-this
is plain if we consider where some people put it; they identify motion
with or 'inequality' or 'not being'; but such things are not
necessarily moved, whether they are 'different' or 'unequal' or
'non-existent'; Nor is change either to or from these rather than to
or from their opposites.
  The reason why they put motion into these genera is that it is
thought to be something indefinite, and the principles in the second
column are indefinite because they are privative: none of them is
either 'this' or 'such' or comes under any of the other modes of
predication. The reason in turn why motion is thought to be indefinite
is that it cannot be classed simply as a potentiality or as an
actuality-a thing that is merely capable of having a certain size is
not undergoing change, nor yet a thing that is actually of a certain
size, and motion is thought to be a sort of actuality, but incomplete,
the reason for this view being that the potential whose actuality it
is is incomplete. This is why it is hard to grasp what motion is. It
is necessary to class it with privation or with potentiality or with
sheer actuality, yet none of these seems possible. There remains
then the suggested mode of definition, namely that it is a sort of
actuality, or actuality of the kind described, hard to grasp, but
not incapable of existing.
  The mover too is moved, as has been said-every mover, that is, which
is capable of motion, and whose immobility is rest-when a thing is
subject to motion its immobility is rest. For to act on the movable as
such is just to move it. But this it does by contact, so that at the
same time it is also acted on. Hence we can define motion as the
fulfilment of the movable qua movable, the cause of the attribute
being contact with what can move so that the mover is also acted on.
The mover or agent will always be the vehicle of a form, either a
'this' or 'such', which, when it acts, will be the source and cause of
the change, e.g. the full-formed man begets man from what is
potentially man.

  The solution of the difficulty that is raised about the
motion-whether it is in the movable-is plain. It is the fulfilment
of this potentiality, and by the action of that which has the power of
causing motion; and the actuality of that which has the power of
causing motion is not other than the actuality of the movable, for
it must be the fulfilment of both. A thing is capable of causing
motion because it can do this, it is a mover because it actually
does it. But it is on the movable that it is capable of acting.
Hence there is a single actuality of both alike, just as one to two
and two to one are the same interval, and the steep ascent and the
steep descent are one-for these are one and the same, although they
can be described in different ways. So it is with the mover and the
  This view has a dialectical difficulty. Perhaps it is necessary that
the actuality of the agent and that of the patient should not be the
same. The one is 'agency' and the other 'patiency'; and the outcome
and completion of the one is an 'action', that of the other a
'passion'. Since then they are both motions, we may ask: in what are
they, if they are different? Either (a) both are in what is acted on
and moved, or (b) the agency is in the agent and the patiency in the
patient. (If we ought to call the latter also 'agency', the word would
be used in two senses.)
  Now, in alternative (b), the motion will be in the mover, for the
same statement will hold of 'mover' and 'moved'. Hence either every
mover will be moved, or, though having motion, it will not be moved.
  If on the other hand (a) both are in what is moved and acted on-both
the agency and the patiency (e.g. both teaching and learning, though
they are two, in the learner), then, first, the actuality of each will
not be present in each, and, a second absurdity, a thing will have two
motions at the same time. How will there be two alterations of quality
in one subject towards one definite quality? The thing is
impossible: the actualization will be one.
  But (some one will say) it is contrary to reason to suppose that
there should be one identical actualization of two things which are
different in kind. Yet there will be, if teaching and learning are the
same, and agency and patiency. To teach will be the same as to
learn, and to act the same as to be acted on-the teacher will
necessarily be learning everything that he teaches, and the agent will
be acted on. One may reply:
  (1) It is not absurd that the actualization of one thing should be
in another. Teaching is the activity of a person who can teach, yet
the operation is performed on some patient-it is not cut adrift from a
subject, but is of A on B.
  (2) There is nothing to prevent two things having one and the same
actualization, provided the actualizations are not described in the
same way, but are related as what can act to what is acting.
  (3) Nor is it necessary that the teacher should learn, even if to
act and to be acted on are one and the same, provided they are not the
same in definition (as 'raiment' and 'dress'), but are the same merely
in the sense in which the road from Thebes to Athens and the road from
Athens to Thebes are the same, as has been explained above. For it
is not things which are in a way the same that have all their
attributes the same, but only such as have the same definition. But
indeed it by no means follows from the fact that teaching is the
same as learning, that to learn is the same as to teach, any more than
it follows from the fact that there is one distance between two things
which are at a distance from each other, that the two vectors AB and
BA, are one and the same. To generalize, teaching is not the same as
learning, or agency as patiency, in the full sense, though they belong
to the same subject, the motion; for the 'actualization of X in Y' and
the 'actualization of Y through the action of X' differ in definition.
  What then Motion is, has been stated both generally and
particularly. It is not difficult to see how each of its types will be
defined-alteration is the fulfillment of the alterable qua alterable
(or, more scientifically, the fulfilment of what can act and what
can be acted on, as such)-generally and again in each particular case,
building, healing, &c. A similar definition will apply to each of
the other kinds of motion.

  The science of nature is concerned with spatial magnitudes and
motion and time, and each of these at least is necessarily infinite or
finite, even if some things dealt with by the science are not, e.g.
a quality or a point-it is not necessary perhaps that such things
should be put under either head. Hence it is incumbent on the person
who specializes in physics to discuss the infinite and to inquire
whether there is such a thing or not, and, if there is, what it is.
  The appropriateness to the science of this problem is clearly
indicated. All who have touched on this kind of science in a way worth
considering have formulated views about the infinite, and indeed, to a
man, make it a principle of things.
  (1) Some, as the Pythagoreans and Plato, make the infinite a
principle in the sense of a self-subsistent substance, and not as a
mere attribute of some other thing. Only the Pythagoreans place the
infinite among the objects of sense (they do not regard number as
separable from these), and assert that what is outside the heaven is
infinite. Plato, on the other hand, holds that there is no body
outside (the Forms are not outside because they are nowhere),yet
that the infinite is present not only in the objects of sense but in
the Forms also.
  Further, the Pythagoreans identify the infinite with the even. For
this, they say, when it is cut off and shut in by the odd, provides
things with the element of infinity. An indication of this is what
happens with numbers. If the gnomons are placed round the one, and
without the one, in the one construction the figure that results is
always different, in the other it is always the same. But Plato has
two infinites, the Great and the Small.
  The physicists, on the other hand, all of them, always regard the
infinite as an attribute of a substance which is different from it and
belongs to the class of the so-called elements-water or air or what is
intermediate between them. Those who make them limited in number never
make them infinite in amount. But those who make the elements infinite
in number, as Anaxagoras and Democritus do, say that the infinite is
continuous by contact-compounded of the homogeneous parts according to
the one, of the seed-mass of the atomic shapes according to the other.
  Further, Anaxagoras held that any part is a mixture in the same
way as the All, on the ground of the observed fact that anything comes
out of anything. For it is probably for this reason that he
maintains that once upon a time all things were together. (This
flesh and this bone were together, and so of any thing: therefore
all things: and at the same time too.) For there is a beginning of
separation, not only for each thing, but for all. Each thing that
comes to be comes from a similar body, and there is a coming to be
of all things, though not, it is true, at the same time. Hence there
must also be an origin of coming to be. One such source there is which
he calls Mind, and Mind begins its work of thinking from some
starting-point. So necessarily all things must have been together at a
certain time, and must have begun to be moved at a certain time.
  Democritus, for his part, asserts the contrary, namely that no
element arises from another element. Nevertheless for him the common
body is a source of all things, differing from part to part in size
and in shape.
  It is clear then from these considerations that the inquiry concerns
the physicist. Nor is it without reason that they all make it a
principle or source. We cannot say that the infinite has no effect,
and the only effectiveness which we can ascribe to it is that of a
principle. Everything is either a source or derived from a source. But
there cannot be a source of the infinite or limitless, for that
would be a limit of it. Further, as it is a beginning, it is both
uncreatable and indestructible. For there must be a point at which
what has come to be reaches completion, and also a termination of
all passing away. That is why, as we say, there is no principle of
this, but it is this which is held to be the principle of other
things, and to encompass all and to steer all, as those assert who
do not recognize, alongside the infinite, other causes, such as Mind
or Friendship. Further they identify it with the Divine, for it is
'deathless and imperishable' as Anaximander says, with the majority of
the physicists.
  Belief in the existence of the infinite comes mainly from five
  (1) From the nature of time-for it is infinite.
  (2) From the division of magnitudes-for the mathematicians also
use the notion of the infinite.
  (3) If coming to be and passing away do not give out, it is only
because that from which things come to be is infinite.
  (4) Because the limited always finds its limit in something, so that
there must be no limit, if everything is always limited by something
different from itself.
  (5) Most of all, a reason which is peculiarly appropriate and
presents the difficulty that is felt by everybody-not only number
but also mathematical magnitudes and what is outside the heaven are
supposed to be infinite because they never give out in our thought.
  The last fact (that what is outside is infinite) leads people to
suppose that body also is infinite, and that there is an infinite
number of worlds. Why should there be body in one part of the void
rather than in another? Grant only that mass is anywhere and it
follows that it must be everywhere. Also, if void and place are
infinite, there must be infinite body too, for in the case of
eternal things what may be must be. But the problem of the infinite is
difficult: many contradictions result whether we suppose it to exist
or not to exist. If it exists, we have still to ask how it exists;
as a substance or as the essential attribute of some entity? Or in
neither way, yet none the less is there something which is infinite or
some things which are infinitely many?
  The problem, however, which specially belongs to the physicist is to
investigate whether there is a sensible magnitude which is infinite.
  We must begin by distinguishing the various senses in which the term
'infinite' is used.
  (1) What is incapable of being gone through, because it is not in
its nature to be gone through (the sense in which the voice is
  (2) What admits of being gone through, the process however having no
termination, or what scarcely admits of being gone through.
  (3) What naturally admits of being gone through, but is not actually
gone through or does not actually reach an end.
  Further, everything that is infinite may be so in respect of
addition or division or both.

  Now it is impossible that the infinite should be a thing which is
itself infinite, separable from sensible objects. If the infinite is
neither a magnitude nor an aggregate, but is itself a substance and
not an attribute, it will be indivisible; for the divisible must be
either a magnitude or an aggregate. But if indivisible, then not
infinite, except in the sense (1) in which the voice is 'invisible'.
But this is not the sense in which it is used by those who say that
the infinite exists, nor that in which we are investigating it, namely
as (2) 'that which cannot be gone through'. But if the infinite exists
as an attribute, it would not be, qua infinite an element in
substances, any more than the invisible would be an element of speech,
though the voice is invisible.
  Further, how can the infinite be itself any thing, unless both
number and magnitude, of which it is an essential attribute, exist
in that way? If they are not substances, a fortiori the infinite is
  It is plain, too, that the infinite cannot be an actual thing and
a substance and principle. For any part of it that is taken will be
infinite, if it has parts: for 'to be infinite' and 'the infinite' are
the same, if it is a substance and not predicated of a subject.
Hence it will be either indivisible or divisible into infinites. But
the same thing cannot be many infinites. (Yet just as part of air is
air, so a part of the infinite would be infinite, if it is supposed to
be a substance and principle.) Therefore the infinite must be
without parts and indivisible. But this cannot be true of what is
infinite in full completion: for it must be a definite quantity.
  Suppose then that infinity belongs to substance as an attribute.
But, if so, it cannot, as we have said, be described as a principle,
but rather that of which it is an attribute-the air or the even
  Thus the view of those who speak after the manner of the
Pythagoreans is absurd. With the same breath they treat the infinite
as substance, and divide it into parts.
  This discussion, however, involves the more general question whether
the infinite can be present in mathematical objects and things which
are intelligible and do not have extension, as well as among
sensible objects. Our inquiry (as physicists) is limited to its
special subject-matter, the objects of sense, and we have to ask
whether there is or is not among them a body which is infinite in
the direction of increase.
  We may begin with a dialectical argument and show as follows that
there is no such thing. If 'bounded by a surface' is the definition of
body there cannot be an infinite body either intelligible or sensible.
Nor can number taken in abstraction be infinite, for number or that
which has number is numerable. If then the numerable can be
numbered, it would also be possible to go through the infinite.
  If, on the other hand, we investigate the question more in
accordance with principles appropriate to physics, we are led as
follows to the same result.
  The infinite body must be either (1) compound, or (2) simple; yet
neither alternative is possible.
  (1) Compound the infinite body will not be, if the elements are
finite in number. For they must be more than one, and the contraries
must always balance, and no one of them can be infinite. If one of the
bodies falls in any degree short of the other in potency-suppose
fire is finite in amount while air is infinite and a given quantity of
fire exceeds in power the same amount of air in any ratio provided
it is numerically definite-the infinite body will obviously prevail
over and annihilate the finite body. On the other hand, it is
impossible that each should be infinite. 'Body' is what has
extension in all directions and the infinite is what is boundlessly
extended, so that the infinite body would be extended in all
directions ad infinitum.
  Nor (2) can the infinite body be one and simple, whether it is, as
some hold, a thing over and above the elements (from which they
generate the elements) or is not thus qualified.
  (a) We must consider the former alternative; for there are some
people who make this the infinite, and not air or water, in order that
the other elements may not be annihilated by the element which is
infinite. They have contrariety with each other-air is cold, water
moist, fire hot; if one were infinite, the others by now would have
ceased to be. As it is, they say, the infinite is different from
them and is their source.
  It is impossible, however, that there should be such a body; not
because it is infinite on that point a general proof can be given
which applies equally to all, air, water, or anything else-but
simply because there is, as a matter of fact, no such sensible body,
alongside the so-called elements. Everything can be resolved into
the elements of which it is composed. Hence the body in question would
have been present in our world here, alongside air and fire and
earth and water: but nothing of the kind is observed.
  (b) Nor can fire or any other of the elements be infinite. For
generally, and apart from the question of how any of them could be
infinite, the All, even if it were limited, cannot either be or become
one of them, as Heraclitus says that at some time all things become
fire. (The same argument applies also to the one which the
physicists suppose to exist alongside the elements: for everything
changes from contrary to contrary, e.g. from hot to cold).
  The preceding consideration of the various cases serves to show us
whether it is or is not possible that there should be an infinite
sensible body. The following arguments give a general demonstration
that it is not possible.
  It is the nature of every kind of sensible body to be somewhere, and
there is a place appropriate to each, the same for the part and for
the whole, e.g. for the whole earth and for a single clod, and for
fire and for a spark.
  Suppose (a) that the infinite sensible body is homogeneous. Then
each part will be either immovable or always being carried along.
Yet neither is possible. For why downwards rather than upwards or in
any other direction? I mean, e.g, if you take a clod, where will it be
moved or where will it be at rest? For ex hypothesi the place of the
body akin to it is infinite. Will it occupy the whole place, then? And
how? What then will be the nature of its rest and of its movement,
or where will they be? It will either be at home everywhere-then it
will not be moved; or it will be moved everywhere-then it will not
come to rest.
  But if (b) the All has dissimilar parts, the proper places of the
parts will be dissimilar also, and the body of the All will have no
unity except that of contact. Then, further, the parts will be
either finite or infinite in variety of kind. (i) Finite they cannot
be, for if the All is to be infinite, some of them would have to be
infinite, while the others were not, e.g. fire or water will be
infinite. But, as we have seen before, such an element would destroy
what is contrary to it. (This indeed is the reason why none of the
physicists made fire or earth the one infinite body, but either
water or air or what is intermediate between them, because the abode
of each of the two was plainly determinate, while the others have an
ambiguous place between up and down.)
  But (ii) if the parts are infinite in number and simple, their
proper places too will be infinite in number, and the same will be
true of the elements themselves. If that is impossible, and the places
are finite, the whole too must be finite; for the place and the body
cannot but fit each other. Neither is the whole place larger than what
can be filled by the body (and then the body would no longer be
infinite), nor is the body larger than the place; for either there
would be an empty space or a body whose nature it is to be nowhere.
  Anaxagoras gives an absurd account of why the infinite is at rest.
He says that the infinite itself is the cause of its being fixed. This
because it is in itself, since nothing else contains it-on the
assumption that wherever anything is, it is there by its own nature.
But this is not true: a thing could be somewhere by compulsion, and
not where it is its nature to be.
  Even if it is true as true can be that the whole is not moved (for
what is fixed by itself and is in itself must be immovable), yet we
must explain why it is not its nature to be moved. It is not enough
just to make this statement and then decamp. Anything else might be in
a state of rest, but there is no reason why it should not be its
nature to be moved. The earth is not carried along, and would not be
carried along if it were infinite, provided it is held together by the
centre. But it would not be because there was no other region in which
it could be carried along that it would remain at the centre, but
because this is its nature. Yet in this case also we may say that it
fixes itself. If then in the case of the earth, supposed to be
infinite, it is at rest, not because it is infinite, but because it
has weight and what is heavy rests at the centre and the earth is at
the centre, similarly the infinite also would rest in itself, not
because it is infinite and fixes itself, but owing to some other
  Another difficulty emerges at the same time. Any part of the
infinite body ought to remain at rest. Just as the infinite remains at
rest in itself because it fixes itself, so too any part of it you
may take will remain in itself. The appropriate places of the whole
and of the part are alike, e.g. of the whole earth and of a clod the
appropriate place is the lower region; of fire as a whole and of a
spark, the upper region. If, therefore, to be in itself is the place
of the infinite, that also will be appropriate to the part.
Therefore it will remain in itself.
  In general, the view that there is an infinite body is plainly
incompatible with the doctrine that there is necessarily a proper
place for each kind of body, if every sensible body has either
weight or lightness, and if a body has a natural locomotion towards
the centre if it is heavy, and upwards if it is light. This would need
to be true of the infinite also. But neither character can belong to
it: it cannot be either as a whole, nor can it be half the one and
half the other. For how should you divide it? or how can the
infinite have the one part up and the other down, or an extremity
and a centre?
  Further, every sensible body is in place, and the kinds or
differences of place are up-down, before-behind, right-left; and these
distinctions hold not only in relation to us and by arbitrary
agreement, but also in the whole itself. But in the infinite body they
cannot exist. In general, if it is impossible that there should be
an infinite place, and if every body is in place, there cannot be an
infinite body.
  Surely what is in a special place is in place, and what is in
place is in a special place. Just, then, as the infinite cannot be
quantity-that would imply that it has a particular quantity, e,g,
two or three cubits; quantity just means these-so a thing's being in
place means that it is somewhere, and that is either up or down or
in some other of the six differences of position: but each of these is
a limit.
  It is plain from these arguments that there is no body which is
actually infinite.

  But on the other hand to suppose that the infinite does not exist in
any way leads obviously to many impossible consequences: there will be
a beginning and an end of time, a magnitude will not be divisible into
magnitudes, number will not be infinite. If, then, in view of the
above considerations, neither alternative seems possible, an arbiter
must be called in; and clearly there is a sense in which the
infinite exists and another in which it does not.
  We must keep in mind that the word 'is' means either what
potentially is or what fully is. Further, a thing is infinite either
by addition or by division.
  Now, as we have seen, magnitude is not actually infinite. But by
division it is infinite. (There is no difficulty in refuting the
theory of indivisible lines.) The alternative then remains that the
infinite has a potential existence.
  But the phrase 'potential existence' is ambiguous. When we speak
of the potential existence of a statue we mean that there will be an
actual statue. It is not so with the infinite. There will not be an
actual infinite. The word 'is' has many senses, and we say that the
infinite 'is' in the sense in which we say 'it is day' or 'it is the
games', because one thing after another is always coming into
existence. For of these things too the distinction between potential
and actual existence holds. We say that there are Olympic games,
both in the sense that they may occur and that they are actually
  The infinite exhibits itself in different ways-in time, in the
generations of man, and in the division of magnitudes. For generally
the infinite has this mode of existence: one thing is always being
taken after another, and each thing that is taken is always finite,
but always different. Again, 'being' has more than one sense, so
that we must not regard the infinite as a 'this', such as a man or a
horse, but must suppose it to exist in the sense in which we speak
of the day or the games as existing things whose being has not come to
them like that of a substance, but consists in a process of coming
to be or passing away; definite if you like at each stage, yet
always different.
  But when this takes place in spatial magnitudes, what is taken
perists, while in the succession of time and of men it takes place
by the passing away of these in such a way that the source of supply
never gives out.
  In a way the infinite by addition is the same thing as the
infinite by division. In a finite magnitude, the infinite by
addition comes about in a way inverse to that of the other. For in
proportion as we see division going on, in the same proportion we
see addition being made to what is already marked off. For if we
take a determinate part of a finite magnitude and add another part
determined by the same ratio (not taking in the same amount of the
original whole), and so on, we shall not traverse the given magnitude.
But if we increase the ratio of the part, so as always to take in
the same amount, we shall traverse the magnitude, for every finite
magnitude is exhausted by means of any determinate quantity however
  The infinite, then, exists in no other way, but in this way it
does exist, potentially and by reduction. It exists fully in the sense
in which we say 'it is day' or 'it is the games'; and potentially as
matter exists, not independently as what is finite does.
  By addition then, also, there is potentially an infinite, namely,
what we have described as being in a sense the same as the infinite in
respect of division. For it will always be possible to take
something ah extra. Yet the sum of the parts taken will not exceed
every determinate magnitude, just as in the direction of division
every determinate magnitude is surpassed in smallness and there will
be a smaller part.
  But in respect of addition there cannot be an infinite which even
potentially exceeds every assignable magnitude, unless it has the
attribute of being actually infinite, as the physicists hold to be
true of the body which is outside the world, whose essential nature is
air or something of the kind. But if there cannot be in this way a
sensible body which is infinite in the full sense, evidently there can
no more be a body which is potentially infinite in respect of
addition, except as the inverse of the infinite by division, as we
have said. It is for this reason that Plato also made the infinites
two in number, because it is supposed to be possible to exceed all
limits and to proceed ad infinitum in the direction both of increase
and of reduction. Yet though he makes the infinites two, he does not
use them. For in the numbers the infinite in the direction of
reduction is not present, as the monad is the smallest; nor is the
infinite in the direction of increase, for the parts number only up to
the decad.
  The infinite turns out to be the contrary of what it is said to
be. It is not what has nothing outside it that is infinite, but what
always has something outside it. This is indicated by the fact that
rings also that have no bezel are described as 'endless', because it
is always possible to take a part which is outside a given part. The
description depends on a certain similarity, but it is not true in the
full sense of the word. This condition alone is not sufficient: it
is necessary also that the next part which is taken should never be
the same. In the circle, the latter condition is not satisfied: it
is only the adjacent part from which the new part is different.
  Our definition then is as follows:
  A quantity is infinite if it is such that we can always take a
part outside what has been already taken. On the other hand, what
has nothing outside it is complete and whole. For thus we define the
whole-that from which nothing is wanting, as a whole man or a whole
box. What is true of each particular is true of the whole as
such-the whole is that of which nothing is outside. On the other
hand that from which something is absent and outside, however small
that may be, is not 'all'. 'Whole' and 'complete' are either quite
identical or closely akin. Nothing is complete (teleion) which has
no end (telos); and the end is a limit.
  Hence Parmenides must be thought to have spoken better than
Melissus. The latter says that the whole is infinite, but the former
describes it as limited, 'equally balanced from the middle'. For to
connect the infinite with the all and the whole is not like joining
two pieces of string; for it is from this they get the dignity they
ascribe to the infinite-its containing all things and holding the
all in itself-from its having a certain similarity to the whole. It is
in fact the matter of the completeness which belongs to size, and what
is potentially a whole, though not in the full sense. It is
divisible both in the direction of reduction and of the inverse
addition. It is a whole and limited; not, however, in virtue of its
own nature, but in virtue of what is other than it. It does not
contain, but, in so far as it is infinite, is contained. Consequently,
also, it is unknowable, qua infinite; for the matter has no form.
(Hence it is plain that the infinite stands in the relation of part
rather than of whole. For the matter is part of the whole, as the
bronze is of the bronze statue.) If it contains in the case of
sensible things, in the case of intelligible things the great and
the small ought to contain them. But it is absurd and impossible to
suppose that the unknowable and indeterminate should contain and

  It is reasonable that there should not be held to be an infinite
in respect of addition such as to surpass every magnitude, but that
there should be thought to be such an infinite in the direction of
division. For the matter and the infinite are contained inside what
contains them, while it is the form which contains. It is natural
too to suppose that in number there is a limit in the direction of the
minimum, and that in the other direction every assigned number is
surpassed. In magnitude, on the contrary, every assigned magnitude
is surpassed in the direction of smallness, while in the other
direction there is no infinite magnitude. The reason is that what is
one is indivisible whatever it may be, e.g. a man is one man, not
many. Number on the other hand is a plurality of 'ones' and a
certain quantity of them. Hence number must stop at the indivisible:
for 'two' and 'three' are merely derivative terms, and so with each of
the other numbers. But in the direction of largeness it is always
possible to think of a larger number: for the number of times a
magnitude can be bisected is infinite. Hence this infinite is
potential, never actual: the number of parts that can be taken
always surpasses any assigned number. But this number is not separable
from the process of bisection, and its infinity is not a permanent
actuality but consists in a process of coming to be, like time and the
number of time.
  With magnitudes the contrary holds. What is continuous is divided ad
infinitum, but there is no infinite in the direction of increase.
For the size which it can potentially be, it can also actually be.
Hence since no sensible magnitude is infinite, it is impossible to
exceed every assigned magnitude; for if it were possible there would
be something bigger than the heavens.
  The infinite is not the same in magnitude and movement and time,
in the sense of a single nature, but its secondary sense depends on
its primary sense, i.e. movement is called infinite in virtue of the
magnitude covered by the movement (or alteration or growth), and
time because of the movement. (I use these terms for the moment. Later
I shall explain what each of them means, and also why every
magnitude is divisible into magnitudes.)
  Our account does not rob the mathematicians of their science, by
disproving the actual existence of the infinite in the direction of
increase, in the sense of the untraversable. In point of fact they
do not need the infinite and do not use it. They postulate only that
the finite straight line may be produced as far as they wish. It is
possible to have divided in the same ratio as the largest quantity
another magnitude of any size you like. Hence, for the purposes of
proof, it will make no difference to them to have such an infinite
instead, while its existence will be in the sphere of real magnitudes.
  In the fourfold scheme of causes, it is plain that the infinite is a
cause in the sense of matter, and that its essence is privation, the
subject as such being what is continuous and sensible. All the other
thinkers, too, evidently treat the infinite as matter-that is why it
is inconsistent in them to make it what contains, and not what is

  It remains to dispose of the arguments which are supposed to support
the view that the infinite exists not only potentially but as a
separate thing. Some have no cogency; others can be met by fresh
objections that are valid.
  (1) In order that coming to be should not fail, it is not
necessary that there should be a sensible body which is actually
infinite. The passing away of one thing may be the coming to be of
another, the All being limited.
  (2) There is a difference between touching and being limited. The
former is relative to something and is the touching of something
(for everything that touches touches something), and further is an
attribute of some one of the things which are limited. On the other
hand, what is limited is not limited in relation to anything. Again,
contact is not necessarily possible between any two things taken at
  (3) To rely on mere thinking is absurd, for then the excess or
defect is not in the thing but in the thought. One might think that
one of us is bigger than he is and magnify him ad infinitum. But it
does not follow that he is bigger than the size we are, just because
some one thinks he is, but only because he is the size he is. The
thought is an accident.
  (a) Time indeed and movement are infinite, and also thinking, in the
sense that each part that is taken passes in succession out of
  (b) Magnitude is not infinite either in the way of reduction or of
magnification in thought.
  This concludes my account of the way in which the infinite exists,
and of the way in which it does not exist, and of what it is.
                              Book IV

  THE physicist must have a knowledge of Place, too, as well as of the
infinite-namely, whether there is such a thing or not, and the
manner of its existence and what it is-both because all suppose that
things which exist are somewhere (the non-existent is nowhere--where
is the goat-stag or the sphinx?), and because 'motion' in its most
general and primary sense is change of place, which we call
  The question, what is place? presents many difficulties. An
examination of all the relevant facts seems to lead to divergent
conclusions. Moreover, we have inherited nothing from previous
thinkers, whether in the way of a statement of difficulties or of a
  The existence of place is held to be obvious from the fact of mutual
replacement. Where water now is, there in turn, when the water has
gone out as from a vessel, air is present. When therefore another body
occupies this same place, the place is thought to be different from
all the bodies which come to be in it and replace one another. What
now contains air formerly contained water, so that clearly the place
or space into which and out of which they passed was something
different from both.
  Further, the typical locomotions of the elementary natural
bodies-namely, fire, earth, and the like-show not only that place is
something, but also that it exerts a certain influence. Each is
carried to its own place, if it is not hindered, the one up, the other
down. Now these are regions or kinds of place-up and down and the rest
of the six directions. Nor do such distinctions (up and down and right
and left, &c.) hold only in relation to us. To us they are not
always the same but change with the direction in which we are
turned: that is why the same thing may be both right and left, up
and down, before and behind. But in nature each is distinct, taken
apart by itself. It is not every chance direction which is 'up', but
where fire and what is light are carried; similarly, too, 'down' is
not any chance direction but where what has weight and what is made of
earth are carried-the implication being that these places do not
differ merely in relative position, but also as possessing distinct
potencies. This is made plain also by the objects studied by
mathematics. Though they have no real place, they nevertheless, in
respect of their position relatively to us, have a right and left as
attributes ascribed to them only in consequence of their relative
position, not having by nature these various characteristics. Again,
the theory that the void exists involves the existence of place: for
one would define void as place bereft of body.
  These considerations then would lead us to suppose that place is
something distinct from bodies, and that every sensible body is in
place. Hesiod too might be held to have given a correct account of
it when he made chaos first. At least he says:

  'First of all things came chaos to being, then broad-breasted

implying that things need to have space first, because he thought,
with most people, that everything is somewhere and in place. If this
is its nature, the potency of place must be a marvellous thing, and
take precedence of all other things. For that without which nothing
else can exist, while it can exist without the others, must needs be
first; for place does not pass out of existence when the things in
it are annihilated.
  True, but even if we suppose its existence settled, the question
of its nature presents difficulty-whether it is some sort of 'bulk' of
body or some entity other than that, for we must first determine its
  (1) Now it has three dimensions, length, breadth, depth, the
dimensions by which all body also is bounded. But the place cannot
be body; for if it were there would be two bodies in the same place.
  (2) Further, if body has a place and space, clearly so too have
surface and the other limits of body; for the same statement will
apply to them: where the bounding planes of the water were, there in
turn will be those of the air. But when we come to a point we cannot
make a distinction between it and its place. Hence if the place of a
point is not different from the point, no more will that of any of the
others be different, and place will not be something different from
each of them.
  (3) What in the world then are we to suppose place to be? If it
has the sort of nature described, it cannot be an element or
composed of elements, whether these be corporeal or incorporeal: for
while it has size, it has not body. But the elements of sensible
bodies are bodies, while nothing that has size results from a
combination of intelligible elements.
  (4) Also we may ask: of what in things is space the cause? None of
the four modes of causation can be ascribed to it. It is neither in
the sense of the matter of existents (for nothing is composed of
it), nor as the form and definition of things, nor as end, nor does it
move existents.
  (5) Further, too, if it is itself an existent, where will it be?
Zeno's difficulty demands an explanation: for if everything that
exists has a place, place too will have a place, and so on ad
  (6) Again, just as every body is in place, so, too, every place
has a body in it. What then shall we say about growing things? It
follows from these premisses that their place must grow with them,
if their place is neither less nor greater than they are.
  By asking these questions, then, we must raise the whole problem
about place-not only as to what it is, but even whether there is
such a thing.

  We may distinguish generally between predicating B of A because it
(A) is itself, and because it is something else; and particularly
between place which is common and in which all bodies are, and the
special place occupied primarily by each. I mean, for instance, that
you are now in the heavens because you are in the air and it is in the
heavens; and you are in the air because you are on the earth; and
similarly on the earth because you are in this place which contains no
more than you.
  Now if place is what primarily contains each body, it would be a
limit, so that the place would be the form or shape of each body by
which the magnitude or the matter of the magnitude is defined: for
this is the limit of each body.
  If, then, we look at the question in this way the place of a thing
is its form. But, if we regard the place as the extension of the
magnitude, it is the matter. For this is different from the magnitude:
it is what is contained and defined by the form, as by a bounding
plane. Matter or the indeterminate is of this nature; when the
boundary and attributes of a sphere are taken away, nothing but the
matter is left.
  This is why Plato in the Timaeus says that matter and space are
the same; for the 'participant' and space are identical. (It is
true, indeed, that the account he gives there of the 'participant'
is different from what he says in his so-called 'unwritten
teaching'. Nevertheless, he did identify place and space.) I mention
Plato because, while all hold place to be something, he alone tried to
say what it is.
  In view of these facts we should naturally expect to find difficulty
in determining what place is, if indeed it is one of these two things,
matter or form. They demand a very close scrutiny, especially as it is
not easy to recognize them apart.
  But it is at any rate not difficult to see that place cannot be
either of them. The form and the matter are not separate from the
thing, whereas the place can be separated. As we pointed out, where
air was, water in turn comes to be, the one replacing the other; and
similarly with other bodies. Hence the place of a thing is neither a
part nor a state of it, but is separable from it. For place is
supposed to be something like a vessel-the vessel being a
transportable place. But the vessel is no part of the thing.
  In so far then as it is separable from the thing, it is not the
form: qua containing, it is different from the matter.
  Also it is held that what is anywhere is both itself something and
that there is a different thing outside it. (Plato of course, if we
may digress, ought to tell us why the form and the numbers are not
in place, if 'what participates' is place-whether what participates is
the Great and the Small or the matter, as he called it in writing in
the Timaeus.)
  Further, how could a body be carried to its own place, if place
was the matter or the form? It is impossible that what has no
reference to motion or the distinction of up and down can be place. So
place must be looked for among things which have these
  If the place is in the thing (it must be if it is either shape or
matter) place will have a place: for both the form and the
indeterminate undergo change and motion along with the thing, and
are not always in the same place, but are where the thing is. Hence
the place will have a place.
  Further, when water is produced from air, the place has been
destroyed, for the resulting body is not in the same place. What
sort of destruction then is that?
  This concludes my statement of the reasons why space must be
something, and again of the difficulties that may be raised about
its essential nature.

  The next step we must take is to see in how many senses one thing is
said to be 'in' another.
  (1) As the finger is 'in' the hand and generally the part 'in' the
  (2) As the whole is 'in' the parts: for there is no whole over and
above the parts.
  (3) As man is 'in' animal and generally species 'in' genus.
  (4) As the genus is 'in' the species and generally the part of the
specific form 'in' the definition of the specific form.
  (5) As health is 'in' the hot and the cold and generally the form
'in' the matter.
  (6) As the affairs of Greece centre 'in' the king, and generally
events centre 'in' their primary motive agent.
  (7) As the existence of a thing centres 'in its good and generally
'in' its end, i.e. in 'that for the sake of which' it exists.
  (8) In the strictest sense of all, as a thing is 'in' a vessel,
and generally 'in' place.
  One might raise the question whether a thing can be in itself, or
whether nothing can be in itself-everything being either nowhere or in
something else.
  The question is ambiguous; we may mean the thing qua itself or qua
something else.
  When there are parts of a whole-the one that in which a thing is,
the other the thing which is in it-the whole will be described as
being in itself. For a thing is described in terms of its parts, as
well as in terms of the thing as a whole, e.g. a man is said to be
white because the visible surface of him is white, or to be scientific
because his thinking faculty has been trained. The jar then will not
be in itself and the wine will not be in itself. But the jar of wine
will: for the contents and the container are both parts of the same
  In this sense then, but not primarily, a thing can be in itself,
namely, as 'white' is in body (for the visible surface is in body),
and science is in the mind.
  It is from these, which are 'parts' (in the sense at least of
being 'in' the man), that the man is called white, &c. But the jar and
the wine in separation are not parts of a whole, though together
they are. So when there are parts, a thing will be in itself, as
'white' is in man because it is in body, and in body because it
resides in the visible surface. We cannot go further and say that it
is in surface in virtue of something other than itself. (Yet it is not
in itself: though these are in a way the same thing,) they differ in
essence, each having a special nature and capacity, 'surface' and
  Thus if we look at the matter inductively we do not find anything to
be 'in' itself in any of the senses that have been distinguished;
and it can be seen by argument that it is impossible. For each of
two things will have to be both, e.g. the jar will have to be both
vessel and wine, and the wine both wine and jar, if it is possible for
a thing to be in itself; so that, however true it might be that they
were in each other, the jar will receive the wine in virtue not of its
being wine but of the wine's being wine, and the wine will be in the
jar in virtue not of its being a jar but of the jar's being a jar. Now
that they are different in respect of their essence is evident; for
'that in which something is' and 'that which is in it' would be
differently defined.
  Nor is it possible for a thing to be in itself even incidentally:
for two things would at the same time in the same thing. The jar would
be in itself-if a thing whose nature it is to receive can be in
itself; and that which it receives, namely (if wine) wine, will be
in it.
  Obviously then a thing cannot be in itself primarily.
  Zeno's problem-that if Place is something it must be in something-is
not difficult to solve. There is nothing to prevent the first place
from being 'in' something else-not indeed in that as 'in' place, but
as health is 'in' the hot as a positive determination of it or as the
hot is 'in' body as an affection. So we escape the infinite regress.
  Another thing is plain: since the vessel is no part of what is in it
(what contains in the strict sense is different from what is
contained), place could not be either the matter or the form of the
thing contained, but must different-for the latter, both the matter
and the shape, are parts of what is contained.
  This then may serve as a critical statement of the difficulties

  What then after all is place? The answer to this question may be
elucidated as follows.
  Let us take for granted about it the various characteristics which
are supposed correctly to belong to it essentially. We assume then-
  (1) Place is what contains that of which it is the place.
  (2) Place is no part of the thing.
  (3) The immediate place of a thing is neither less nor greater
than the thing.
  (4) Place can be left behind by the thing and is separable. In
  (5) All place admits of the distinction of up and down, and each
of the bodies is naturally carried to its appropriate place and
rests there, and this makes the place either up or down.
  Having laid these foundations, we must complete the theory. We ought
to try to make our investigation such as will render an account of
place, and will not only solve the difficulties connected with it, but
will also show that the attributes supposed to belong to it do
really belong to it, and further will make clear the cause of the
trouble and of the difficulties about it. Such is the most
satisfactory kind of exposition.
  First then we must understand that place would not have been thought
of, if there had not been a special kind of motion, namely that with
respect to place. It is chiefly for this reason that we suppose the
heaven also to be in place, because it is in constant movement. Of
this kind of change there are two species-locomotion on the one hand
and, on the other, increase and diminution. For these too involve
variation of place: what was then in this place has now in turn
changed to what is larger or smaller.
  Again, when we say a thing is 'moved', the predicate either (1)
belongs to it actually, in virtue of its own nature, or (2) in
virtue of something conjoined with it. In the latter case it may be
either (a) something which by its own nature is capable of being
moved, e.g. the parts of the body or the nail in the ship, or (b)
something which is not in itself capable of being moved, but is always
moved through its conjunction with something else, as 'whiteness' or
'science'. These have changed their place only because the subjects to
which they belong do so.
  We say that a thing is in the world, in the sense of in place,
because it is in the air, and the air is in the world; and when we say
it is in the air, we do not mean it is in every part of the air, but
that it is in the air because of the outer surface of the air which
surrounds it; for if all the air were its place, the place of a
thing would not be equal to the thing-which it is supposed to be,
and which the primary place in which a thing is actually is.
  When what surrounds, then, is not separate from the thing, but is in
continuity with it, the thing is said to be in what surrounds it,
not in the sense of in place, but as a part in a whole. But when the
thing is separate and in contact, it is immediately 'in' the inner
surface of the surrounding body, and this surface is neither a part of
what is in it nor yet greater than its extension, but equal to it; for
the extremities of things which touch are coincident.
  Further, if one body is in continuity with another, it is not
moved in that but with that. On the other hand it is moved in that
if it is separate. It makes no difference whether what contains is
moved or not.
  Again, when it is not separate it is described as a part in a whole,
as the pupil in the eye or the hand in the body: when it is
separate, as the water in the cask or the wine in the jar. For the
hand is moved with the body and the water in the cask.
  It will now be plain from these considerations what place is.
There are just four things of which place must be one-the shape, or
the matter, or some sort of extension between the bounding surfaces of
the containing body, or this boundary itself if it contains no
extension over and above the bulk of the body which comes to be in it.
  Three of these it obviously cannot be:
  (1) The shape is supposed to be place because it surrounds, for
the extremities of what contains and of what is contained are
coincident. Both the shape and the place, it is true, are
boundaries. But not of the same thing: the form is the boundary of the
thing, the place is the boundary of the body which contains it.
  (2) The extension between the extremities is thought to be
something, because what is contained and separate may often be changed
while the container remains the same (as water may be poured from a
vessel)-the assumption being that the extension is something over
and above the body displaced. But there is no such extension. One of
the bodies which change places and are naturally capable of being in
contact with the container falls in whichever it may chance to be.
  If there were an extension which were such as to exist independently
and be permanent, there would be an infinity of places in the same
thing. For when the water and the air change places, all the
portions of the two together will play the same part in the whole
which was previously played by all the water in the vessel; at the
same time the place too will be undergoing change; so that there
will be another place which is the place of the place, and many places
will be coincident. There is not a different place of the part, in
which it is moved, when the whole vessel changes its place: it is
always the same: for it is in the (proximate) place where they are
that the air and the water (or the parts of the water) succeed each
other, not in that place in which they come to be, which is part of
the place which is the place of the whole world.
  (3) The matter, too, might seem to be place, at least if we consider
it in what is at rest and is thus separate but in continuity. For just
as in change of quality there is something which was formerly black
and is now white, or formerly soft and now hard-this is just why we
say that the matter exists-so place, because it presents a similar
phenomenon, is thought to exist-only in the one case we say so because
what was air is now water, in the other because where air formerly was
there a is now water. But the matter, as we said before, is neither
separable from the thing nor contains it, whereas place has both
  Well, then, if place is none of the three-neither the form nor the
matter nor an extension which is always there, different from, and
over and above, the extension of the thing which is displaced-place
necessarily is the one of the four which is left, namely, the boundary
of the containing body at which it is in contact with the contained
body. (By the contained body is meant what can be moved by way of
  Place is thought to be something important and hard to grasp, both
because the matter and the shape present themselves along with it, and
because the displacement of the body that is moved takes place in a
stationary container, for it seems possible that there should be an
interval which is other than the bodies which are moved. The air, too,
which is thought to be incorporeal, contributes something to the
belief: it is not only the boundaries of the vessel which seem to be
place, but also what is between them, regarded as empty. Just, in
fact, as the vessel is transportable place, so place is a non-portable
vessel. So when what is within a thing which is moved, is moved and
changes its place, as a boat on a river, what contains plays the
part of a vessel rather than that of place. Place on the other hand is
rather what is motionless: so it is rather the whole river that is
place, because as a whole it is motionless.
  Hence we conclude that the innermost motionless boundary of what
contains is place.
  This explains why the middle of the heaven and the surface which
faces us of the rotating system are held to be 'up' and 'down' in
the strict and fullest sense for all men: for the one is always at
rest, while the inner side of the rotating body remains always
coincident with itself. Hence since the light is what is naturally
carried up, and the heavy what is carried down, the boundary which
contains in the direction of the middle of the universe, and the
middle itself, are down, and that which contains in the direction of
the outermost part of the universe, and the outermost part itself, are
  For this reason, too, place is thought to be a kind of surface,
and as it were a vessel, i.e. a container of the thing.
  Further, place is coincident with the thing, for boundaries are
coincident with the bounded.

  If then a body has another body outside it and containing it, it
is in place, and if not, not. That is why, even if there were to be
water which had not a container, the parts of it, on the one hand,
will be moved (for one part is contained in another), while, on the
other hand, the whole will be moved in one sense, but not in
another. For as a whole it does not simultaneously change its place,
though it will be moved in a circle: for this place is the place of
its parts. (Some things are moved, not up and down, but in a circle;
others up and down, such things namely as admit of condensation and
  As was explained, some things are potentially in place, others
actually. So, when you have a homogeneous substance which is
continuous, the parts are potentially in place: when the parts are
separated, but in contact, like a heap, they are actually in place.
  Again, (1) some things are per se in place, namely every body
which is movable either by way of locomotion or by way of increase
is per se somewhere, but the heaven, as has been said, is not anywhere
as a whole, nor in any place, if at least, as we must suppose, no body
contains it. On the line on which it is moved, its parts have place:
for each is contiguous the next.
  But (2) other things are in place indirectly, through something
conjoined with them, as the soul and the heaven. The latter is, in a
way, in place, for all its parts are: for on the orb one part contains
another. That is why the upper part is moved in a circle, while the
All is not anywhere. For what is somewhere is itself something, and
there must be alongside it some other thing wherein it is and which
contains it. But alongside the All or the Whole there is nothing
outside the All, and for this reason all things are in the heaven; for
the heaven, we may say, is the All. Yet their place is not the same as
the heaven. It is part of it, the innermost part of it, which is in
contact with the movable body; and for this reason the earth is in
water, and this in the air, and the air in the aether, and the
aether in heaven, but we cannot go on and say that the heaven is in
anything else.
  It is clear, too, from these considerations that all the problems
which were raised about place will be solved when it is explained in
this way:
  (1) There is no necessity that the place should grow with the body
in it,
  (2) Nor that a point should have a place,
  (3) Nor that two bodies should be in the same place,
  (4) Nor that place should be a corporeal interval: for what is
between the boundaries of the place is any body which may chance to be
there, not an interval in body.
  Further, (5) place is also somewhere, not in the sense of being in a
place, but as the limit is in the limited; for not everything that
is is in place, but only movable body.
  Also (6) it is reasonable that each kind of body should be carried
to its own place. For a body which is next in the series and in
contact (not by compulsion) is akin, and bodies which are united do
not affect each other, while those which are in contact interact on
each other.
  Nor (7) is it without reason that each should remain naturally in
its proper place. For this part has the same relation to its place, as
a separable part to its whole, as when one moves a part of water or
air: so, too, air is related to water, for the one is like matter, the
other form-water is the matter of air, air as it were the actuality of
water, for water is potentially air, while air is potentially water,
though in another way.
  These distinctions will be drawn more carefully later. On the
present occasion it was necessary to refer to them: what has now
been stated obscurely will then be made more clear. If the matter
and the fulfilment are the same thing (for water is both, the one
potentially, the other completely), water will be related to air in
a way as part to whole. That is why these have contact: it is
organic union when both become actually one.
  This concludes my account of place-both of its existence and of
its nature.

  The investigation of similar questions about the void, also, must be
held to belong to the physicist-namely whether it exists or not, and
how it exists or what it is-just as about place. The views taken of it
involve arguments both for and against, in much the same sort of
way. For those who hold that the void exists regard it as a sort of
place or vessel which is supposed to be 'full' when it holds the
bulk which it is capable of containing, 'void' when it is deprived
of that-as if 'void' and 'full' and 'place' denoted the same thing,
though the essence of the three is different.
  We must begin the inquiry by putting down the account given by those
who say that it exists, then the account of those who say that it does
not exist, and third the current view on these questions.
  Those who try to show that the void does not exist do not disprove
what people really mean by it, but only their erroneous way of
speaking; this is true of Anaxagoras and of those who refute the
existence of the void in this way. They merely give an ingenious
demonstration that air is something--by straining wine-skins and
showing the resistance of the air, and by cutting it off in
clepsydras. But people really mean that there is an empty interval
in which there is no sensible body. They hold that everything which is
in body is body and say that what has nothing in it at all is void (so
what is full of air is void). It is not then the existence of air that
needs to be proved, but the non-existence of an interval, different
from the bodies, either separable or actual-an interval which
divides the whole body so as to break its continuity, as Democritus
and Leucippus hold, and many other physicists-or even perhaps as
something which is outside the whole body, which remains continuous.
  These people, then, have not reached even the threshold of the
problem, but rather those who say that the void exists.
  (1) They argue, for one thing, that change in place (i.e. locomotion
and increase) would not be. For it is maintained that motion would
seem not to exist, if there were no void, since what is full cannot
contain anything more. If it could, and there were two bodies in the
same place, it would also be true that any number of bodies could be
together; for it is impossible to draw a line of division beyond which
the statement would become untrue. If this were possible, it would
follow also that the smallest body would contain the greatest; for
'many a little makes a mickle': thus if many equal bodies can be
together, so also can many unequal bodies.
  Melissus, indeed, infers from these considerations that the All is
immovable; for if it were moved there must, he says, be void, but void
is not among the things that exist.
  This argument, then, is one way in which they show that there is a
  (2) They reason from the fact that some things are observed to
contract and be compressed, as people say that a cask will hold the
wine which formerly filled it, along with the skins into which the
wine has been decanted, which implies that the compressed body
contracts into the voids present in it.
  Again (3) increase, too, is thought to take always by means of void,
for nutriment is body, and it is impossible for two bodies to be
together. A proof of this they find also in what happens to ashes,
which absorb as much water as the empty vessel.
  The Pythagoreans, too, (4) held that void exists and that it
enters the heaven itself, which as it were inhales it, from the
infinite air. Further it is the void which distinguishes the natures
of things, as if it were like what separates and distinguishes the
terms of a series. This holds primarily in the numbers, for the void
distinguishes their nature.
  These, then, and so many, are the main grounds on which people
have argued for and against the existence of the void.

  As a step towards settling which view is true, we must determine the
meaning of the name.
  The void is thought to be place with nothing in it. The reason for
this is that people take what exists to be body, and hold that while
every body is in place, void is place in which there is no body, so
that where there is no body, there must be void.
  Every body, again, they suppose to be tangible; and of this nature
is whatever has weight or lightness.
  Hence, by a syllogism, what has nothing heavy or light in it, is
  This result, then, as I have said, is reached by syllogism. It would
be absurd to suppose that the point is void; for the void must be
place which has in it an interval in tangible body.
  But at all events we observe then that in one way the void is
described as what is not full of body perceptible to touch; and what
has heaviness and lightness is perceptible to touch. So we would raise
the question: what would they say of an interval that has colour or
sound-is it void or not? Clearly they would reply that if it could
receive what is tangible it was void, and if not, not.
  In another way void is that in which there is no 'this' or corporeal
substance. So some say that the void is the matter of the body (they
identify the place, too, with this), and in this they speak
incorrectly; for the matter is not separable from the things, but they
are inquiring about the void as about something separable.
  Since we have determined the nature of place, and void must, if it
exists, be place deprived of body, and we have stated both in what
sense place exists and in what sense it does not, it is plain that
on this showing void does not exist, either unseparated or
separated; the void is meant to be, not body but rather an interval in
body. This is why the void is thought to be something, viz. because
place is, and for the same reasons. For the fact of motion in
respect of place comes to the aid both of those who maintain that
place is something over and above the bodies that come to occupy it,
and of those who maintain that the void is something. They state
that the void is the condition of movement in the sense of that in
which movement takes place; and this would be the kind of thing that
some say place is.
  But there is no necessity for there being a void if there is
movement. It is not in the least needed as a condition of movement
in general, for a reason which, incidentally, escaped Melissus; viz.
that the full can suffer qualitative change.
  But not even movement in respect of place involves a void; for
bodies may simultaneously make room for one another, though there is
no interval separate and apart from the bodies that are in movement.
And this is plain even in the rotation of continuous things, as in
that of liquids.
  And things can also be compressed not into a void but because they
squeeze out what is contained in them (as, for instance, when water is
compressed the air within it is squeezed out); and things can increase
in size not only by the entrance of something but also by
qualitative change; e.g. if water were to be transformed into air.
  In general, both the argument about increase of size and that
about water poured on to the ashes get in their own way. For either
not any and every part of the body is increased, or bodies may be
increased otherwise than by the addition of body, or there may be
two bodies in the same place (in which case they are claiming to solve
a quite general difficulty, but are not proving the existence of
void), or the whole body must be void, if it is increased in every
part and is increased by means of void. The same argument applies to
the ashes.
  It is evident, then, that it is easy to refute the arguments by
which they prove the existence of the void.

  Let us explain again that there is no void existing separately, as
some maintain. If each of the simple bodies has a natural
locomotion, e.g. fire upward and earth downward and towards the middle
of the universe, it is clear that it cannot be the void that is the
condition of locomotion. What, then, will the void be the condition
of? It is thought to be the condition of movement in respect of place,
and it is not the condition of this.
  Again, if void is a sort of place deprived of body, when there is
a void where will a body placed in it move to? It certainly cannot
move into the whole of the void. The same argument applies as
against those who think that place is something separate, into which
things are carried; viz. how will what is placed in it move, or
rest? Much the same argument will apply to the void as to the 'up' and
'down' in place, as is natural enough since those who maintain the
existence of the void make it a place.
  And in what way will things be present either in place-or in the
void? For the expected result does not take place when a body is
placed as a whole in a place conceived of as separate and permanent;
for a part of it, unless it be placed apart, will not be in a place
but in the whole. Further, if separate place does not exist, neither
will void.
  If people say that the void must exist, as being necessary if
there is to be movement, what rather turns out to be the case, if
one the matter, is the opposite, that not a single thing can be
moved if there is a void; for as with those who for a like reason
say the earth is at rest, so, too, in the void things must be at rest;
for there is no place to which things can move more or less than to
another; since the void in so far as it is void admits no difference.
  The second reason is this: all movement is either compulsory or
according to nature, and if there is compulsory movement there must
also be natural (for compulsory movement is contrary to nature, and
movement contrary to nature is posterior to that according to
nature, so that if each of the natural bodies has not a natural
movement, none of the other movements can exist); but how can there be
natural movement if there is no difference throughout the void or
the infinite? For in so far as it is infinite, there will be no up
or down or middle, and in so far as it is a void, up differs no whit
from down; for as there is no difference in what is nothing, there
is none in the void (for the void seems to be a non-existent and a
privation of being), but natural locomotion seems to be
differentiated, so that the things that exist by nature must be
differentiated. Either, then, nothing has a natural locomotion, or
else there is no void.
  Further, in point of fact things that are thrown move though that
which gave them their impulse is not touching them, either by reason
of mutual replacement, as some maintain, or because the air that has
been pushed pushes them with a movement quicker than the natural
locomotion of the projectile wherewith it moves to its proper place.
But in a void none of these things can take place, nor can anything be
moved save as that which is carried is moved.
  Further, no one could say why a thing once set in motion should stop
anywhere; for why should it stop here rather than here? So that a
thing will either be at rest or must be moved ad infinitum, unless
something more powerful get in its way.
  Further, things are now thought to move into the void because it
yields; but in a void this quality is present equally everywhere, so
that things should move in all directions.
  Further, the truth of what we assert is plain from the following
considerations. We see the same weight or body moving faster than
another for two reasons, either because there is a difference in
what it moves through, as between water, air, and earth, or because,
other things being equal, the moving body differs from the other owing
to excess of weight or of lightness.
  Now the medium causes a difference because it impedes the moving
thing, most of all if it is moving in the opposite direction, but in a
secondary degree even if it is at rest; and especially a medium that
is not easily divided, i.e. a medium that is somewhat dense. A,
then, will move through B in time G, and through D, which is
thinner, in time E (if the length of B is egual to D), in proportion
to the density of the hindering body. For let B be water and D air;
then by so much as air is thinner and more incorporeal than water, A
will move through D faster than through B. Let the speed have the same
ratio to the speed, then, that air has to water. Then if air is
twice as thin, the body will traverse B in twice the time that it does
D, and the time G will be twice the time E. And always, by so much
as the medium is more incorporeal and less resistant and more easily
divided, the faster will be the movement.
  Now there is no ratio in which the void is exceeded by body, as
there is no ratio of 0 to a number. For if 4 exceeds 3 by 1, and 2
by more than 1, and 1 by still more than it exceeds 2, still there
is no ratio by which it exceeds 0; for that which exceeds must be
divisible into the excess + that which is exceeded, so that will be
what it exceeds 0 by + 0. For this reason, too, a line does not exceed
a point unless it is composed of points! Similarly the void can bear
no ratio to the full, and therefore neither can movement through the
one to movement through the other, but if a thing moves through the
thickest medium such and such a distance in such and such a time, it
moves through the void with a speed beyond any ratio. For let Z be
void, equal in magnitude to B and to D. Then if A is to traverse and
move through it in a certain time, H, a time less than E, however, the
void will bear this ratio to the full. But in a time equal to H, A
will traverse the part O of A. And it will surely also traverse in
that time any substance Z which exceeds air in thickness in the
ratio which the time E bears to the time H. For if the body Z be as
much thinner than D as E exceeds H, A, if it moves through Z, will
traverse it in a time inverse to the speed of the movement, i.e. in
a time equal to H. If, then, there is no body in Z, A will traverse
Z still more quickly. But we supposed that its traverse of Z when Z
was void occupied the time H. So that it will traverse Z in an equal
time whether Z be full or void. But this is impossible. It is plain,
then, that if there is a time in which it will move through any part
of the void, this impossible result will follow: it will be found to
traverse a certain distance, whether this be full or void, in an equal
time; for there will be some body which is in the same ratio to the
other body as the time is to the time.
  To sum the matter up, the cause of this result is obvious, viz. that
between any two movements there is a ratio (for they occupy time,
and there is a ratio between any two times, so long as both are
finite), but there is no ratio of void to full.
  These are the consequences that result from a difference in the
media; the following depend upon an excess of one moving body over
another. We see that bodies which have a greater impulse either of
weight or of lightness, if they are alike in other respects, move
faster over an equal space, and in the ratio which their magnitudes
bear to each other. Therefore they will also move through the void
with this ratio of speed. But that is impossible; for why should one
move faster? (In moving through plena it must be so; for the greater
divides them faster by its force. For a moving thing cleaves the
medium either by its shape, or by the impulse which the body that is
carried along or is projected possesses.) Therefore all will possess
equal velocity. But this is impossible.
  It is evident from what has been said, then, that, if there is a
void, a result follows which is the very opposite of the reason for
which those who believe in a void set it up. They think that if
movement in respect of place is to exist, the void cannot exist,
separated all by itself; but this is the same as to say that place
is a separate cavity; and this has already been stated to be
  But even if we consider it on its own merits the so-called vacuum
will be found to be really vacuous. For as, if one puts a cube in
water, an amount of water equal to the cube will be displaced; so
too in air; but the effect is imperceptible to sense. And indeed
always in the case of any body that can be displaced, must, if it is
not compressed, be displaced in the direction in which it is its
nature to be displaced-always either down, if its locomotion is
downwards as in the case of earth, or up, if it is fire, or in both
directions-whatever be the nature of the inserted body. Now in the
void this is impossible; for it is not body; the void must have
penetrated the cube to a distance equal to that which this portion
of void formerly occupied in the void, just as if the water or air had
not been displaced by the wooden cube, but had penetrated right
through it.
  But the cube also has a magnitude equal to that occupied by the
void; a magnitude which, if it is also hot or cold, or heavy or light,
is none the less different in essence from all its attributes, even if
it is not separable from them; I mean the volume of the wooden cube.
So that even if it were separated from everything else and were
neither heavy nor light, it will occupy an equal amount of void, and
fill the same place, as the part of place or of the void equal to
itself. How then will the body of the cube differ from the void or
place that is equal to it? And if there can be two such things, why
cannot there be any number coinciding?
  This, then, is one absurd and impossible implication of the
theory. It is also evident that the cube will have this same volume
even if it is displaced, which is an attribute possessed by all
other bodies also. Therefore if this differs in no respect from its
place, why need we assume a place for bodies over and above the volume
of each, if their volume be conceived of as free from attributes? It
contributes nothing to the situation if there is an equal interval
attached to it as well. [Further it ought to be clear by the study
of moving things what sort of thing void is. But in fact it is found
nowhere in the world. For air is something, though it does not seem to
be so-nor, for that matter, would water, if fishes were made of
iron; for the discrimination of the tangible is by touch.]
  It is clear, then, from these considerations that there is no
separate void.

  There are some who think that the existence of rarity and density
shows that there is a void. If rarity and density do not exist, they
say, neither can things contract and be compressed. But if this were
not to take place, either there would be no movement at all, or the
universe would bulge, as Xuthus said, or air and water must always
change into equal amounts (e.g. if air has been made out of a cupful
of water, at the same time out of an equal amount of air a cupful of
water must have been made), or void must necessarily exist; for
compression and expansion cannot take place otherwise.
  Now, if they mean by the rare that which has many voids existing
separately, it is plain that if void cannot exist separate any more
than a place can exist with an extension all to itself, neither can
the rare exist in this sense. But if they mean that there is void, not
separately existent, but still present in the rare, this is less
impossible, yet, first, the void turns out not to be a condition of
all movement, but only of movement upwards (for the rare is light,
which is the reason why they say fire is rare); second, the void turns
out to be a condition of movement not as that in which it takes place,
but in that the void carries things up as skins by being carried up
themselves carry up what is continuous with them. Yet how can void
have a local movement or a place? For thus that into which void
moves is till then void of a void.
  Again, how will they explain, in the case of what is heavy, its
movement downwards? And it is plain that if the rarer and more void
a thing is the quicker it will move upwards, if it were completely
void it would move with a maximum speed! But perhaps even this is
impossible, that it should move at all; the same reason which showed
that in the void all things are incapable of moving shows that the
void cannot move, viz. the fact that the speeds are incomparable.
  Since we deny that a void exists, but for the rest the problem has
been truly stated, that either there will be no movement, if there
is not to be condensation and rarefaction, or the universe will bulge,
or a transformation of water into air will always be balanced by an
equal transformation of air into water (for it is clear that the air
produced from water is bulkier than the water): it is necessary
therefore, if compression does not exist, either that the next portion
will be pushed outwards and make the outermost part bulge, or that
somewhere else there must be an equal amount of water produced out
of air, so that the entire bulk of the whole may be equal, or that
nothing moves. For when anything is displaced this will always happen,
unless it comes round in a circle; but locomotion is not always
circular, but sometimes in a straight line.
  These then are the reasons for which they might say that there is
a void; our statement is based on the assumption that there is a
single matter for contraries, hot and cold and the other natural
contrarieties, and that what exists actually is produced from a
potential existent, and that matter is not separable from the
contraries but its being is different, and that a single matter may
serve for colour and heat and cold.
  The same matter also serves for both a large and a small body.
This is evident; for when air is produced from water, the same
matter has become something different, not by acquiring an addition to
it, but has become actually what it was potentially, and, again, water
is produced from air in the same way, the change being sometimes
from smallness to greatness, and sometimes from greatness to
smallness. Similarly, therefore, if air which is large in extent comes
to have a smaller volume, or becomes greater from being smaller, it is
the matter which is potentially both that comes to be each of the two.
  For as the same matter becomes hot from being cold, and cold from
being hot, because it was potentially both, so too from hot it can
become more hot, though nothing in the matter has become hot that
was not hot when the thing was less hot; just as, if the arc or
curve of a greater circle becomes that of a smaller, whether it
remains the same or becomes a different curve, convexity has not
come to exist in anything that was not convex but straight (for
differences of degree do not depend on an intermission of the
quality); nor can we get any portion of a flame, in which both heat
and whiteness are not present. So too, then, is the earlier heat
related to the later. So that the greatness and smallness, also, of
the sensible volume are extended, not by the matter's acquiring
anything new, but because the matter is potentially matter for both
states; so that the same thing is dense and rare, and the two
qualities have one matter.
  The dense is heavy, and the rare is light. [Again, as the arc of a
circle when contracted into a smaller space does not acquire a new
part which is convex, but what was there has been contracted; and as
any part of fire that one takes will be hot; so, too, it is all a
question of contraction and expansion of the same matter.] There are
two types in each case, both in the dense and in the rare; for both
the heavy and the hard are thought to be dense, and contrariwise
both the light and the soft are rare; and weight and hardness fail
to coincide in the case of lead and iron.
  From what has been said it is evident, then, that void does not
exist either separate (either absolutely separate or as a separate
element in the rare) or potentially, unless one is willing to call the
condition of movement void, whatever it may be. At that rate the
matter of the heavy and the light, qua matter of them, would be the
void; for the dense and the rare are productive of locomotion in
virtue of this contrariety, and in virtue of their hardness and
softness productive of passivity and impassivity, i.e. not of
locomotion but rather of qualitative change.
  So much, then, for the discussion of the void, and of the sense in
which it exists and the sense in which it does not exist.

  Next for discussion after the subjects mentioned is Time. The best
plan will be to begin by working out the difficulties connected with
it, making use of the current arguments. First, does it belong to
the class of things that exist or to that of things that do not exist?
Then secondly, what is its nature? To start, then: the following
considerations would make one suspect that it either does not exist at
all or barely, and in an obscure way. One part of it has been and is
not, while the other is going to be and is not yet. Yet time-both
infinite time and any time you like to take-is made up of these. One
would naturally suppose that what is made up of things which do not
exist could have no share in reality.
  Further, if a divisible thing is to exist, it is necessary that,
when it exists, all or some of its parts must exist. But of time
some parts have been, while others have to be, and no part of it is
though it is divisible. For what is 'now' is not a part: a part is a
measure of the whole, which must be made up of parts. Time, on the
other hand, is not held to be made up of 'nows'.
  Again, the 'now' which seems to bound the past and the future-does
it always remain one and the same or is it always other and other?
It is hard to say.
  (1) If it is always different and different, and if none of the
parts in time which are other and other are simultaneous (unless the
one contains and the other is contained, as the shorter time is by the
longer), and if the 'now' which is not, but formerly was, must have
ceased-to-be at some time, the 'nows' too cannot be simultaneous
with one another, but the prior 'now' must always have ceased-to-be.
But the prior 'now' cannot have ceased-to-be in itself (since it
then existed); yet it cannot have ceased-to-be in another 'now'. For
we may lay it down that one 'now' cannot be next to another, any
more than point to point. If then it did not cease-to-be in the next
'now' but in another, it would exist simultaneously with the
innumerable 'nows' between the two-which is impossible.
  Yes, but (2) neither is it possible for the 'now' to remain always
the same. No determinate divisible thing has a single termination,
whether it is continuously extended in one or in more than one
dimension: but the 'now' is a termination, and it is possible to cut
off a determinate time. Further, if coincidence in time (i.e. being
neither prior nor posterior) means to be 'in one and the same
"now"', then, if both what is before and what is after are in this
same 'now', things which happened ten thousand years ago would be
simultaneous with what has happened to-day, and nothing would be
before or after anything else.
  This may serve as a statement of the difficulties about the
attributes of time.
  As to what time is or what is its nature, the traditional accounts
give us as little light as the preliminary problems which we have
worked through.
  Some assert that it is (1) the movement of the whole, others that it
is (2) the sphere itself.
  (1) Yet part, too, of the revolution is a time, but it certainly
is not a revolution: for what is taken is part of a revolution, not
a revolution. Besides, if there were more heavens than one, the
movement of any of them equally would be time, so that there would
be many times at the same time.
  (2) Those who said that time is the sphere of the whole thought
so, no doubt, on the ground that all things are in time and all things
are in the sphere of the whole. The view is too naive for it to be
worth while to consider the impossibilities implied in it.
  But as time is most usually supposed to be (3) motion and a kind
of change, we must consider this view.
  Now (a) the change or movement of each thing is only in the thing
which changes or where the thing itself which moves or changes may
chance to be. But time is present equally everywhere and with all
  Again, (b) change is always faster or slower, whereas time is not:
for 'fast' and 'slow' are defined by time-'fast' is what moves much in
a short time, 'slow' what moves little in a long time; but time is not
defined by time, by being either a certain amount or a certain kind of
  Clearly then it is not movement. (We need not distinguish at present
between 'movement' and 'change'.)

  But neither does time exist without change; for when the state of
our own minds does not change at all, or we have not noticed its
changing, we do not realize that time has elapsed, any more than those
who are fabled to sleep among the heroes in Sardinia do when they
are awakened; for they connect the earlier 'now' with the later and
make them one, cutting out the interval because of their failure to
notice it. So, just as, if the 'now' were not different but one and
the same, there would not have been time, so too when its difference
escapes our notice the interval does not seem to be time. If, then,
the non-realization of the existence of time happens to us when we
do not distinguish any change, but the soul seems to stay in one
indivisible state, and when we perceive and distinguish we say time
has elapsed, evidently time is not independent of movement and change.
It is evident, then, that time is neither movement nor independent
of movement.
  We must take this as our starting-point and try to discover-since we
wish to know what time is-what exactly it has to do with movement.
  Now we perceive movement and time together: for even when it is dark
and we are not being affected through the body, if any movement
takes place in the mind we at once suppose that some time also has
elapsed; and not only that but also, when some time is thought to have
passed, some movement also along with it seems to have taken place.
Hence time is either movement or something that belongs to movement.
Since then it is not movement, it must be the other.
  But what is moved is moved from something to something, and all
magnitude is continuous. Therefore the movement goes with the
magnitude. Because the magnitude is continuous, the movement too
must be continuous, and if the movement, then the time; for the time
that has passed is always thought to be in proportion to the movement.
  The distinction of 'before' and 'after' holds primarily, then, in
place; and there in virtue of relative position. Since then 'before'
and 'after' hold in magnitude, they must hold also in movement,
these corresponding to those. But also in time the distinction of
'before' and 'after' must hold, for time and movement always
correspond with each other. The 'before' and 'after' in motion is
identical in substratum with motion yet differs from it in definition,
and is not identical with motion.
  But we apprehend time only when we have marked motion, marking it by
'before' and 'after'; and it is only when we have perceived 'before'
and 'after' in motion that we say that time has elapsed. Now we mark
them by judging that A and B are different, and that some third
thing is intermediate to them. When we think of the extremes as
different from the middle and the mind pronounces that the 'nows'
are two, one before and one after, it is then that we say that there
is time, and this that we say is time. For what is bounded by the
'now' is thought to be time-we may assume this.
  When, therefore, we perceive the 'now' one, and neither as before
and after in a motion nor as an identity but in relation to a 'before'
and an 'after', no time is thought to have elapsed, because there
has been no motion either. On the other hand, when we do perceive a
'before' and an 'after', then we say that there is time. For time is
just this-number of motion in respect of 'before' and 'after'.
  Hence time is not movement, but only movement in so far as it admits
of enumeration. A proof of this: we discriminate the more or the
less by number, but more or less movement by time. Time then is a kind
of number. (Number, we must note, is used in two senses-both of what
is counted or the countable and also of that with which we count. Time
obviously is what is counted, not that with which we count: there
are different kinds of thing.) Just as motion is a perpetual
succession, so also is time. But every simultaneous time is
self-identical; for the 'now' as a subject is an identity, but it
accepts different attributes. The 'now' measures time, in so far as
time involves the 'before and after'.
  The 'now' in one sense is the same, in another it is not the same.
In so far as it is in succession, it is different (which is just
what its being was supposed to mean), but its substratum is an
identity: for motion, as was said, goes with magnitude, and time, as
we maintain, with motion. Similarly, then, there corresponds to the
point the body which is carried along, and by which we are aware of
the motion and of the 'before and after' involved in it. This is an
identical substratum (whether a point or a stone or something else
of the kind), but it has different attributes as the sophists assume
that Coriscus' being in the Lyceum is a different thing from Coriscus'
being in the market-place. And the body which is carried along is
different, in so far as it is at one time here and at another there.
But the 'now' corresponds to the body that is carried along, as time
corresponds to the motion. For it is by means of the body that is
carried along that we become aware of the 'before and after' the
motion, and if we regard these as countable we get the 'now'. Hence in
these also the 'now' as substratum remains the same (for it is what is
before and after in movement), but what is predicated of it is
different; for it is in so far as the 'before and after' is
numerable that we get the 'now'. This is what is most knowable: for,
similarly, motion is known because of that which is moved,
locomotion because of that which is carried. what is carried is a real
thing, the movement is not. Thus what is called 'now' in one sense
is always the same; in another it is not the same: for this is true
also of what is carried.
  Clearly, too, if there were no time, there would be no 'now', and
vice versa. just as the moving body and its locomotion involve each
other mutually, so too do the number of the moving body and the number
of its locomotion. For the number of the locomotion is time, while the
'now' corresponds to the moving body, and is like the unit of number.
  Time, then, also is both made continuous by the 'now' and divided at
it. For here too there is a correspondence with the locomotion and the
moving body. For the motion or locomotion is made one by the thing
which is moved, because it is one-not because it is one in its own
nature (for there might be pauses in the movement of such a thing)-but
because it is one in definition: for this determines the movement as
'before' and 'after'. Here, too there is a correspondence with the
point; for the point also both connects and terminates the length-it
is the beginning of one and the end of another. But when you take it
in this way, using the one point as two, a pause is necessary, if
the same point is to be the beginning and the end. The 'now' on the
other hand, since the body carried is moving, is always different.
  Hence time is not number in the sense in which there is 'number'
of the same point because it is beginning and end, but rather as the
extremities of a line form a number, and not as the parts of the
line do so, both for the reason given (for we can use the middle point
as two, so that on that analogy time might stand still), and further
because obviously the 'now' is no part of time nor the section any
part of the movement, any more than the points are parts of the
line-for it is two lines that are parts of one line.
  In so far then as the 'now' is a boundary, it is not time, but an
attribute of it; in so far as it numbers, it is number; for boundaries
belong only to that which they bound, but number (e.g. ten) is the
number of these horses, and belongs also elsewhere.
  It is clear, then, that time is 'number of movement in respect of
the before and after', and is continuous since it is an attribute of
what is continuous.

  The smallest number, in the strict sense of the word 'number', is
two. But of number as concrete, sometimes there is a minimum,
sometimes not: e.g. of a 'line', the smallest in respect of
multiplicity is two (or, if you like, one), but in respect of size
there is no minimum; for every line is divided ad infinitum. Hence
it is so with time. In respect of number the minimum is one (or
two); in point of extent there is no minimum.
  It is clear, too, that time is not described as fast or slow, but as
many or few and as long or short. For as continuous it is long or
short and as a number many or few, but it is not fast or slow-any more
than any number with which we number is fast or slow.
  Further, there is the same time everywhere at once, but not the same
time before and after, for while the present change is one, the change
which has happened and that which will happen are different. Time is
not number with which we count, but the number of things which are
counted, and this according as it occurs before or after is always
different, for the 'nows' are different. And the number of a hundred
horses and a hundred men is the same, but the things numbered are
different-the horses from the men. Further, as a movement can be one
and the same again and again, so too can time, e.g. a year or a spring
or an autumn.
  Not only do we measure the movement by the time, but also the time
by the movement, because they define each other. The time marks the
movement, since it is its number, and the movement the time. We
describe the time as much or little, measuring it by the movement,
just as we know the number by what is numbered, e.g. the number of the
horses by one horse as the unit. For we know how many horses there are
by the use of the number; and again by using the one horse as unit
we know the number of the horses itself. So it is with the time and
the movement; for we measure the movement by the time and vice
versa. It is natural that this should happen; for the movement goes
with the distance and the time with the movement, because they are
quanta and continuous and divisible. The movement has these attributes
because the distance is of this nature, and the time has them
because of the movement. And we measure both the distance by the
movement and the movement by the distance; for we say that the road is
long, if the journey is long, and that this is long, if the road is
long-the time, too, if the movement, and the movement, if the time.
  Time is a measure of motion and of being moved, and it measures
the motion by determining a motion which will measure exactly the
whole motion, as the cubit does the length by determining an amount
which will measure out the whole. Further 'to be in time' means for
movement, that both it and its essence are measured by time (for
simultaneously it measures both the movement and its essence, and this
is what being in time means for it, that its essence should be
  Clearly then 'to be in time' has the same meaning for other things
also, namely, that their being should be measured by time. 'To be in
time' is one of two things: (1) to exist when time exists, (2) as we
say of some things that they are 'in number'. The latter means
either what is a part or mode of number-in general, something which
belongs to number-or that things have a number.
  Now, since time is number, the 'now' and the 'before' and the like
are in time, just as 'unit' and 'odd' and 'even' are in number, i.e.
in the sense that the one set belongs to number, the other to time.
But things are in time as they are in number. If this is so, they
are contained by time as things in place are contained by place.
  Plainly, too, to be in time does not mean to co-exist with time, any
more than to be in motion or in place means to co-exist with motion or
place. For if 'to be in something' is to mean this, then all things
will be in anything, and the heaven will be in a grain; for when the
grain is, then also is the heaven. But this is a merely incidental
conjunction, whereas the other is necessarily involved: that which
is in time necessarily involves that there is time when it is, and
that which is in motion that there is motion when it is.
  Since what is 'in time' is so in the same sense as what is in number
is so, a time greater than everything in time can be found. So it is
necessary that all the things in time should be contained by time,
just like other things also which are 'in anything', e.g. the things
'in place' by place.
  A thing, then, will be affected by time, just as we are accustomed
to say that time wastes things away, and that all things grow old
through time, and that there is oblivion owing to the lapse of time,
but we do not say the same of getting to know or of becoming young
or fair. For time is by its nature the cause rather of decay, since it
is the number of change, and change removes what is.
  Hence, plainly, things which are always are not, as such, in time,
for they are not contained time, nor is their being measured by
time. A proof of this is that none of them is affected by time,
which indicates that they are not in time.
  Since time is the measure of motion, it will be the measure of
rest too-indirectly. For all rest is in time. For it does not follow
that what is in time is moved, though what is in motion is necessarily
moved. For time is not motion, but 'number of motion': and what is
at rest, also, can be in the number of motion. Not everything that
is not in motion can be said to be 'at rest'-but only that which can
be moved, though it actually is not moved, as was said above.
  'To be in number' means that there is a number of the thing, and
that its being is measured by the number in which it is. Hence if a
thing is 'in time' it will be measured by time. But time will
measure what is moved and what is at rest, the one qua moved, the
other qua at rest; for it will measure their motion and rest
  Hence what is moved will not be measurable by the time simply in
so far as it has quantity, but in so far as its motion has quantity.
Thus none of the things which are neither moved nor at rest are in
time: for 'to be in time' is 'to be measured by time', while time is
the measure of motion and rest.
  Plainly, then, neither will everything that does not exist be in
time, i.e. those non-existent things that cannot exist, as the
diagonal cannot be commensurate with the side.
  Generally, if time is directly the measure of motion and
indirectly of other things, it is clear that a thing whose existence
is measured by it will have its existence in rest or motion. Those
things therefore which are subject to perishing and
becoming-generally, those which at one time exist, at another do
not-are necessarily in time: for there is a greater time which will
extend both beyond their existence and beyond the time which
measures their existence. Of things which do not exist but are
contained by time some were, e.g. Homer once was, some will be, e.g. a
future event; this depends on the direction in which time contains
them; if on both, they have both modes of existence. As to such things
as it does not contain in any way, they neither were nor are nor
will be. These are those nonexistents whose opposites always are, as
the incommensurability of the diagonal always is-and this will not
be in time. Nor will the commensurability, therefore; hence this
eternally is not, because it is contrary to what eternally is. A thing
whose contrary is not eternal can be and not be, and it is of such
things that there is coming to be and passing away.

  The 'now' is the link of time, as has been said (for it connects
past and future time), and it is a limit of time (for it is the
beginning of the one and the end of the other). But this is not
obvious as it is with the point, which is fixed. It divides
potentially, and in so far as it is dividing the 'now' is always
different, but in so far as it connects it is always the same, as it
is with mathematical lines. For the intellect it is not always one and
the same point, since it is other and other when one divides the line;
but in so far as it is one, it is the same in every respect.
  So the 'now' also is in one way a potential dividing of time, in
another the termination of both parts, and their unity. And the
dividing and the uniting are the same thing and in the same reference,
but in essence they are not the same.
  So one kind of 'now' is described in this way: another is when the
time is near this kind of 'now'. 'He will come now' because he will
come to-day; 'he has come now' because he came to-day. But the
things in the Iliad have not happened 'now', nor is the flood
'now'-not that the time from now to them is not continuous, but
because they are not near.
  'At some time' means a time determined in relation to the first of
the two types of 'now', e.g. 'at some time' Troy was taken, and 'at
some time' there will be a flood; for it must be determined with
reference to the 'now'. There will thus be a determinate time from
this 'now' to that, and there was such in reference to the past event.
But if there be no time which is not 'sometime', every time will be
  Will time then fail? Surely not, if motion always exists. Is time
then always different or does the same time recur? Clearly time is, in
the same way as motion is. For if one and the same motion sometimes
recurs, it will be one and the same time, and if not, not.
  Since the 'now' is an end and a beginning of time, not of the same
time however, but the end of that which is past and the beginning of
that which is to come, it follows that, as the circle has its
convexity and its concavity, in a sense, in the same thing, so time is
always at a beginning and at an end. And for this reason it seems to
be always different; for the 'now' is not the beginning and the end of
the same thing; if it were, it would be at the same time and in the
same respect two opposites. And time will not fail; for it is always
at a beginning.
  'Presently' or 'just' refers to the part of future time which is
near the indivisible present 'now' ('When do you walk? 'Presently',
because the time in which he is going to do so is near), and to the
part of past time which is not far from the 'now' ('When do you walk?'
'I have just been walking'). But to say that Troy has just been
taken-we do not say that, because it is too far from the 'now'.
'Lately', too, refers to the part of past time which is near the
present 'now'. 'When did you go?' 'Lately', if the time is near the
existing now. 'Long ago' refers to the distant past.
  'Suddenly' refers to what has departed from its former condition
in a time imperceptible because of its smallness; but it is the nature
of all change to alter things from their former condition. In time all
things come into being and pass away; for which reason some called
it the wisest of all things, but the Pythagorean Paron called it the
most stupid, because in it we also forget; and his was the truer view.
It is clear then that it must be in itself, as we said before, the
condition of destruction rather than of coming into being (for change,
in itself, makes things depart from their former condition), and
only incidentally of coming into being, and of being. A sufficient
evidence of this is that nothing comes into being without itself
moving somehow and acting, but a thing can be destroyed even if it
does not move at all. And this is what, as a rule, we chiefly mean
by a thing's being destroyed by time. Still, time does not work even
this change; even this sort of change takes place incidentally in
  We have stated, then, that time exists and what it is, and in how
many senses we speak of the 'now', and what 'at some time',
'lately', 'presently' or 'just', 'long ago', and 'suddenly' mean.

  These distinctions having been drawn, it is evident that every
change and everything that moves is in time; for the distinction of
faster and slower exists in reference to all change, since it is found
in every instance. In the phrase 'moving faster' I refer to that which
changes before another into the condition in question, when it moves
over the same interval and with a regular movement; e.g. in the case
of locomotion, if both things move along the circumference of a
circle, or both along a straight line; and similarly in all other
cases. But what is before is in time; for we say 'before' and
'after' with reference to the distance from the 'now', and the 'now'
is the boundary of the past and the future; so that since 'nows' are
in time, the before and the after will be in time too; for in that
in which the 'now' is, the distance from the 'now' will also be. But
'before' is used contrariwise with reference to past and to future
time; for in the past we call 'before' what is farther from the 'now',
and 'after' what is nearer, but in the future we call the nearer
'before' and the farther 'after'. So that since the 'before' is in
time, and every movement involves a 'before', evidently every change
and every movement is in time.
  It is also worth considering how time can be related to the soul;
and why time is thought to be in everything, both in earth and in
sea and in heaven. Is because it is an attribute, or state, or
movement (since it is the number of movement) and all these things are
movable (for they are all in place), and time and movement are
together, both in respect of potentiality and in respect of actuality?
  Whether if soul did not exist time would exist or not, is a question
that may fairly be asked; for if there cannot be some one to count
there cannot be anything that can be counted, so that evidently
there cannot be number; for number is either what has been, or what
can be, counted. But if nothing but soul, or in soul reason, is
qualified to count, there would not be time unless there were soul,
but only that of which time is an attribute, i.e. if movement can
exist without soul, and the before and after are attributes of
movement, and time is these qua numerable.
  One might also raise the question what sort of movement time is
the number of. Must we not say 'of any kind'? For things both come
into being in time and pass away, and grow, and are altered in time,
and are moved locally; thus it is of each movement qua movement that
time is the number. And so it is simply the number of continuous
movement, not of any particular kind of it.
  But other things as well may have been moved now, and there would be
a number of each of the two movements. Is there another time, then,
and will there be two equal times at once? Surely not. For a time that
is both equal and simultaneous is one and the same time, and even
those that are not simultaneous are one in kind; for if there were
dogs, and horses, and seven of each, it would be the same number.
So, too, movements that have simultaneous limits have the same time,
yet the one may in fact be fast and the other not, and one may be
locomotion and the other alteration; still the time of the two changes
is the same if their number also is equal and simultaneous; and for
this reason, while the movements are different and separate, the
time is everywhere the same, because the number of equal and
simultaneous movements is everywhere one and the same.
  Now there is such a thing as locomotion, and in locomotion there
is included circular movement, and everything is measured by some
one thing homogeneous with it, units by a unit, horses by a horse, and
similarly times by some definite time, and, as we said, time is
measured by motion as well as motion by time (this being so because by
a motion definite in time the quantity both of the motion and of the
time is measured): if, then, what is first is the measure of
everything homogeneous with it, regular circular motion is above all
else the measure, because the number of this is the best known. Now
neither alteration nor increase nor coming into being can be
regular, but locomotion can be. This also is why time is thought to be
the movement of the sphere, viz. because the other movements are
measured by this, and time by this movement.
  This also explains the common saying that human affairs form a
circle, and that there is a circle in all other things that have a
natural movement and coming into being and passing away. This is
because all other things are discriminated by time, and end and
begin as though conforming to a cycle; for even time itself is thought
to be a circle. And this opinion again is held because time is the
measure of this kind of locomotion and is itself measured by such.
So that to say that the things that come into being form a circle is
to say that there is a circle of time; and this is to say that it is
measured by the circular movement; for apart from the measure
nothing else to be measured is observed; the whole is just a plurality
of measures.
  It is said rightly, too, that the number of the sheep and of the
dogs is the same number if the two numbers are equal, but not the same
decad or the same ten; just as the equilateral and the scalene are not
the same triangle, yet they are the same figure, because they are both
triangles. For things are called the same so-and-so if they do not
differ by a differentia of that thing, but not if they do; e.g.
triangle differs from triangle by a differentia of triangle, therefore
they are different triangles; but they do not differ by a
differentia of figure, but are in one and the same division of it. For
a figure of the one kind is a circle and a figure of another kind of
triangle, and a triangle of one kind is equilateral and a triangle
of another kind scalene. They are the same figure, then, that,
triangle, but not the same triangle. Therefore the number of two
groups also-is the same number (for their number does not differ by
a differentia of number), but it is not the same decad; for the things
of which it is asserted differ; one group are dogs, and the other
  We have now discussed time-both time itself and the matters
appropriate to the consideration of it.
                              Book V

  EVERYTHING which changes does so in one of three senses. It may
change (1) accidentally, as for instance when we say that something
musical walks, that which walks being something in which aptitude
for music is an accident. Again (2) a thing is said without
qualification to change because something belonging to it changes,
i.e. in statements which refer to part of the thing in question:
thus the body is restored to health because the eye or the chest, that
is to say a part of the whole body, is restored to health. And above
all there is (3) the case of a thing which is in motion neither
accidentally nor in respect of something else belonging to it, but
in virtue of being itself directly in motion. Here we have a thing
which is essentially movable: and that which is so is a different
thing according to the particular variety of motion: for instance it
may be a thing capable of alteration: and within the sphere of
alteration it is again a different thing according as it is capable of
being restored to health or capable of being heated. And there are the
same distinctions in the case of the mover: (1) one thing causes
motion accidentally, (2) another partially (because something
belonging to it causes motion), (3) another of itself directly, as,
for instance, the physician heals, the hand strikes. We have, then,
the following factors: (a) on the one hand that which directly
causes motion, and (b) on the other hand that which is in motion:
further, we have (c) that in which motion takes place, namely time,
and (distinct from these three) (d) that from which and (e) that to
which it proceeds: for every motion proceeds from something and to
something, that which is directly in motion being distinct from that
to which it is in motion and that from which it is in motion: for
instance, we may take the three things 'wood', 'hot', and 'cold', of
which the first is that which is in motion, the second is that to
which the motion proceeds, and the third is that from which it
proceeds. This being so, it is clear that the motion is in the wood,
not in its form: for the motion is neither caused nor experienced by
the form or the place or the quantity. So we are left with a mover,
a moved, and a goal of motion. I do not include the starting-point
of motion: for it is the goal rather than the starting-point of motion
that gives its name to a particular process of change. Thus
'perishing' is change to not-being, though it is also true that that
that which perishes changes from being: and 'becoming' is change to
being, though it is also change from not-being.
  Now a definition of motion has been given above, from which it
will be seen that every goal of motion, whether it be a form, an
affection, or a place, is immovable, as, for instance, knowledge and
heat. Here, however, a difficulty may be raised. Affections, it may be
said, are motions, and whiteness is an affection: thus there may be
change to a motion. To this we may reply that it is not whiteness
but whitening that is a motion. Here also the same distinctions are to
be observed: a goal of motion may be so accidentally, or partially and
with reference to something other than itself, or directly and with no
reference to anything else: for instance, a thing which is becoming
white changes accidentally to an object of thought, the colour being
only accidentally the object of thought; it changes to colour, because
white is a part of colour, or to Europe, because Athens is a part of
Europe; but it changes essentially to white colour. It is now clear in
what sense a thing is in motion essentially, accidentally, or in
respect of something other than itself, and in what sense the phrase
'itself directly' is used in the case both of the mover and of the
moved: and it is also clear that the motion is not in the form but
in that which is in motion, that is to say 'the movable in
activity'. Now accidental change we may leave out of account: for it
is to be found in everything, at any time, and in any respect.
Change which is not accidental on the other hand is not to be found in
everything, but only in contraries, in things intermediate contraries,
and in contradictories, as may be proved by induction. An intermediate
may be a starting-point of change, since for the purposes of the
change it serves as contrary to either of two contraries: for the
intermediate is in a sense the extremes. Hence we speak of the
intermediate as in a sense a contrary relatively to the extremes and
of either extreme as a contrary relatively to the intermediate: for
instance, the central note is low relatively-to the highest and high
relatively to the lowest, and grey is light relatively to black and
dark relatively to white.
  And since every change is from something to something-as the word
itself (metabole) indicates, implying something 'after' (meta)
something else, that is to say something earlier and something
later-that which changes must change in one of four ways: from subject
to subject, from subject to nonsubject, from non-subject to subject,
or from non-subject to non-subject, where by 'subject' I mean what
is affirmatively expressed. So it follows necessarily from what has
been said above that there are only three kinds of change, that from
subject to subject, that from subject to non-subject, and that from
non-subject to subject: for the fourth conceivable kind, that from
non-subject to nonsubject, is not change, as in that case there is
no opposition either of contraries or of contradictories.
  Now change from non-subject to subject, the relation being that of
contradiction, is 'coming to be'-'unqualified coming to be' when the
change takes place in an unqualified way, 'particular coming to be'
when the change is change in a particular character: for instance, a
change from not-white to white is a coming to be of the particular
thing, white, while change from unqualified not-being to being is
coming to be in an unqualified way, in respect of which we say that
a thing 'comes to be' without qualification, not that it 'comes to be'
some particular thing. Change from subject to non-subject is
'perishing'-'unqualified perishing' when the change is from being to
not-being, 'particular perishing' when the change is to the opposite
negation, the distinction being the same as that made in the case of
coming to be.
  Now the expression 'not-being' is used in several senses: and
there can be motion neither of that which 'is not' in respect of the
affirmation or negation of a predicate, nor of that which 'is not'
in the sense that it only potentially 'is', that is to say the
opposite of that which actually 'is' in an unqualified sense: for
although that which is 'not-white' or 'not-good' may nevertheless he
in motion accidentally (for example that which is 'not-white' might be
a man), yet that which is without qualification 'not-so-and-so' cannot
in any sense be in motion: therefore it is impossible for that which
is not to be in motion. This being so, it follows that 'becoming'
cannot be a motion: for it is that which 'is not' that 'becomes'.
For however true it may be that it accidentally 'becomes', it is
nevertheless correct to say that it is that which 'is not' that in
an unqualified sense 'becomes'. And similarly it is impossible for
that which 'is not' to be at rest.
  There are these difficulties, then, in the way of the assumption
that that which 'is not' can be in motion: and it may be further
objected that, whereas everything which is in motion is in space, that
which 'is not' is not in space: for then it would be somewhere.
  So, too, 'perishing' is not a motion: for a motion has for its
contrary either another motion or rest, whereas 'perishing' is the
contrary of 'becoming'.
  Since, then, every motion is a kind of change, and there are only
the three kinds of change mentioned above, and since of these three
those which take the form of 'becoming' and 'perishing', that is to
say those which imply a relation of contradiction, are not motions: it
necessarily follows that only change from subject to subject is
motion. And every such subject is either a contrary or an intermediate
(for a privation may be allowed to rank as a contrary) and can be
affirmatively expressed, as naked, toothless, or black. If, then,
the categories are severally distinguished as Being, Quality, Place,
Time, Relation, Quantity, and Activity or Passivity, it necessarily
follows that there are three kinds of motion-qualitative,
quantitative, and local.

  In respect of Substance there is no motion, because Substance has no
contrary among things that are. Nor is there motion in respect of
Relation: for it may happen that when one correlative changes, the
other, although this does not itself change, is no longer
applicable, so that in these cases the motion is accidental. Nor is
there motion in respect of Agent and Patient-in fact there can never
be motion of mover and moved, because there cannot be motion of motion
or becoming of becoming or in general change of change.
  For in the first place there are two senses in which motion of
motion is conceivable. (1) The motion of which there is motion might
be conceived as subject; e.g. a man is in motion because he changes
from fair to dark. Can it be that in this sense motion grows hot or
cold, or changes place, or increases or decreases? Impossible: for
change is not a subject. Or (2) can there be motion of motion in the
sense that some other subject changes from a change to another mode of
being, as e.g. a man changes from falling ill to getting well? Even
this is possible only in an accidental sense. For, whatever the
subject may be, movement is change from one form to another. (And
the same holds good of becoming and perishing, except that in these
processes we have a change to a particular kind of opposite, while the
other, motion, is a change to a different kind.) So, if there is to be
motion of motion, that which is changing from health to sickness
must simultaneously be changing from this very change to another. It
is clear, then, that by the time that it has become sick, it must also
have changed to whatever may be the other change concerned (for that
it should be at rest, though logically possible, is excluded by the
theory). Moreover this other can never be any casual change, but
must be a change from something definite to some other definite thing.
So in this case it must be the opposite change, viz. convalescence. It
is only accidentally that there can be change of change, e.g. there is
a change from remembering to forgetting only because the subject of
this change changes at one time to knowledge, at another to ignorance.
  In the second place, if there is to be change of change and becoming
of becoming, we shall have an infinite regress. Thus if one of a
series of changes is to be a change of change, the preceding change
must also be so: e.g. if simple becoming was ever in process of
becoming, then that which was becoming simple becoming was also in
process of becoming, so that we should not yet have arrived at what
was in process of simple becoming but only at what was already in
process of becoming in process of becoming. And this again was
sometime in process of becoming, so that even then we should not
have arrived at what was in process of simple becoming. And since in
an infinite series there is no first term, here there will be no first
stage and therefore no following stage either. On this hypothesis,
then, nothing can become or be moved or change.
  Thirdly, if a thing is capable of any particular motion, it is
also capable of the corresponding contrary motion or the corresponding
coming to rest, and a thing that is capable of becoming is also
capable of perishing: consequently, if there be becoming of
becoming, that which is in process of becoming is in process of
perishing at the very moment when it has reached the stage of
becoming: since it cannot be in process of perishing when it is just
beginning to become or after it has ceased to become: for that which
is in process of perishing must be in existence.
  Fourthly, there must be a substrate underlying all processes of
becoming and changing. What can this be in the present case? It is
either the body or the soul that undergoes alteration: what is it that
correspondingly becomes motion or becoming? And again what is the goal
of their motion? It must be the motion or becoming of something from
something to something else. But in what sense can this be so? For the
becoming of learning cannot be learning: so neither can the becoming
of becoming be becoming, nor can the becoming of any process be that
  Finally, since there are three kinds of motion, the substratum and
the goal of motion must be one or other of these, e.g. locomotion will
have to be altered or to be locally moved.
  To sum up, then, since everything that is moved is moved in one of
three ways, either accidentally, or partially, or essentially,
change can change only accidentally, as e.g. when a man who is being
restored to health runs or learns: and accidental change we have
long ago decided to leave out of account.
  Since, then, motion can belong neither to Being nor to Relation
nor to Agent and Patient, it remains that there can be motion only
in respect of Quality, Quantity, and Place: for with each of these
we have a pair of contraries. Motion in respect of Quality let us call
alteration, a general designation that is used to include both
contraries: and by Quality I do not here mean a property of
substance (in that sense that which constitutes a specific distinction
is a quality) but a passive quality in virtue of which a thing is said
to be acted on or to be incapable of being acted on. Motion in respect
of Quantity has no name that includes both contraries, but it is
called increase or decrease according as one or the other is
designated: that is to say motion in the direction of complete
magnitude is increase, motion in the contrary direction is decrease.
Motion in respect of Place has no name either general or particular:
but we may designate it by the general name of locomotion, though
strictly the term 'locomotion' is applicable to things that change
their place only when they have not the power to come to a stand,
and to things that do not move themselves locally.
  Change within the same kind from a lesser to a greater or from a
greater to a lesser degree is alteration: for it is motion either from
a contrary or to a contrary, whether in an unqualified or in a
qualified sense: for change to a lesser degree of a quality will be
called change to the contrary of that quality, and change to a greater
degree of a quality will be regarded as change from the contrary of
that quality to the quality itself. It makes no difference whether the
change be qualified or unqualified, except that in the former case the
contraries will have to be contrary to one another only in a qualified
sense: and a thing's possessing a quality in a greater or in a
lesser degree means the presence or absence in it of more or less of
the opposite quality. It is now clear, then, that there are only these
three kinds of motion.
  The term 'immovable' we apply in the first place to that which is
absolutely incapable of being moved (just as we correspondingly
apply the term invisible to sound); in the second place to that
which is moved with difficulty after a long time or whose movement
is slow at the start-in fact, what we describe as hard to move; and in
the third place to that which is naturally designed for and capable of
motion, but is not in motion when, where, and as it naturally would be
so. This last is the only kind of immovable thing of which I use the
term 'being at rest': for rest is contrary to motion, so that rest
will be negation of motion in that which is capable of admitting
  The foregoing remarks are sufficient to explain the essential nature
of motion and rest, the number of kinds of change, and the different
varieties of motion.

  Let us now proceed to define the terms 'together' and 'apart', 'in
contact', 'between', 'in succession', 'contiguous', and
'continuous', and to show in what circumstances each of these terms is
naturally applicable.
  Things are said to be together in place when they are in one place
(in the strictest sense of the word 'place') and to be apart when they
are in different places.
  Things are said to be in contact when their extremities are
  That which a changing thing, if it changes continuously in a natural
manner, naturally reaches before it reaches that to which it changes
last, is between. Thus 'between' implies the presence of at least
three things: for in a process of change it is the contrary that is
'last': and a thing is moved continuously if it leaves no gap or
only the smallest possible gap in the material-not in the time (for
a gap in the time does not prevent things having a 'between', while,
on the other hand, there is nothing to prevent the highest note
sounding immediately after the lowest) but in the material in which
the motion takes place. This is manifestly true not only in local
changes but in every other kind as well. (Now every change implies a
pair of opposites, and opposites may be either contraries or
contradictories; since then contradiction admits of no mean term, it
is obvious that 'between' must imply a pair of contraries) That is
locally contrary which is most distant in a straight line: for the
shortest line is definitely limited, and that which is definitely
limited constitutes a measure.
  A thing is 'in succession' when it is after the beginning in
position or in form or in some other respect in which it is definitely
so regarded, and when further there is nothing of the same kind as
itself between it and that to which it is in succession, e.g. a line
or lines if it is a line, a unit or units if it is a unit, a house
if it is a house (there is nothing to prevent something of a different
kind being between). For that which is in succession is in
succession to a particular thing, and is something posterior: for
one is not 'in succession' to two, nor is the first day of the month
to be second: in each case the latter is 'in succession' to the
  A thing that is in succession and touches is 'contiguous'. The
'continuous' is a subdivision of the contiguous: things are called
continuous when the touching limits of each become one and the same
and are, as the word implies, contained in each other: continuity is
impossible if these extremities are two. This definition makes it
plain that continuity belongs to things that naturally in virtue of
their mutual contact form a unity. And in whatever way that which
holds them together is one, so too will the whole be one, e.g. by a
rivet or glue or contact or organic union.
  It is obvious that of these terms 'in succession' is first in
order of analysis: for that which touches is necessarily in
succession, but not everything that is in succession touches: and so
succession is a property of things prior in definition, e.g.
numbers, while contact is not. And if there is continuity there is
necessarily contact, but if there is contact, that alone does not
imply continuity: for the extremities of things may be 'together'
without necessarily being one: but they cannot be one without being
necessarily together. So natural junction is last in coming to be: for
the extremities must necessarily come into contact if they are to be
naturally joined: but things that are in contact are not all naturally
joined, while there is no contact clearly there is no natural junction
either. Hence, if as some say 'point' and 'unit' have an independent
existence of their own, it is impossible for the two to be
identical: for points can touch while units can only be in succession.
Moreover, there can always be something between points (for all
lines are intermediate between points), whereas it is not necessary
that there should possibly be anything between units: for there can be
nothing between the numbers one and two.
  We have now defined what is meant by 'together' and 'apart',
'contact', 'between' and 'in succession', 'contiguous' and
'continuous': and we have shown in what circumstances each of these
terms is applicable.

  There are many senses in which motion is said to be 'one': for we
use the term 'one' in many senses.
  Motion is one generically according to the different categories to
which it may be assigned: thus any locomotion is one generically
with any other locomotion, whereas alteration is different generically
from locomotion.
  Motion is one specifically when besides being one generically it
also takes place in a species incapable of subdivision: e.g. colour
has specific differences: therefore blackening and whitening differ
specifically; but at all events every whitening will be specifically
the same with every other whitening and every blackening with every
other blackening. But white is not further subdivided by specific
differences: hence any whitening is specifically one with any other
whitening. Where it happens that the genus is at the same time a
species, it is clear that the motion will then in a sense be one
specifically though not in an unqualified sense: learning is an
example of this, knowledge being on the one hand a species of
apprehension and on the other hand a genus including the various
knowledges. A difficulty, however, may be raised as to whether a
motion is specifically one when the same thing changes from the same
to the same, e.g. when one point changes again and again from a
particular place to a particular place: if this motion is specifically
one, circular motion will be the same as rectilinear motion, and
rolling the same as walking. But is not this difficulty removed by the
principle already laid down that if that in which the motion takes
place is specifically different (as in the present instance the
circular path is specifically different from the straight) the
motion itself is also different? We have explained, then, what is
meant by saying that motion is one generically or one specifically.
  Motion is one in an unqualified sense when it is one essentially
or numerically: and the following distinctions will make clear what
this kind of motion is. There are three classes of things in connexion
with which we speak of motion, the 'that which', the 'that in
which', and the 'that during which'. I mean that there must he
something that is in motion, e.g. a man or gold, and it must be in
motion in something, e.g. a place or an affection, and during
something, for all motion takes place during a time. Of these three it
is the thing in which the motion takes place that makes it one
generically or specifically, it is the thing moved that makes the
motion one in subject, and it is the time that makes it consecutive:
but it is the three together that make it one without qualification:
to effect this, that in which the motion takes place (the species)
must be one and incapable of subdivision, that during which it takes
place (the time) must be one and unintermittent, and that which is
in motion must be one-not in an accidental sense (i.e. it must be
one as the white that blackens is one or Coriscus who walks is one,
not in the accidental sense in which Coriscus and white may be one),
nor merely in virtue of community of nature (for there might be a case
of two men being restored to health at the same time in the same
way, e.g. from inflammation of the eye, yet this motion is not
really one, but only specifically one).
  Suppose, however, that Socrates undergoes an alteration specifically
the same but at one time and again at another: in this case if it is
possible for that which ceased to be again to come into being and
remain numerically the same, then this motion too will be one:
otherwise it will be the same but not one. And akin to this difficulty
there is another; viz. is health one? and generally are the states and
affections in bodies severally one in essence although (as is clear)
the things that contain them are obviously in motion and in flux? Thus
if a person's health at daybreak and at the present moment is one
and the same, why should not this health be numerically one with
that which he recovers after an interval? The same argument applies in
each case. There is, however, we may answer, this difference: that
if the states are two then it follows simply from this fact that the
activities must also in point of number be two (for only that which is
numerically one can give rise to an activity that is numerically one),
but if the state is one, this is not in itself enough to make us
regard the activity also as one: for when a man ceases walking, the
walking no longer is, but it will again be if he begins to walk again.
But, be this as it may, if in the above instance the health is one and
the same, then it must be possible for that which is one and the
same to come to be and to cease to be many times. However, these
difficulties lie outside our present inquiry.
  Since every motion is continuous, a motion that is one in an
unqualified sense must (since every motion is divisible) be
continuous, and a continuous motion must be one. There will not be
continuity between any motion and any other indiscriminately any
more than there is between any two things chosen at random in any
other sphere: there can be continuity only when the extremities of the
two things are one. Now some things have no extremities at all: and
the extremities of others differ specifically although we give them
the same name of 'end': how should e.g. the 'end' of a line and the
'end' of walking touch or come to be one? Motions that are not the
same either specifically or generically may, it is true, be
consecutive (e.g. a man may run and then at once fall ill of a fever),
and again, in the torch-race we have consecutive but not continuous
locomotion: for according to our definition there can be continuity
only when the ends of the two things are one. Hence motions may be
consecutive or successive in virtue of the time being continuous,
but there can be continuity only in virtue of the motions themselves
being continuous, that is when the end of each is one with the end
of the other. Motion, therefore, that is in an unqualified sense
continuous and one must be specifically the same, of one thing, and in
one time. Unity is required in respect of time in order that there may
be no interval of immobility, for where there is intermission of
motion there must be rest, and a motion that includes intervals of
rest will be not one but many, so that a motion that is interrupted by
stationariness is not one or continuous, and it is so interrupted if
there is an interval of time. And though of a motion that is not
specifically one (even if the time is unintermittent) the time is one,
the motion is specifically different, and so cannot really be one, for
motion that is one must be specifically one, though motion that is
specifically one is not necessarily one in an unqualified sense. We
have now explained what we mean when we call a motion one without
  Further, a motion is also said to be one generically,
specifically, or essentially when it is complete, just as in other
cases completeness and wholeness are characteristics of what is one:
and sometimes a motion even if incomplete is said to be one,
provided only that it is continuous.
  And besides the cases already mentioned there is another in which
a motion is said to be one, viz. when it is regular: for in a sense
a motion that is irregular is not regarded as one, that title
belonging rather to that which is regular, as a straight line is
regular, the irregular being as such divisible. But the difference
would seem to be one of degree. In every kind of motion we may have
regularity or irregularity: thus there may be regular alteration,
and locomotion in a regular path, e.g. in a circle or on a straight
line, and it is the same with regard to increase and decrease. The
difference that makes a motion irregular is sometimes to be found in
its path: thus a motion cannot be regular if its path is an
irregular magnitude, e.g. a broken line, a spiral, or any other
magnitude that is not such that any part of it taken at random fits on
to any other that may be chosen. Sometimes it is found neither in
the place nor in the time nor in the goal but in the manner of the
motion: for in some cases the motion is differentiated by quickness
and slowness: thus if its velocity is uniform a motion is regular,
if not it is irregular. So quickness and slowness are not species of
motion nor do they constitute specific differences of motion,
because this distinction occurs in connexion with all the distinct
species of motion. The same is true of heaviness and lightness when
they refer to the same thing: e.g. they do not specifically
distinguish earth from itself or fire from itself. Irregular motion,
therefore, while in virtue of being continuous it is one, is so in a
lesser degree, as is the case with locomotion in a broken line: and
a lesser degree of something always means an admixture of its
contrary. And since every motion that is one can be both regular and
irregular, motions that are consecutive but not specifically the
same cannot be one and continuous: for how should a motion composed of
alteration and locomotion be regular? If a motion is to be regular its
parts ought to fit one another.

  We have further to determine what motions are contrary to each
other, and to determine similarly how it is with rest. And we have
first to decide whether contrary motions are motions respectively from
and to the same thing, e.g. a motion from health and a motion to
health (where the opposition, it would seem, is of the same kind as
that between coming to be and ceasing to be); or motions
respectively from contraries, e.g. a motion from health and a motion
from disease; or motions respectively to contraries, e.g. a motion
to health and a motion to disease; or motions respectively from a
contrary and to the opposite contrary, e.g. a motion from health and a
motion to disease; or motions respectively from a contrary to the
opposite contrary and from the latter to the former, e.g. a motion
from health to disease and a motion from disease to health: for
motions must be contrary to one another in one or more of these
ways, as there is no other way in which they can be opposed.
  Now motions respectively from a contrary and to the opposite
contrary, e.g. a motion from health and a motion to disease, are not
contrary motions: for they are one and the same. (Yet their essence is
not the same, just as changing from health is different from
changing to disease.) Nor are motion respectively from a contrary
and from the opposite contrary contrary motions, for a motion from a
contrary is at the same time a motion to a contrary or to an
intermediate (of this, however, we shall speak later), but changing to
a contrary rather than changing from a contrary would seem to be the
cause of the contrariety of motions, the latter being the loss, the
former the gain, of contrariness. Moreover, each several motion
takes its name rather from the goal than from the starting-point of
change, e.g. motion to health we call convalescence, motion to disease
sickening. Thus we are left with motions respectively to contraries,
and motions respectively to contraries from the opposite contraries.
Now it would seem that motions to contraries are at the same time
motions from contraries (though their essence may not be the same; 'to
health' is distinct, I mean, from 'from disease', and 'from health'
from 'to disease').
  Since then change differs from motion (motion being change from a
particular subject to a particular subject), it follows that
contrary motions are motions respectively from a contrary to the
opposite contrary and from the latter to the former, e.g. a motion
from health to disease and a motion from disease to health.
Moreover, the consideration of particular examples will also show what
kinds of processes are generally recognized as contrary: thus
falling ill is regarded as contrary to recovering one's health,
these processes having contrary goals, and being taught as contrary to
being led into error by another, it being possible to acquire error,
like knowledge, either by one's own agency or by that of another.
Similarly we have upward locomotion and downward locomotion, which are
contrary lengthwise, locomotion to the right and locomotion to the
left, which are contrary breadthwise, and forward locomotion and
backward locomotion, which too are contraries. On the other hand, a
process simply to a contrary, e.g. that denoted by the expression
'becoming white', where no starting-point is specified, is a change
but not a motion. And in all cases of a thing that has no contrary
we have as contraries change from and change to the same thing. Thus
coming to be is contrary to ceasing to be, and losing to gaining.
But these are changes and not motions. And wherever a pair of
contraries admit of an intermediate, motions to that intermediate must
be held to be in a sense motions to one or other of the contraries:
for the intermediate serves as a contrary for the purposes of the
motion, in whichever direction the change may be, e.g. grey in a
motion from grey to white takes the place of black as
starting-point, in a motion from white to grey it takes the place of
black as goal, and in a motion from black to grey it takes the place
of white as goal: for the middle is opposed in a sense to either of
the extremes, as has been said above. Thus we see that two motions are
contrary to each other only when one is a motion from a contrary to
the opposite contrary and the other is a motion from the latter to the

  But since a motion appears to have contrary to it not only another
motion but also a state of rest, we must determine how this is so. A
motion has for its contrary in the strict sense of the term another
motion, but it also has for an opposite a state of rest (for rest is
the privation of motion and the privation of anything may be called
its contrary), and motion of one kind has for its opposite rest of
that kind, e.g. local motion has local rest. This statement,
however, needs further qualification: there remains the question, is
the opposite of remaining at a particular place motion from or
motion to that place? It is surely clear that since there are two
subjects between which motion takes place, motion from one of these
(A) to its contrary (B) has for its opposite remaining in A while
the reverse motion has for its opposite remaining in B. At the same
time these two are also contrary to each other: for it would be absurd
to suppose that there are contrary motions and not opposite states
of rest. States of rest in contraries are opposed. To take an example,
a state of rest in health is (1) contrary to a state of rest in
disease, and (2) the motion to which it is contrary is that from
health to disease. For (2) it would be absurd that its contrary motion
should be that from disease to health, since motion to that in which a
thing is at rest is rather a coming to rest, the coming to rest
being found to come into being simultaneously with the motion; and one
of these two motions it must be. And (1) rest in whiteness is of
course not contrary to rest in health.
  Of all things that have no contraries there are opposite changes
(viz. change from the thing and change to the thing, e.g. change
from being and change to being), but no motion. So, too, of such
things there is no remaining though there is absence of change. Should
there be a particular subject, absence of change in its being will
be contrary to absence of change in its not-being. And here a
difficulty may be raised: if not-being is not a particular
something, what is it, it may be asked, that is contrary to absence of
change in a thing's being? and is this absence of change a state of
rest? If it is, then either it is not true that every state of rest is
contrary to a motion or else coming to be and ceasing to be are
motion. It is clear then that, since we exclude these from among
motions, we must not say that this absence of change is a state of
rest: we must say that it is similar to a state of rest and call it
absence of change. And it will have for its contrary either nothing or
absence of change in the thing's not-being, or the ceasing to be of
the thing: for such ceasing to be is change from it and the thing's
coming to be is change to it.
  Again, a further difficulty may be raised. How is it, it may be
asked, that whereas in local change both remaining and moving may be
natural or unnatural, in the other changes this is not so? e.g.
alteration is not now natural and now unnatural, for convalescence
is no more natural or unnatural than falling ill, whitening no more
natural or unnatural than blackening; so, too, with increase and
decrease: these are not contrary to each other in the sense that
either of them is natural while the other is unnatural, nor is one
increase contrary to another in this sense; and the same account may
be given of becoming and perishing: it is not true that becoming is
natural and perishing unnatural (for growing old is natural), nor do
we observe one becoming to be natural and another unnatural. We answer
that if what happens under violence is unnatural, then violent
perishing is unnatural and as such contrary to natural perishing.
Are there then also some becomings that are violent and not the result
of natural necessity, and are therefore contrary to natural becomings,
and violent increases and decreases, e.g. the rapid growth to maturity
of profligates and the rapid ripening of seeds even when not packed
close in the earth? And how is it with alterations? Surely just the
same: we may say that some alterations are violent while others are
natural, e.g. patients alter naturally or unnaturally according as
they throw off fevers on the critical days or not. But, it may be
objected, then we shall have perishings contrary to one another, not
to becoming. Certainly: and why should not this in a sense be so? Thus
it is so if one perishing is pleasant and another painful: and so
one perishing will be contrary to another not in an unqualified sense,
but in so far as one has this quality and the other that.
  Now motions and states of rest universally exhibit contrariety in
the manner described above, e.g. upward motion and rest above are
respectively contrary to downward motion and rest below, these being
instances of local contrariety; and upward locomotion belongs
naturally to fire and downward to earth, i.e. the locomotions of the
two are contrary to each other. And again, fire moves up naturally and
down unnaturally: and its natural motion is certainly contrary to
its unnatural motion. Similarly with remaining: remaining above is
contrary to motion from above downwards, and to earth this remaining
comes unnaturally, this motion naturally. So the unnatural remaining
of a thing is contrary to its natural motion, just as we find a
similar contrariety in the motion of the same thing: one of its
motions, the upward or the downward, will be natural, the other
  Here, however, the question arises, has every state of rest that
is not permanent a becoming, and is this becoming a coming to a
standstill? If so, there must be a becoming of that which is at rest
unnaturally, e.g. of earth at rest above: and therefore this earth
during the time that it was being carried violently upward was
coming to a standstill. But whereas the velocity of that which comes
to a standstill seems always to increase, the velocity of that which
is carried violently seems always to decrease: so it will he in a
state of rest without having become so. Moreover 'coming to a
standstill' is generally recognized to be identical or at least
concomitant with the locomotion of a thing to its proper place.
  There is also another difficulty involved in the view that remaining
in a particular place is contrary to motion from that place. For
when a thing is moving from or discarding something, it still
appears to have that which is being discarded, so that if a state of
rest is itself contrary to the motion from the state of rest to its
contrary, the contraries rest and motion will be simultaneously
predicable of the same thing. May we not say, however, that in so
far as the thing is still stationary it is in a state of rest in a
qualified sense? For, in fact, whenever a thing is in motion, part
of it is at the starting-point while part is at the goal to which it
is changing: and consequently a motion finds its true contrary
rather in another motion than in a state of rest.
  With regard to motion and rest, then, we have now explained in
what sense each of them is one and under what conditions they
exhibit contrariety.
  [With regard to coming to a standstill the question may be raised
whether there is an opposite state of rest to unnatural as well as
to natural motions. It would be absurd if this were not the case:
for a thing may remain still merely under violence: thus we shall have
a thing being in a non-permanent state of rest without having become
so. But it is clear that it must be the case: for just as there is
unnatural motion, so, too, a thing may be in an unnatural state of
rest. Further, some things have a natural and an unnatural motion,
e.g. fire has a natural upward motion and an unnatural downward
motion: is it, then, this unnatural downward motion or is it the
natural downward motion of earth that is contrary to the natural
upward motion? Surely it is clear that both are contrary to it
though not in the same sense: the natural motion of earth is
contrary inasmuch as the motion of fire is also natural, whereas the
upward motion of fire as being natural is contrary to the downward
motion of fire as being unnatural. The same is true of the
corresponding cases of remaining. But there would seem to be a sense
in which a state of rest and a motion are opposites.]
                              Book VI

  Now if the terms 'continuous', 'in contact', and 'in succession' are
understood as defined above things being 'continuous' if their
extremities are one, 'in contact' if their extremities are together,
and 'in succession' if there is nothing of their own kind intermediate
between them-nothing that is continuous can be composed 'of
indivisibles': e.g. a line cannot be composed of points, the line
being continuous and the point indivisible. For the extremities of two
points can neither be one (since of an indivisible there can be no
extremity as distinct from some other part) nor together (since that
which has no parts can have no extremity, the extremity and the
thing of which it is the extremity being distinct).
  Moreover, if that which is continuous is composed of points, these
points must be either continuous or in contact with one another: and
the same reasoning applies in the case of all indivisibles. Now for
the reason given above they cannot be continuous: and one thing can be
in contact with another only if whole is in contact with whole or part
with part or part with whole. But since indivisibles have no parts,
they must be in contact with one another as whole with whole. And if
they are in contact with one another as whole with whole, they will
not be continuous: for that which is continuous has distinct parts:
and these parts into which it is divisible are different in this
way, i.e. spatially separate.
  Nor, again, can a point be in succession to a point or a moment to a
moment in such a way that length can be composed of points or time
of moments: for things are in succession if there is nothing of
their own kind intermediate between them, whereas that which is
intermediate between points is always a line and that which is
intermediate between moments is always a period of time.
  Again, if length and time could thus be composed of indivisibles,
they could be divided into indivisibles, since each is divisible
into the parts of which it is composed. But, as we saw, no
continuous thing is divisible into things without parts. Nor can there
be anything of any other kind intermediate between the parts or
between the moments: for if there could be any such thing it is
clear that it must be either indivisible or divisible, and if it is
divisible, it must be divisible either into indivisibles or into
divisibles that are infinitely divisible, in which case it is
  Moreover, it is plain that everything continuous is divisible into
divisibles that are infinitely divisible: for if it were divisible
into indivisibles, we should have an indivisible in contact with an
indivisible, since the extremities of things that are continuous
with one another are one and are in contact.
  The same reasoning applies equally to magnitude, to time, and to
motion: either all of these are composed of indivisibles and are
divisible into indivisibles, or none. This may be made clear as
follows. If a magnitude is composed of indivisibles, the motion over
that magnitude must be composed of corresponding indivisible
motions: e.g. if the magnitude ABG is composed of the indivisibles
A, B, G, each corresponding part of the motion DEZ of O over ABG is
indivisible. Therefore, since where there is motion there must be
something that is in motion, and where there is something in motion
there must be motion, therefore the being-moved will also be
composed of indivisibles. So O traversed A when its motion was D, B
when its motion was E, and G similarly when its motion was Z. Now a
thing that is in motion from one place to another cannot at the moment
when it was in motion both be in motion and at the same time have
completed its motion at the place to which it was in motion: e.g. if a
man is walking to Thebes, he cannot be walking to Thebes and at the
same time have completed his walk to Thebes: and, as we saw, O
traverses a the partless section A in virtue of the presence of the
motion D. Consequently, if O actually passed through A after being
in process of passing through, the motion must be divisible: for at
the time when O was passing through, it neither was at rest nor had
completed its passage but was in an intermediate state: while if it is
passing through and has completed its passage at the same moment, then
that which is walking will at the moment when it is walking have
completed its walk and will be in the place to which it is walking;
that is to say, it will have completed its motion at the place to
which it is in motion. And if a thing is in motion over the whole
KBG and its motion is the three D, E, and Z, and if it is not in
motion at all over the partless section A but has completed its motion
over it, then the motion will consist not of motions but of starts,
and will take place by a thing's having completed a motion without
being in motion: for on this assumption it has completed its passage
through A without passing through it. So it will be possible for a
thing to have completed a walk without ever walking: for on this
assumption it has completed a walk over a particular distance
without walking over that distance. Since, then, everything must be
either at rest or in motion, and O is therefore at rest in each of the
sections A, B, and G, it follows that a thing can be continuously at
rest and at the same time in motion: for, as we saw, O is in motion
over the whole ABG and at rest in any part (and consequently in the
whole) of it. Moreover, if the indivisibles composing DEZ are motions,
it would be possible for a thing in spite of the presence in it of
motion to be not in motion but at rest, while if they are not motions,
it would be possible for motion to be composed of something other than
  And if length and motion are thus indivisible, it is neither more
nor less necessary that time also be similarly indivisible, that is to
say be composed of indivisible moments: for if the whole distance is
divisible and an equal velocity will cause a thing to pass through
less of it in less time, the time must also be divisible, and
conversely, if the time in which a thing is carried over the section A
is divisible, this section A must also be divisible.

  And since every magnitude is divisible into magnitudes-for we have
shown that it is impossible for anything continuous to be composed
of indivisible parts, and every magnitude is continuous-it necessarily
follows that the quicker of two things traverses a greater magnitude
in an equal time, an equal magnitude in less time, and a greater
magnitude in less time, in conformity with the definition sometimes
given of 'the quicker'. Suppose that A is quicker than B. Now since of
two things that which changes sooner is quicker, in the time ZH, in
which A has changed from G to D, B will not yet have arrived at D
but will be short of it: so that in an equal time the quicker will
pass over a greater magnitude. More than this, it will pass over a
greater magnitude in less time: for in the time in which A has arrived
at D, B being the slower has arrived, let us say, at E. Then since A
has occupied the whole time ZH in arriving at D, will have arrived
at O in less time than this, say ZK. Now the magnitude GO that A has
passed over is greater than the magnitude GE, and the time ZK is
less than the whole time ZH: so that the quicker will pass over a
greater magnitude in less time. And from this it is also clear that
the quicker will pass over an equal magnitude in less time than the
slower. For since it passes over the greater magnitude in less time
than the slower, and (regarded by itself) passes over LM the greater
in more time than LX the lesser, the time PRh in which it passes
over LM will be more than the time PS, which it passes over LX: so
that, the time PRh being less than the time PCh in which the slower
passes over LX, the time PS will also be less than the time PX: for it
is less than the time PRh, and that which is less than something
else that is less than a thing is also itself less than that thing.
Hence it follows that the quicker will traverse an equal magnitude
in less time than the slower. Again, since the motion of anything must
always occupy either an equal time or less or more time in
comparison with that of another thing, and since, whereas a thing is
slower if its motion occupies more time and of equal velocity if its
motion occupies an equal time, the quicker is neither of equal
velocity nor slower, it follows that the motion of the quicker can
occupy neither an equal time nor more time. It can only be, then, that
it occupies less time, and thus we get the necessary consequence
that the quicker will pass over an equal magnitude (as well as a
greater) in less time than the slower.
  And since every motion is in time and a motion may occupy any
time, and the motion of everything that is in motion may be either
quicker or slower, both quicker motion and slower motion may occupy
any time: and this being so, it necessarily follows that time also
is continuous. By continuous I mean that which is divisible into
divisibles that are infinitely divisible: and if we take this as the
definition of continuous, it follows necessarily that time is
continuous. For since it has been shown that the quicker will pass
over an equal magnitude in less time than the slower, suppose that A
is quicker and B slower, and that the slower has traversed the
magnitude GD in the time ZH. Now it is clear that the quicker will
traverse the same magnitude in less time than this: let us say in
the time ZO. Again, since the quicker has passed over the whole D in
the time ZO, the slower will in the same time pass over GK, say, which
is less than GD. And since B, the slower, has passed over GK in the
time ZO, the quicker will pass over it in less time: so that the
time ZO will again be divided. And if this is divided the magnitude GK
will also be divided just as GD was: and again, if the magnitude is
divided, the time will also be divided. And we can carry on this
process for ever, taking the slower after the quicker and the
quicker after the slower alternately, and using what has been
demonstrated at each stage as a new point of departure: for the
quicker will divide the time and the slower will divide the length.
If, then, this alternation always holds good, and at every turn
involves a division, it is evident that all time must be continuous.
And at the same time it is clear that all magnitude is also
continuous; for the divisions of which time and magnitude respectively
are susceptible are the same and equal.
  Moreover, the current popular arguments make it plain that, if
time is continuous, magnitude is continuous also, inasmuch as a
thing asses over half a given magnitude in half the time taken to
cover the whole: in fact without qualification it passes over a less
magnitude in less time; for the divisions of time and of magnitude
will be the same. And if either is infinite, so is the other, and
the one is so in the same way as the other; i.e. if time is infinite
in respect of its extremities, length is also infinite in respect of
its extremities: if time is infinite in respect of divisibility,
length is also infinite in respect of divisibility: and if time is
infinite in both respects, magnitude is also infinite in both
  Hence Zeno's argument makes a false assumption in asserting that
it is impossible for a thing to pass over or severally to come in
contact with infinite things in a finite time. For there are two
senses in which length and time and generally anything continuous
are called 'infinite': they are called so either in respect of
divisibility or in respect of their extremities. So while a thing in a
finite time cannot come in contact with things quantitatively
infinite, it can come in contact with things infinite in respect of
divisibility: for in this sense the time itself is also infinite:
and so we find that the time occupied by the passage over the infinite
is not a finite but an infinite time, and the contact with the
infinites is made by means of moments not finite but infinite in
  The passage over the infinite, then, cannot occupy a finite time,
and the passage over the finite cannot occupy an infinite time: if the
time is infinite the magnitude must be infinite also, and if the
magnitude is infinite, so also is the time. This may be shown as
follows. Let AB be a finite magnitude, and let us suppose that it is
traversed in infinite time G, and let a finite period GD of the time
be taken. Now in this period the thing in motion will pass over a
certain segment of the magnitude: let BE be the segment that it has
thus passed over. (This will be either an exact measure of AB or
less or greater than an exact measure: it makes no difference which it
is.) Then, since a magnitude equal to BE will always be passed over in
an equal time, and BE measures the whole magnitude, the whole time
occupied in passing over AB will be finite: for it will be divisible
into periods equal in number to the segments into which the
magnitude is divisible. Moreover, if it is the case that infinite time
is not occupied in passing over every magnitude, but it is possible to
ass over some magnitude, say BE, in a finite time, and if this BE
measures the whole of which it is a part, and if an equal magnitude is
passed over in an equal time, then it follows that the time like the
magnitude is finite. That infinite time will not be occupied in
passing over BE is evident if the time be taken as limited in one
direction: for as the part will be passed over in less time than the
whole, the time occupied in traversing this part must be finite, the
limit in one direction being given. The same reasoning will also
show the falsity of the assumption that infinite length can be
traversed in a finite time. It is evident, then, from what has been
said that neither a line nor a surface nor in fact anything continuous
can be indivisible.
  This conclusion follows not only from the present argument but
from the consideration that the opposite assumption implies the
divisibility of the indivisible. For since the distinction of
quicker and slower may apply to motions occupying any period of time
and in an equal time the quicker passes over a greater length, it
may happen that it will pass over a length twice, or one and a half
times, as great as that passed over by the slower: for their
respective velocities may stand to one another in this proportion.
Suppose, then, that the quicker has in the same time been carried over
a length one and a half times as great as that traversed by the
slower, and that the respective magnitudes are divided, that of the
quicker, the magnitude ABGD, into three indivisibles, and that of
the slower into the two indivisibles EZ, ZH. Then the time may also be
divided into three indivisibles, for an equal magnitude will be passed
over in an equal time. Suppose then that it is thus divided into KL,
LM, MN. Again, since in the same time the slower has been carried over
EZ, ZH, the time may also be similarly divided into two. Thus the
indivisible will be divisible, and that which has no parts will be
passed over not in an indivisible but in a greater time. It is
evident, therefore, that nothing continuous is without parts.

  The present also is necessarily indivisible-the present, that is,
not in the sense in which the word is applied to one thing in virtue
of another, but in its proper and primary sense; in which sense it
is inherent in all time. For the present is something that is an
extremity of the past (no part of the future being on this side of it)
and also of the future (no part of the past being on the other side of
it): it is, as we have said, a limit of both. And if it is once
shown that it is essentially of this character and one and the same,
it will at once be evident also that it is indivisible.
  Now the present that is the extremity of both times must be one
and the same: for if each extremity were different, the one could
not be in succession to the other, because nothing continuous can be
composed of things having no parts: and if the one is apart from the
other, there will be time intermediate between them, because
everything continuous is such that there is something intermediate
between its limits and described by the same name as itself. But if
the intermediate thing is time, it will be divisible: for all time has
been shown to be divisible. Thus on this assumption the present is
divisible. But if the present is divisible, there will be part of
the past in the future and part of the future in the past: for past
time will be marked off from future time at the actual point of
division. Also the present will be a present not in the proper sense
but in virtue of something else: for the division which yields it will
not be a division proper. Furthermore, there will be a part of the
present that is past and a part that is future, and it will not always
be the same part that is past or future: in fact one and the same
present will not be simultaneous: for the time may be divided at
many points. If, therefore, the present cannot possibly have these
characteristics, it follows that it must be the same present that
belongs to each of the two times. But if this is so it is evident that
the present is also indivisible: for if it is divisible it will be
involved in the same implications as before. It is clear, then, from
what has been said that time contains something indivisible, and
this is what we call a present.
  We will now show that nothing can be in motion in a present. For
if this is possible, there can be both quicker and slower motion in
the present. Suppose then that in the present N the quicker has
traversed the distance AB. That being so, the slower will in the
same present traverse a distance less than AB, say AG. But since the
slower will have occupied the whole present in traversing AG, the
quicker will occupy less than this in traversing it. Thus we shall
have a division of the present, whereas we found it to be indivisible.
It is impossible, therefore, for anything to be in motion in a
  Nor can anything be at rest in a present: for, as we were saying,
only can be at rest which is naturally designed to be in motion but is
not in motion when, where, or as it would naturally be so: since,
therefore, nothing is naturally designed to be in motion in a present,
it is clear that nothing can be at rest in a present either.
  Moreover, inasmuch as it is the same present that belongs to both
the times, and it is possible for a thing to be in motion throughout
one time and to be at rest throughout the other, and that which is
in motion or at rest for the whole of a time will be in motion or at
rest as the case may be in any part of it in which it is naturally
designed to be in motion or at rest: this being so, the assumption
that there can be motion or rest in a present will carry with it the
implication that the same thing can at the same time be at rest and in
motion: for both the times have the same extremity, viz. the present.
  Again, when we say that a thing is at rest, we imply that its
condition in whole and in part is at the time of speaking uniform with
what it was previously: but the present contains no 'previously':
consequently, there can be no rest in it.
  It follows then that the motion of that which is in motion and the
rest of that which is at rest must occupy time.

  Further, everything that changes must be divisible. For since
every change is from something to something, and when a thing is at
the goal of its change it is no longer changing, and when both it
itself and all its parts are at the starting-point of its change it is
not changing (for that which is in whole and in part in an unvarying
condition is not in a state of change); it follows, therefore, that
part of that which is changing must be at the starting-point and
part at the goal: for as a whole it cannot be in both or in neither.
(Here by 'goal of change' I mean that which comes first in the process
of change: e.g. in a process of change from white the goal in question
will be grey, not black: for it is not necessary that that that
which is changing should be at either of the extremes.) It is evident,
therefore, that everything that changes must be divisible.
  Now motion is divisible in two senses. In the first place it is
divisible in virtue of the time that it occupies. In the second
place it is divisible according to the motions of the several parts of
that which is in motion: e.g. if the whole AG is in motion, there will
be a motion of AB and a motion of BG. That being so, let DE be the
motion of the part AB and EZ the motion of the part BG. Then the whole
DZ must be the motion of AG: for DZ must constitute the motion of AG
inasmuch as DE and EZ severally constitute the motions of each of
its parts. But the motion of a thing can never be constituted by the
motion of something else: consequently the whole motion is the
motion of the whole magnitude.
  Again, since every motion is a motion of something, and the whole
motion DZ is not the motion of either of the parts (for each of the
parts DE, EZ is the motion of one of the parts AB, BG) or of
anything else (for, the whole motion being the motion of a whole,
the parts of the motion are the motions of the parts of that whole:
and the parts of DZ are the motions of AB, BG and of nothing else:
for, as we saw, a motion that is one cannot be the motion of more
things than one): since this is so, the whole motion will be the
motion of the magnitude ABG.
  Again, if there is a motion of the whole other than DZ, say the
the of each of the arts may be subtracted from it: and these motions
will be equal to DE, EZ respectively: for the motion of that which
is one must be one. So if the whole motion OI may be divided into
the motions of the parts, OI will be equal to DZ: if on the other hand
there is any remainder, say KI, this will be a motion of nothing:
for it can be the motion neither of the whole nor of the parts (as the
motion of that which is one must be one) nor of anything else: for a
motion that is continuous must be the motion of things that are
continuous. And the same result follows if the division of OI
reveals a surplus on the side of the motions of the parts.
Consequently, if this is impossible, the whole motion must be the same
as and equal to DZ.
  This then is what is meant by the division of motion according to
the motions of the parts: and it must be applicable to everything that
is divisible into parts.
  Motion is also susceptible of another kind of division, that
according to time. For since all motion is in time and all time is
divisible, and in less time the motion is less, it follows that
every motion must be divisible according to time. And since everything
that is in motion is in motion in a certain sphere and for a certain
time and has a motion belonging to it, it follows that the time, the
motion, the being-in-motion, the thing that is in motion, and the
sphere of the motion must all be susceptible of the same divisions
(though spheres of motion are not all divisible in a like manner: thus
quantity is essentially, quality accidentally divisible). For
suppose that A is the time occupied by the motion B. Then if all the
time has been occupied by the whole motion, it will take less of the
motion to occupy half the time, less again to occupy a further
subdivision of the time, and so on to infinity. Again, the time will
be divisible similarly to the motion: for if the whole motion occupies
all the time half the motion will occupy half the time, and less of
the motion again will occupy less of the time.
  In the same way the being-in-motion will also be divisible. For
let G be the whole being-in-motion. Then the being-in-motion that
corresponds to half the motion will be less than the whole
being-in-motion, that which corresponds to a quarter of the motion
will be less again, and so on to infinity. Moreover by setting out
successively the being-in-motion corresponding to each of the two
motions DG (say) and GE, we may argue that the whole being-in-motion
will correspond to the whole motion (for if it were some other
being-in-motion that corresponded to the whole motion, there would
be more than one being-in motion corresponding to the same motion),
the argument being the same as that whereby we showed that the
motion of a thing is divisible into the motions of the parts of the
thing: for if we take separately the being-in motion corresponding
to each of the two motions, we shall see that the whole being-in
motion is continuous.
  The same reasoning will show the divisibility of the length, and
in fact of everything that forms a sphere of change (though some of
these are only accidentally divisible because that which changes is
so): for the division of one term will involve the division of all.
So, too, in the matter of their being finite or infinite, they will
all alike be either the one or the other. And we now see that in
most cases the fact that all the terms are divisible or infinite is
a direct consequence of the fact that the thing that changes is
divisible or infinite: for the attributes 'divisible' and 'infinite'
belong in the first instance to the thing that changes. That
divisibility does so we have already shown: that infinity does so will
be made clear in what follows?

  Since everything that changes changes from something to something,
that which has changed must at the moment when it has first changed be
in that to which it has changed. For that which changes retires from
or leaves that from which it changes: and leaving, if not identical
with changing, is at any rate a consequence of it. And if leaving is a
consequence of changing, having left is a consequence of having
changed: for there is a like relation between the two in each case.
  One kind of change, then, being change in a relation of
contradiction, where a thing has changed from not-being to being it
has left not-being. Therefore it will be in being: for everything must
either be or not be. It is evident, then, that in contradictory change
that which has changed must be in that to which it has changed. And if
this is true in this kind of change, it will be true in all other
kinds as well: for in this matter what holds good in the case of one
will hold good likewise in the case of the rest.
  Moreover, if we take each kind of change separately, the truth of
our conclusion will be equally evident, on the ground that that that
which has changed must be somewhere or in something. For, since it has
left that from which it has changed and must be somewhere, it must
be either in that to which it has changed or in something else. If,
then, that which has changed to B is in something other than B, say G,
it must again be changing from G to B: for it cannot be assumed that
there is no interval between G and B, since change is continuous. Thus
we have the result that the thing that has changed, at the moment when
it has changed, is changing to that to which it has changed, which
is impossible: that which has changed, therefore, must be in that to
which it has changed. So it is evident likewise that that that which
has come to be, at the moment when it has come to be, will be, and
that which has ceased to be will not-be: for what we have said applies
universally to every kind of change, and its truth is most obvious
in the case of contradictory change. It is clear, then, that that
which has changed, at the moment when it has first changed, is in that
to which it has changed.
  We will now show that the 'primary when' in which that which has
changed effected the completion of its change must be indivisible,
where by 'primary' I mean possessing the characteristics in question
of itself and not in virtue of the possession of them by something
else belonging to it. For let AG be divisible, and let it be divided
at B. If then the completion of change has been effected in AB or
again in BG, AG cannot be the primary thing in which the completion of
change has been effected. If, on the other hand, it has been
changing in both AB and BG (for it must either have changed or be
changing in each of them), it must have been changing in the whole AG:
but our assumption was that AG contains only the completion of the
change. It is equally impossible to suppose that one part of AG
contains the process and the other the completion of the change: for
then we shall have something prior to what is primary. So that in
which the completion of change has been effected must be
indivisible. It is also evident, therefore, that that that in which
that which has ceased to be has ceased to be and that in which that
which has come to be has come to be are indivisible.
  But there are two senses of the expression 'the primary when in
which something has changed'. On the one hand it may mean the
primary when containing the completion of the process of change- the
moment when it is correct to say 'it has changed': on the other hand
it may mean the primary when containing the beginning of the process
of change. Now the primary when that has reference to the end of the
change is something really existent: for a change may really be
completed, and there is such a thing as an end of change, which we
have in fact shown to be indivisible because it is a limit. But that
which has reference to the beginning is not existent at all: for there
is no such thing as a beginning of a process of change, and the time
occupied by the change does not contain any primary when in which
the change began. For suppose that AD is such a primary when. Then
it cannot be indivisible: for, if it were, the moment immediately
preceding the change and the moment in which the change begins would
be consecutive (and moments cannot be consecutive). Again, if the
changing thing is at rest in the whole preceding time GA (for we may
suppose that it is at rest), it is at rest in A also: so if AD is
without parts, it will simultaneously be at rest and have changed: for
it is at rest in A and has changed in D. Since then AD is not
without parts, it must be divisible, and the changing thing must
have changed in every part of it (for if it has changed in neither
of the two parts into which AD is divided, it has not changed in the
whole either: if, on the other hand, it is in process of change in
both parts, it is likewise in process of change in the whole: and
if, again, it has changed in one of the two parts, the whole is not
the primary when in which it has changed: it must therefore have
changed in every part). It is evident, then, that with reference to
the beginning of change there is no primary when in which change has
been effected: for the divisions are infinite.
  So, too, of that which has changed there is no primary part that has
changed. For suppose that of AE the primary part that has changed is
AZ (everything that changes having been shown to be divisible): and
let OI be the time in which DZ has changed. If, then, in the whole
time DZ has changed, in half the time there will be a part that has
changed, less than and therefore prior to DZ: and again there will
be another part prior to this, and yet another, and so on to infinity.
Thus of that which changes there cannot be any primary part that has
changed. It is evident, then, from what has been said, that neither of
that which changes nor of the time in which it changes is there any
primary part.
  With regard, however, to the actual subject of change-that is to say
that in respect of which a thing changes-there is a difference to be
observed. For in a process of change we may distinguish three
terms-that which changes, that in which it changes, and the actual
subject of change, e.g. the man, the time, and the fair complexion. Of
these the man and the time are divisible: but with the fair complexion
it is otherwise (though they are all divisible accidentally, for
that in which the fair complexion or any other quality is an
accident is divisible). For of actual subjects of change it will be
seen that those which are classed as essentially, not accidentally,
divisible have no primary part. Take the case of magnitudes: let AB be
a magnitude, and suppose that it has moved from B to a primary 'where'
G. Then if BG is taken to be indivisible, two things without parts
will have to be contiguous (which is impossible): if on the other hand
it is taken to be divisible, there will be something prior to G to
which the magnitude has changed, and something else again prior to
that, and so on to infinity, because the process of division may be
continued without end. Thus there can be no primary 'where' to which a
thing has changed. And if we take the case of quantitative change,
we shall get a like result, for here too the change is in something
continuous. It is evident, then, that only in qualitative motion can
there be anything essentially indivisible.

  Now everything that changes changes time, and that in two senses:
for the time in which a thing is said to change may be the primary
time, or on the other hand it may have an extended reference, as
e.g. when we say that a thing changes in a particular year because
it changes in a particular day. That being so, that which changes must
be changing in any part of the primary time in which it changes.
This is clear from our definition of 'primary', in which the word is
said to express just this: it may also, however, be made evident by
the following argument. Let ChRh be the primary time in which that
which is in motion is in motion: and (as all time is divisible) let it
be divided at K. Now in the time ChK it either is in motion or is
not in motion, and the same is likewise true of the time KRh. Then
if it is in motion in neither of the two parts, it will be at rest
in the whole: for it is impossible that it should be in motion in a
time in no part of which it is in motion. If on the other hand it is
in motion in only one of the two parts of the time, ChRh cannot be the
primary time in which it is in motion: for its motion will have
reference to a time other than ChRh. It must, then, have been in
motion in any part of ChRh.
  And now that this has been proved, it is evident that everything
that is in motion must have been in motion before. For if that which
is in motion has traversed the distance KL in the primary time ChRh,
in half the time a thing that is in motion with equal velocity and
began its motion at the same time will have traversed half the
distance. But if this second thing whose velocity is equal has
traversed a certain distance in a certain time, the original thing
that is in motion must have traversed the same distance in the same
time. Hence that which is in motion must have been in motion before.
  Again, if by taking the extreme moment of the time-for it is the
moment that defines the time, and time is that which is intermediate
between moments-we are enabled to say that motion has taken place in
the whole time ChRh or in fact in any period of it, motion may
likewise be said to have taken place in every other such period. But
half the time finds an extreme in the point of division. Therefore
motion will have taken place in half the time and in fact in any
part of it: for as soon as any division is made there is always a time
defined by moments. If, then, all time is divisible, and that which is
intermediate between moments is time, everything that is changing must
have completed an infinite number of changes.
  Again, since a thing that changes continuously and has not
perished or ceased from its change must either be changing or have
changed in any part of the time of its change, and since it cannot
be changing in a moment, it follows that it must have changed at every
moment in the time: consequently, since the moments are infinite in
number, everything that is changing must have completed an infinite
number of changes.
  And not only must that which is changing have changed, but that
which has changed must also previously have been changing, since
everything that has changed from something to something has changed in
a period of time. For suppose that a thing has changed from A to B
in a moment. Now the moment in which it has changed cannot be the same
as that in which it is at A (since in that case it would be in A and B
at once): for we have shown above that that that which has changed,
when it has changed, is not in that from which it has changed. If,
on the other hand, it is a different moment, there will be a period of
time intermediate between the two: for, as we saw, moments are not
consecutive. Since, then, it has changed in a period of time, and
all time is divisible, in half the time it will have completed another
change, in a quarter another, and so on to infinity: consequently when
it has changed, it must have previously been changing.
  Moreover, the truth of what has been said is more evident in the
case of magnitude, because the magnitude over which what is changing
changes is continuous. For suppose that a thing has changed from G
to D. Then if GD is indivisible, two things without parts will be
consecutive. But since this is impossible, that which is
intermediate between them must be a magnitude and divisible into an
infinite number of segments: consequently, before the change is
completed, the thing changes to those segments. Everything that has
changed, therefore, must previously have been changing: for the same
proof also holds good of change with respect to what is not
continuous, changes, that is to say, between contraries and between
contradictories. In such cases we have only to take the time in
which a thing has changed and again apply the same reasoning. So
that which has changed must have been changing and that which is
changing must have changed, and a process of change is preceded by a
completion of change and a completion by a process: and we can never
take any stage and say that it is absolutely the first. The reason
of this is that no two things without parts can be contiguous, and
therefore in change the process of division is infinite, just as lines
may be infinitely divided so that one part is continually increasing
and the other continually decreasing.
  So it is evident also that that that which has become must
previously have been in process of becoming, and that which is in
process of becoming must previously have become, everything (that
is) that is divisible and continuous: though it is not always the
actual thing that is in process of becoming of which this is true:
sometimes it is something else, that is to say, some part of the thing
in question, e.g. the foundation-stone of a house. So, too, in the
case of that which is perishing and that which has perished: for
that which becomes and that which perishes must contain an element
of infiniteness as an immediate consequence of the fact that they
are continuous things: and so a thing cannot be in process of becoming
without having become or have become without having been in process of
becoming. So, too, in the case of perishing and having perished:
perishing must be preceded by having perished, and having perished
must be preceded by perishing. It is evident, then, that that which
has become must previously have been in process of becoming, and
that which is in process of becoming must previously have become:
for all magnitudes and all periods of time are infinitely divisible.
  Consequently no absolutely first stage of change can be
represented by any particular part of space or time which the changing
thing may occupy.

  Now since the motion of everything that is in motion occupies a
period of time, and a greater magnitude is traversed in a longer time,
it is impossible that a thing should undergo a finite motion in an
infinite time, if this is understood to mean not that the same
motion or a part of it is continually repeated, but that the whole
infinite time is occupied by the whole finite motion. In all cases
where a thing is in motion with uniform velocity it is clear that
the finite magnitude is traversed in a finite time. For if we take a
part of the motion which shall be a measure of the whole, the whole
motion is completed in as many equal periods of the time as there
are parts of the motion. Consequently, since these parts are finite,
both in size individually and in number collectively, the whole time
must also be finite: for it will be a multiple of the portion, equal
to the time occupied in completing the aforesaid part multiplied by
the number of the parts.
  But it makes no difference even if the velocity is not uniform.
For let us suppose that the line AB represents a finite stretch over
which a thing has been moved in the given time, and let GD be the
infinite time. Now if one part of the stretch must have been traversed
before another part (this is clear, that in the earlier and in the
later part of the time a different part of the stretch has been
traversed: for as the time lengthens a different part of the motion
will always be completed in it, whether the thing in motion changes
with uniform velocity or not: and whether the rate of motion increases
or diminishes or remains stationary this is none the less so), let
us then take AE a part of the whole stretch of motion AB which shall
be a measure of AB. Now this part of the motion occupies a certain
period of the infinite time: it cannot itself occupy an infinite time,
for we are assuming that that is occupied by the whole AB. And if
again I take another part equal to AE, that also must occupy a
finite time in consequence of the same assumption. And if I go on
taking parts in this way, on the one hand there is no part which
will be a measure of the infinite time (for the infinite cannot be
composed of finite parts whether equal or unequal, because there
must be some unity which will be a measure of things finite in
multitude or in magnitude, which, whether they are equal or unequal,
are none the less limited in magnitude); while on the other hand the
finite stretch of motion AB is a certain multiple of AE:
consequently the motion AB must be accomplished in a finite time.
Moreover it is the same with coming to rest as with motion. And so
it is impossible for one and the same thing to be infinitely in
process of becoming or of perishing. The reasoning he will prove
that in a finite time there cannot be an infinite extent of motion
or of coming to rest, whether the motion is regular or irregular.
For if we take a part which shall be a measure of the whole time, in
this part a certain fraction, not the whole, of the magnitude will
be traversed, because we assume that the traversing of the whole
occupies all the time. Again, in another equal part of the time
another part of the magnitude will be traversed: and similarly in each
part of the time that we take, whether equal or unequal to the part
originally taken. It makes no difference whether the parts are equal
or not, if only each is finite: for it is clear that while the time is
exhausted by the subtraction of its parts, the infinite magnitude will
not be thus exhausted, since the process of subtraction is finite both
in respect of the quantity subtracted and of the number of times a
subtraction is made. Consequently the infinite magnitude will not be
traversed in finite time: and it makes no difference whether the
magnitude is infinite in only one direction or in both: for the same
reasoning will hold good.
  This having been proved, it is evident that neither can a finite
magnitude traverse an infinite magnitude in a finite time, the
reason being the same as that given above: in part of the time it will
traverse a finite magnitude and in each several part likewise, so that
in the whole time it will traverse a finite magnitude.
  And since a finite magnitude will not traverse an infinite in a
finite time, it is clear that neither will an infinite traverse a
finite in a finite time. For if the infinite could traverse the
finite, the finite could traverse the infinite; for it makes no
difference which of the two is the thing in motion; either case
involves the traversing of the infinite by the finite. For when the
infinite magnitude A is in motion a part of it, say GD, will occupy
the finite and then another, and then another, and so on to
infinity. Thus the two results will coincide: the infinite will have
completed a motion over the finite and the finite will have
traversed the infinite: for it would seem to be impossible for the
motion of the infinite over the finite to occur in any way other
than by the finite traversing the infinite either by locomotion over
it or by measuring it. Therefore, since this is impossible, the
infinite cannot traverse the finite.
  Nor again will the infinite traverse the infinite in a finite
time. Otherwise it would also traverse the finite, for the infinite
includes the finite. We can further prove this in the same way by
taking the time as our starting-point.
  Since, then, it is established that in a finite time neither will
the finite traverse the infinite, nor the infinite the finite, nor the
infinite the infinite, it is evident also that in a finite time
there cannot be infinite motion: for what difference does it make
whether we take the motion or the magnitude to be infinite? If
either of the two is infinite, the other must be so likewise: for
all locomotion is in space.

  Since everything to which motion or rest is natural is in motion
or at rest in the natural time, place, and manner, that which is
coming to a stand, when it is coming to a stand, must be in motion:
for if it is not in motion it must be at rest: but that which is at
rest cannot be coming to rest. From this it evidently follows that
coming to a stand must occupy a period of time: for the motion of that
which is in motion occupies a period of time, and that which is coming
to a stand has been shown to be in motion: consequently coming to a
stand must occupy a period of time.
  Again, since the terms 'quicker' and 'slower' are used only of
that which occupies a period of time, and the process of coming to a
stand may be quicker or slower, the same conclusion follows.
  And that which is coming to a stand must be coming to a stand in any
part of the primary time in which it is coming to a stand. For if it
is coming to a stand in neither of two parts into which the time may
be divided, it cannot be coming to a stand in the whole time, with the
result that that that which is coming to a stand will not be coming to
a stand. If on the other hand it is coming to a stand in only one of
the two parts of the time, the whole cannot be the primary time in
which it is coming to a stand: for it is coming to a stand in the
whole time not primarily but in virtue of something distinct from
itself, the argument being the same as that which we used above
about things in motion.
  And just as there is no primary time in which that which is in
motion is in motion, so too there is no primary time in which that
which is coming to a stand is coming to a stand, there being no
primary stage either of being in motion or of coming to a stand. For
let AB be the primary time in which a thing is coming to a stand.
Now AB cannot be without parts: for there cannot be motion in that
which is without parts, because the moving thing would necessarily
have been already moved for part of the time of its movement: and that
which is coming to a stand has been shown to be in motion. But since
AB is therefore divisible, the thing is coming to a stand in every one
of the parts of AB: for we have shown above that it is coming to a
stand in every one of the parts in which it is primarily coming to a
stand. Since then, that in which primarily a thing is coming to a
stand must be a period of time and not something indivisible, and
since all time is infinitely divisible, there cannot be anything in
which primarily it is coming to a stand.
  Nor again can there be a primary time at which the being at rest
of that which is at rest occurred: for it cannot have occurred in that
which has no parts, because there cannot be motion in that which is
indivisible, and that in which rest takes place is the same as that in
which motion takes place: for we defined a state of rest to be the
state of a thing to which motion is natural but which is not in motion
when (that is to say in that in which) motion would be natural to
it. Again, our use of the phrase 'being at rest' also implies that the
previous state of a thing is still unaltered, not one point only but
two at least being thus needed to determine its presence: consequently
that in which a thing is at rest cannot be without parts. Since,
then it is divisible, it must be a period of time, and the thing
must be at rest in every one of its parts, as may be shown by the same
method as that used above in similar demonstrations.
  So there can be no primary part of the time: and the reason is
that rest and motion are always in a period of time, and a period of
time has no primary part any more than a magnitude or in fact anything
continuous: for everything continuous is divisible into an infinite
number of parts.
  And since everything that is in motion is in motion in a period of
time and changes from something to something, when its motion is
comprised within a particular period of time essentially-that is to
say when it fills the whole and not merely a part of the time in
question-it is impossible that in that time that which is in motion
should be over against some particular thing primarily. For if a
thing-itself and each of its parts-occupies the same space for a
definite period of time, it is at rest: for it is in just these
circumstances that we use the term 'being at rest'-when at one
moment after another it can be said with truth that a thing, itself
and its parts, occupies the same space. So if this is being at rest it
is impossible for that which is changing to be as a whole, at the time
when it is primarily changing, over against any particular thing
(for the whole period of time is divisible), so that in one part of it
after another it will be true to say that the thing, itself and its
parts, occupies the same space. If this is not so and the aforesaid
proposition is true only at a single moment, then the thing will be
over against a particular thing not for any period of time but only at
a moment that limits the time. It is true that at any moment it is
always over against something stationary: but it is not at rest: for
at a moment it is not possible for anything to be either in motion
or at rest. So while it is true to say that that which is in motion is
at a moment not in motion and is opposite some particular thing, it
cannot in a period of time be over against that which is at rest:
for that would involve the conclusion that that which is in locomotion
is at rest.

  Zeno's reasoning, however, is fallacious, when he says that if
everything when it occupies an equal space is at rest, and if that
which is in locomotion is always occupying such a space at any moment,
the flying arrow is therefore motionless. This is false, for time is
not composed of indivisible moments any more than any other
magnitude is composed of indivisibles.
  Zeno's arguments about motion, which cause so much disquietude to
those who try to solve the problems that they present, are four in
number. The first asserts the non-existence of motion on the ground
that that which is in locomotion must arrive at the half-way stage
before it arrives at the goal. This we have discussed above.
  The second is the so-called 'Achilles', and it amounts to this, that
in a race the quickest runner can never overtake the slowest, since
the pursuer must first reach the point whence the pursued started,
so that the slower must always hold a lead. This argument is the
same in principle as that which depends on bisection, though it
differs from it in that the spaces with which we successively have
to deal are not divided into halves. The result of the argument is
that the slower is not overtaken: but it proceeds along the same lines
as the bisection-argument (for in both a division of the space in a
certain way leads to the result that the goal is not reached, though
the 'Achilles' goes further in that it affirms that even the
quickest runner in legendary tradition must fail in his pursuit of the
slowest), so that the solution must be the same. And the axiom that
that which holds a lead is never overtaken is false: it is not
overtaken, it is true, while it holds a lead: but it is overtaken
nevertheless if it is granted that it traverses the finite distance
prescribed. These then are two of his arguments.
  The third is that already given above, to the effect that the flying
arrow is at rest, which result follows from the assumption that time
is composed of moments: if this assumption is not granted, the
conclusion will not follow.
  The fourth argument is that concerning the two rows of bodies,
each row being composed of an equal number of bodies of equal size,
passing each other on a race-course as they proceed with equal
velocity in opposite directions, the one row originally occupying
the space between the goal and the middle point of the course and
the other that between the middle point and the starting-post. This,
he thinks, involves the conclusion that half a given time is equal
to double that time. The fallacy of the reasoning lies in the
assumption that a body occupies an equal time in passing with equal
velocity a body that is in motion and a body of equal size that is
at rest; which is false. For instance (so runs the argument), let A, the stationary bodies of equal size, B, B...the bodies, equal
in number and in size to A, A...,originally occupying the half of
the course from the starting-post to the middle of the A's, and G,
G...those originally occupying the other half from the goal to the
middle of the A's, equal in number, size, and velocity to B, B....Then
three consequences follow:
  First, as the B's and the G's pass one another, the first B
reaches the last G at the same moment as the first G reaches the
last B. Secondly at this moment the first G has passed all the A's,
whereas the first B has passed only half the A's, and has consequently
occupied only half the time occupied by the first G, since each of the
two occupies an equal time in passing each A. Thirdly, at the same
moment all the B's have passed all the G's: for the first G and the
first B will simultaneously reach the opposite ends of the course,
since (so says Zeno) the time occupied by the first G in passing
each of the B's is equal to that occupied by it in passing each of the
A's, because an equal time is occupied by both the first B and the
first G in passing all the A's. This is the argument, but it
presupposed the aforesaid fallacious assumption.
  Nor in reference to contradictory change shall we find anything
unanswerable in the argument that if a thing is changing from
not-white, say, to white, and is in neither condition, then it will be
neither white nor not-white: for the fact that it is not wholly in
either condition will not preclude us from calling it white or
not-white. We call a thing white or not-white not necessarily
because it is be one or the other, but cause most of its parts or
the most essential parts of it are so: not being in a certain
condition is different from not being wholly in that condition. So,
too, in the case of being and not-being and all other conditions which
stand in a contradictory relation: while the changing thing must of
necessity be in one of the two opposites, it is never wholly in
  Again, in the case of circles and spheres and everything whose
motion is confined within the space that it occupies, it is not true
to say the motion can be nothing but rest, on the ground that such
things in motion, themselves and their parts, will occupy the same
position for a period of time, and that therefore they will be at once
at rest and in motion. For in the first place the parts do not
occupy the same position for any period of time: and in the second
place the whole also is always changing to a different position: for
if we take the orbit as described from a point A on a circumference,
it will not be the same as the orbit as described from B or G or any
other point on the same circumference except in an accidental sense,
the sense that is to say in which a musical man is the same as a
man. Thus one orbit is always changing into another, and the thing
will never be at rest. And it is the same with the sphere and
everything else whose motion is confined within the space that it

  Our next point is that that which is without parts cannot be in
motion except accidentally: i.e. it can be in motion only in so far as
the body or the magnitude is in motion and the partless is in motion
by inclusion therein, just as that which is in a boat may be in motion
in consequence of the locomotion of the boat, or a part may be in
motion in virtue of the motion of the whole. (It must be remembered,
however, that by 'that which is without parts' I mean that which is
quantitatively indivisible (and that the case of the motion of a
part is not exactly parallel): for parts have motions belonging
essentially and severally to themselves distinct from the motion of
the whole. The distinction may be seen most clearly in the case of a
revolving sphere, in which the velocities of the parts near the centre
and of those on the surface are different from one another and from
that of the whole; this implies that there is not one motion but
many). As we have said, then, that which is without parts can be in
motion in the sense in which a man sitting in a boat is in motion when
the boat is travelling, but it cannot be in motion of itself. For
suppose that it is changing from AB to BG-either from one magnitude to
another, or from one form to another, or from some state to its
contradictory-and let D be the primary time in which it undergoes
the change. Then in the time in which it is changing it must be either
in AB or in BG or partly in one and partly in the other: for this,
as we saw, is true of everything that is changing. Now it cannot be
partly in each of the two: for then it would be divisible into
parts. Nor again can it be in BG: for then it will have completed
the change, whereas the assumption is that the change is in process.
It remains, then, that in the time in which it is changing, it is in
AB. That being so, it will be at rest: for, as we saw, to be in the
same condition for a period of time is to be at rest. So it is not
possible for that which has no parts to be in motion or to change in
any way: for only one condition could have made it possible for it
to have motion, viz. that time should be composed of moments, in which
case at any moment it would have completed a motion or a change, so
that it would never be in motion, but would always have been in
motion. But this we have already shown above to be impossible: time is
not composed of moments, just as a line is not composed of points, and
motion is not composed of starts: for this theory simply makes
motion consist of indivisibles in exactly the same way as time is made
to consist of moments or a length of points.
  Again, it may be shown in the following way that there can be no
motion of a point or of any other indivisible. That which is in motion
can never traverse a space greater than itself without first
traversing a space equal to or less than itself. That being so, it
is evident that the point also must first traverse a space equal to or
less than itself. But since it is indivisible, there can be no space
less than itself for it to traverse first: so it will have to traverse
a distance equal to itself. Thus the line will be composed of
points, for the point, as it continually traverses a distance equal to
itself, will be a measure of the whole line. But since this is
impossible, it is likewise impossible for the indivisible to be in
  Again, since motion is always in a period of time and never in a
moment, and all time is divisible, for everything that is in motion
there must be a time less than that in which it traverses a distance
as great as itself. For that in which it is in motion will be a
time, because all motion is in a period of time; and all time has been
shown above to be divisible. Therefore, if a point is in motion, there
must be a time less than that in which it has itself traversed any
distance. But this is impossible, for in less time it must traverse
less distance, and thus the indivisible will be divisible into
something less than itself, just as the time is so divisible: the fact
being that the only condition under which that which is without
parts and indivisible could be in motion would have been the
possibility of the infinitely small being in motion in a moment: for
in the two questions-that of motion in a moment and that of motion
of something indivisible-the same principle is involved.
  Our next point is that no process of change is infinite: for every
change, whether between contradictories or between contraries, is a
change from something to something. Thus in contradictory changes
the positive or the negative, as the case may be, is the limit, e.g.
being is the limit of coming to be and not-being is the limit of
ceasing to be: and in contrary changes the particular contraries are
the limits, since these are the extreme points of any such process
of change, and consequently of every process of alteration: for
alteration is always dependent upon some contraries. Similarly
contraries are the extreme points of processes of increase and
decrease: the limit of increase is to be found in the complete
magnitude proper to the peculiar nature of the thing that is
increasing, while the limit of decrease is the complete loss of such
magnitude. Locomotion, it is true, we cannot show to be finite in this
way, since it is not always between contraries. But since that which
cannot be cut (in the sense that it is inconceivable that it should be
cut, the term 'cannot' being used in several senses)-since it is
inconceivable that that which in this sense cannot be cut should be in
process of being cut, and generally that that which cannot come to
be should be in process of coming to be, it follows that it is
inconceivable that that which cannot complete a change should be in
process of changing to that to which it cannot complete a change.
If, then, it is to be assumed that that which is in locomotion is in
process of changing, it must be capable of completing the change.
Consequently its motion is not infinite, and it will not be in
locomotion over an infinite distance, for it cannot traverse such a
  It is evident, then, that a process of change cannot be infinite
in the sense that it is not defined by limits. But it remains to be
considered whether it is possible in the sense that one and the same
process of change may be infinite in respect of the time which it
occupies. If it is not one process, it would seem that there is
nothing to prevent its being infinite in this sense; e.g. if a process
of locomotion be succeeded by a process of alteration and that by a
process of increase and that again by a process of coming to be: in
this way there may be motion for ever so far as the time is concerned,
but it will not be one motion, because all these motions do not
compose one. If it is to be one process, no motion can be infinite
in respect of the time that it occupies, with the single exception
of rotatory locomotion.
                              Book VII

  EVERYTHING that is in motion must be moved by something. For if it
has not the source of its motion in itself it is evident that it is
moved by something other than itself, for there must be something else
that moves it. If on the other hand it has the source of its motion in
itself, let AB be taken to represent that which is in motion
essentially of itself and not in virtue of the fact that something
belonging to it is in motion. Now in the first place to assume that
AB, because it is in motion as a whole and is not moved by anything
external to itself, is therefore moved by itself-this is just as if,
supposing that KL is moving LM and is also itself in motion, we were
to deny that KM is moved by anything on the ground that it is not
evident which is the part that is moving it and which the part that is
moved. In the second place that which is in motion without being moved
by anything does not necessarily cease from its motion because
something else is at rest, but a thing must be moved by something if
the fact of something else having ceased from its motion causes it
to be at rest. Thus, if this is accepted, everything that is in motion
must be moved by something. For AB, which has been taken to
represent that which is in motion, must be divisible since
everything that is in motion is divisible. Let it be divided, then, at
G. Now if GB is not in motion, then AB will not be in motion: for if
it is, it is clear that AG would be in motion while BG is at rest, and
thus AB cannot be in motion essentially and primarily. But ex
hypothesi AB is in motion essentially and primarily. Therefore if GB
is not in motion AB will be at rest. But we have agreed that that
which is at rest if something else is not in motion must be moved by
something. Consequently, everything that is in motion must be moved by
something: for that which is in motion will always be divisible, and
if a part of it is not in motion the whole must be at rest.
  Since everything that is in motion must be moved by something, let
us take the case in which a thing is in locomotion and is moved by
something that is itself in motion, and that again is moved by
something else that is in motion, and that by something else, and so
on continually: then the series cannot go on to infinity, but there
must be some first movent. For let us suppose that this is not so
and take the series to be infinite. Let A then be moved by B, B by
G, G by D, and so on, each member of the series being moved by that
which comes next to it. Then since ex hypothesi the movent while
causing motion is also itself in motion, and the motion of the moved
and the motion of the movent must proceed simultaneously (for the
movent is causing motion and the moved is being moved
simultaneously) it is evident that the respective motions of A, B,
G, and each of the other moved movents are simultaneous. Let us take
the motion of each separately and let E be the motion of A, Z of B,
and H and O respectively the motions of G and D: for though they are
all moved severally one by another, yet we may still take the motion
of each as numerically one, since every motion is from something to
something and is not infinite in respect of its extreme points. By a
motion that is numerically one I mean a motion that proceeds from
something numerically one and the same to something numerically one
and the same in a period of time numerically one and the same: for a
motion may be the same generically, specifically, or numerically: it
is generically the same if it belongs to the same category, e.g.
substance or quality: it is specifically the same if it proceeds
from something specifically the same to something specifically the
same, e.g. from white to black or from good to bad, which is not of
a kind specifically distinct: it is numerically the same if it
proceeds from something numerically one to something numerically one
in the same period of time, e.g. from a particular white to a
particular black, or from a particular place to a particular place, in
a particular period of time: for if the period of time were not one
and the same, the motion would no longer be numerically one though
it would still be specifically one.
  We have dealt with this question above. Now let us further take
the time in which A has completed its motion, and let it be
represented by K. Then since the motion of A is finite the time will
also be finite. But since the movents and the things moved are
infinite, the motion EZHO, i.e. the motion that is composed of all the
individual motions, must be infinite. For the motions of A, B, and the
others may be equal, or the motions of the others may be greater:
but assuming what is conceivable, we find that whether they are
equal or some are greater, in both cases the whole motion is infinite.
And since the motion of A and that of each of the others are
simultaneous, the whole motion must occupy the same time as the motion
of A: but the time occupied by the motion of A is finite: consequently
the motion will be infinite in a finite time, which is impossible.
  It might be thought that what we set out to prove has thus been
shown, but our argument so far does not prove it, because it does
not yet prove that anything impossible results from the contrary
supposition: for in a finite time there may be an infinite motion,
though not of one thing, but of many: and in the case that we are
considering this is so: for each thing accomplishes its own motion,
and there is no impossibility in many things being in motion
simultaneously. But if (as we see to be universally the case) that
which primarily is moved locally and corporeally must be either in
contact with or continuous with that which moves it, the things
moved and the movents must be continuous or in contact with one
another, so that together they all form a single unity: whether this
unity is finite or infinite makes no difference to our present
argument; for in any case since the things in motion are infinite in
number the whole motion will be infinite, if, as is theoretically
possible, each motion is either equal to or greater than that which
follows it in the series: for we shall take as actual that which is
theoretically possible. If, then, A, B, G, D form an infinite
magnitude that passes through the motion EZHO in the finite time K,
this involves the conclusion that an infinite motion is passed through
in a finite time: and whether the magnitude in question is finite or
infinite this is in either case impossible. Therefore the series
must come to an end, and there must be a first movent and a first
moved: for the fact that this impossibility results only from the
assumption of a particular case is immaterial, since the case
assumed is theoretically possible, and the assumption of a
theoretically possible case ought not to give rise to any impossible

  That which is the first movement of a thing-in the sense that it
supplies not 'that for the sake of which' but the source of the
motion-is always together with that which is moved by it by 'together'
I mean that there is nothing intermediate between them). This is
universally true wherever one thing is moved by another. And since
there are three kinds of motion, local, qualitative, and quantitative,
there must also be three kinds of movent, that which causes
locomotion, that which causes alteration, and that which causes
increase or decrease.
  Let us begin with locomotion, for this is the primary motion.
Everything that is in locomotion is moved either by itself or by
something else. In the case of things that are moved by themselves
it is evident that the moved and the movent are together: for they
contain within themselves their first movent, so that there is nothing
in between. The motion of things that are moved by something else must
proceed in one of four ways: for there are four kinds of locomotion
caused by something other than that which is in motion, viz.
pulling, pushing, carrying, and twirling. All forms of locomotion
are reducible to these. Thus pushing on is a form of pushing in
which that which is causing motion away from itself follows up that
which it pushes and continues to push it: pushing off occurs when
the movent does not follow up the thing that it has moved: throwing
when the movent causes a motion away from itself more violent than the
natural locomotion of the thing moved, which continues its course so
long as it is controlled by the motion imparted to it. Again,
pushing apart and pushing together are forms respectively of pushing
off and pulling: pushing apart is pushing off, which may be a motion
either away from the pusher or away from something else, while pushing
together is pulling, which may be a motion towards something else as
well as the puller. We may similarly classify all the varieties of
these last two, e.g. packing and combing: the former is a form of
pushing together, the latter a form of pushing apart. The same is true
of the other processes of combination and separation (they will all be
found to be forms of pushing apart or of pushing together), except
such as are involved in the processes of becoming and perishing. (At
same time it is evident that there is no other kind of motion but
combination and separation: for they may all be apportioned to one
or other of those already mentioned.) Again, inhaling is a form of
pulling, exhaling a form of pushing: and the same is true of
spitting and of all other motions that proceed through the body,
whether secretive or assimilative, the assimilative being forms of
pulling, the secretive of pushing off. All other kinds of locomotion
must be similarly reduced, for they all fall under one or other of our
four heads. And again, of these four, carrying and twirling are to
pulling and pushing. For carrying always follows one of the other
three methods, for that which is carried is in motion accidentally,
because it is in or upon something that is in motion, and that which
carries it is in doing so being either pulled or pushed or twirled;
thus carrying belongs to all the other three kinds of motion in
common. And twirling is a compound of pulling and pushing, for that
which is twirling a thing must be pulling one part of the thing and
pushing another part, since it impels one part away from itself and
another part towards itself. If, therefore, it can be shown that
that which is pushing and that which is pushing and pulling are
adjacent respectively to that which is being pushed and that which
is being pulled, it will be evident that in all locomotion there is
nothing intermediate between moved and movent. But the former fact
is clear even from the definitions of pushing and pulling, for pushing
is motion to something else from oneself or from something else, and
pulling is motion from something else to oneself or to something else,
when the motion of that which is pulling is quicker than the motion
that would separate from one another the two things that are
continuous: for it is this that causes one thing to be pulled on along
with the other. (It might indeed be thought that there is a form of
pulling that arises in another way: that wood, e.g. pulls fire in a
manner different from that described above. But it makes no difference
whether that which pulls is in motion or is stationary when it is
pulling: in the latter case it pulls to the place where it is, while
in the former it pulls to the place where it was.) Now it is
impossible to move anything either from oneself to something else or
something else to oneself without being in contact with it: it is
evident, therefore, that in all locomotion there is nothing
intermediate between moved and movent.
  Nor again is there anything intermediate between that which
undergoes and that which causes alteration: this can be proved by
induction: for in every case we find that the respective extremities
of that which causes and that which undergoes alteration are adjacent.
For our assumption is that things that are undergoing alteration are
altered in virtue of their being affected in respect of their
so-called affective qualities, since that which is of a certain
quality is altered in so far as it is sensible, and the
characteristics in which bodies differ from one another are sensible
characteristics: for every body differs from another in possessing a
greater or lesser number of sensible characteristics or in
possessing the same sensible characteristics in a greater or lesser
degree. But the alteration of that which undergoes alteration is
also caused by the above-mentioned characteristics, which are
affections of some particular underlying quality. Thus we say that a
thing is altered by becoming hot or sweet or thick or dry or white:
and we make these assertions alike of what is inanimate and of what is
animate, and further, where animate things are in question, we make
them both of the parts that have no power of sense-perception and of
the senses themselves. For in a way even the senses undergo
alteration, since the active sense is a motion through the body in the
course of which the sense is affected in a certain way. We see,
then, that the animate is capable of every kind of alteration of which
the inanimate is capable: but the inanimate is not capable of every
kind of alteration of which the animate is capable, since it is not
capable of alteration in respect of the senses: moreover the inanimate
is unconscious of being affected by alteration, whereas the animate is
conscious of it, though there is nothing to prevent the animate also
being unconscious of it when the process of the alteration does not
concern the senses. Since, then, the alteration of that which
undergoes alteration is caused by sensible things, in every case of
such alteration it is evident that the respective extremities of
that which causes and that which undergoes alteration are adjacent.
Thus the air is continuous with that which causes the alteration,
and the body that undergoes alteration is continuous with the air.
Again, the colour is continuous with the light and the light with
the sight. And the same is true of hearing and smelling: for the
primary movent in respect to the moved is the air. Similarly, in the
case of tasting, the flavour is adjacent to the sense of taste. And it
is just the same in the case of things that are inanimate and
incapable of sense-perception. Thus there can be nothing
intermediate between that which undergoes and that which causes
  Nor, again, can there be anything intermediate between that which
suffers and that which causes increase: for the part of the latter
that starts the increase does so by becoming attached in such a way to
the former that the whole becomes one. Again, the decrease of that
which suffers decrease is caused by a part of the thing becoming
detached. So that which causes increase and that which causes decrease
must be continuous with that which suffers increase and that which
suffers decrease respectively: and if two things are continuous with
one another there can be nothing intermediate between them.
    It is evident, therefore, that between the extremities of the
moved and the movent that are respectively first and last in reference
to the moved there is nothing intermediate.

  Everything, we say, that undergoes alteration is altered by sensible
causes, and there is alteration only in things that are said to be
essentially affected by sensible things. The truth of this is to be
seen from the following considerations. Of all other things it would
be most natural to suppose that there is alteration in figures and
shapes, and in acquired states and in the processes of acquiring and
losing these: but as a matter of fact in neither of these two
classes of things is there alteration.
  In the first place, when a particular formation of a thing is
completed, we do not call it by the name of its material: e.g. we do
not call the statue 'bronze' or the pyramid 'wax' or the bed 'wood',
but we use a derived expression and call them 'of bronze', 'waxen',
and 'wooden' respectively. But when a thing has been affected and
altered in any way we still call it by the original name: thus we
speak of the bronze or the wax being dry or fluid or hard or hot.
  And not only so: we also speak of the particular fluid or hot
substance as being bronze, giving the material the same name as that
which we use to describe the affection.
  Since, therefore, having regard to the figure or shape of a thing we
no longer call that which has become of a certain figure by the name
of the material that exhibits the figure, whereas having regard to a
thing's affections or alterations we still call it by the name of
its material, it is evident that becomings of the former kind cannot
be alterations.
  Moreover it would seem absurd even to speak in this way, to speak,
that is to say, of a man or house or anything else that has come
into existence as having been altered. Though it may be true that
every such becoming is necessarily the result of something's being
altered, the result, e.g. of the material's being condensed or
rarefied or heated or cooled, nevertheless it is not the things that
are coming into existence that are altered, and their becoming is
not an alteration.
  Again, acquired states, whether of the body or of the soul, are
not alterations. For some are excellences and others are defects,
and neither excellence nor defect is an alteration: excellence is a
perfection (for when anything acquires its proper excellence we call
it perfect, since it is then if ever that we have a thing in its
natural state: e.g. we have a perfect circle when we have one as
good as possible), while defect is a perishing of or departure from
this condition. So as when speaking of a house we do not call its
arrival at perfection an alteration (for it would be absurd to suppose
that the coping or the tiling is an alteration or that in receiving
its coping or its tiling a house is altered and not perfected), the
same also holds good in the case of excellences and defects and of the
persons or things that possess or acquire them: for excellences are
perfections of a thing's nature and defects are departures from it:
consequently they are not alterations.
  Further, we say that all excellences depend upon particular
relations. Thus bodily excellences such as health and a good state
of body we regard as consisting in a blending of hot and cold elements
within the body in due proportion, in relation either to one another
or to the surrounding atmosphere: and in like manner we regard beauty,
strength, and all the other bodily excellences and defects. Each of
them exists in virtue of a particular relation and puts that which
possesses it in a good or bad condition with regard to its proper
affections, where by 'proper' affections I mean those influences
that from the natural constitution of a thing tend to promote or
destroy its existence. Since then, relatives are neither themselves
alterations nor the subjects of alteration or of becoming or in fact
of any change whatever, it is evident that neither states nor the
processes of losing and acquiring states are alterations, though it
may be true that their becoming or perishing is necessarily, like
the becoming or perishing of a specific character or form, the
result of the alteration of certain other things, e.g. hot and cold or
dry and wet elements or the elements, whatever they may be, on which
the states primarily depend. For each several bodily defect or
excellence involves a relation with those things from which the
possessor of the defect or excellence is naturally subject to
alteration: thus excellence disposes its possessor to be unaffected by
these influences or to be affected by those of them that ought to be
admitted, while defect disposes its possessor to be affected by them
or to be unaffected by those of them that ought to be admitted.
  And the case is similar in regard to the states of the soul, all
of which (like those of body) exist in virtue of particular relations,
the excellences being perfections of nature and the defects departures
from it: moreover, excellence puts its possessor in good condition,
while defect puts its possessor in a bad condition, to meet his proper
affections. Consequently these cannot any more than the bodily
states be alterations, nor can the processes of losing and acquiring
them be so, though their becoming is necessarily the result of an
alteration of the sensitive part of the soul, and this is altered by
sensible objects: for all moral excellence is concerned with bodily
pleasures and pains, which again depend either upon acting or upon
remembering or upon anticipating. Now those that depend upon action
are determined by sense-perception, i.e. they are stimulated by
something sensible: and those that depend upon memory or
anticipation are likewise to be traced to sense-perception, for in
these cases pleasure is felt either in remembering what one has
experienced or in anticipating what one is going to experience. Thus
all pleasure of this kind must be produced by sensible things: and
since the presence in any one of moral defect or excellence involves
the presence in him of pleasure or pain (with which moral excellence
and defect are always concerned), and these pleasures and pains are
alterations of the sensitive part, it is evident that the loss and
acquisition of these states no less than the loss and acquisition of
the states of the body must be the result of the alteration of
something else. Consequently, though their becoming is accompanied
by an alteration, they are not themselves alterations.
  Again, the states of the intellectual part of the soul are not
alterations, nor is there any becoming of them. In the first place
it is much more true of the possession of knowledge that it depends
upon a particular relation. And further, it is evident that there is
no becoming of these states. For that which is potentially possessed
of knowledge becomes actually possessed of it not by being set in
motion at all itself but by reason of the presence of something
else: i.e. it is when it meets with the particular object that it
knows in a manner the particular through its knowledge of the
universal. (Again, there is no becoming of the actual use and activity
of these states, unless it is thought that there is a becoming of
vision and touching and that the activity in question is similar to
these.) And the original acquisition of knowledge is not a becoming or
an alteration: for the terms 'knowing' and 'understanding' imply
that the intellect has reached a state of rest and come to a
standstill, and there is no becoming that leads to a state of rest,
since, as we have said above, change at all can have a becoming.
Moreover, just as to say, when any one has passed from a state of
intoxication or sleep or disease to the contrary state, that he has
become possessed of knowledge again is incorrect in spite of the
fact that he was previously incapable of using his knowledge, so, too,
when any one originally acquires the state, it is incorrect to say
that he becomes possessed of knowledge: for the possession of
understanding and knowledge is produced by the soul's settling down
out of the restlessness natural to it. Hence, too, in learning and
in forming judgements on matters relating to their sense-perceptions
children are inferior to adults owing to the great amount of
restlessness and motion in their souls. Nature itself causes the
soul to settle down and come to a state of rest for the performance of
some of its functions, while for the performance of others other
things do so: but in either case the result is brought about through
the alteration of something in the body, as we see in the case of
the use and activity of the intellect arising from a man's becoming
sober or being awakened. It is evident, then, from the preceding
argument that alteration and being altered occur in sensible things
and in the sensitive part of the soul, and, except accidentally, in
nothing else.

  A difficulty may be raised as to whether every motion is
commensurable with every other or not. Now if they are all
commensurable and if two things to have the same velocity must
accomplish an equal motion in an equal time, then we may have a
circumference equal to a straight line, or, of course, the one may
be greater or less than the other. Further, if one thing alters and
another accomplishes a locomotion in an equal time, we may have an
alteration and a locomotion equal to one another: thus an affection
will be equal to a length, which is impossible. But is it not only
when an equal motion is accomplished by two things in an equal time
that the velocities of the two are equal? Now an affection cannot be
equal to a length. Therefore there cannot be an alteration equal to or
less than a locomotion: and consequently it is not the case that every
motion is commensurable with every other.
  But how will our conclusion work out in the case of the circle and
the straight line? It would be absurd to suppose that the motion of
one in a circle and of another in a straight line cannot be similar,
but that the one must inevitably move more quickly or more slowly than
the other, just as if the course of one were downhill and of the other
uphill. Moreover it does not as a matter of fact make any difference
to the argument to say that the one motion must inevitably be
quicker or slower than the other: for then the circumference can be
greater or less than the straight line; and if so it is possible for
the two to be equal. For if in the time A the quicker (B) passes
over the distance B' and the slower (G) passes over the distance G',
B' will be greater than G': for this is what we took 'quicker' to
mean: and so quicker motion also implies that one thing traverses an
equal distance in less time than another: consequently there will be a
part of A in which B will pass over a part of the circle equal to
G', while G will occupy the whole of A in passing over G'. None the
less, if the two motions are commensurable, we are confronted with the
consequence stated above, viz. that there may be a straight line equal
to a circle. But these are not commensurable: and so the corresponding
motions are not commensurable either.
  But may we say that things are always commensurable if the same
terms are applied to them without equivocation? e.g. a pen, a wine,
and the highest note in a scale are not commensurable: we cannot say
whether any one of them is sharper than any other: and why is this?
they are incommensurable because it is only equivocally that the
same term 'sharp' is applied to them: whereas the highest note in a
scale is commensurable with the leading-note, because the term 'sharp'
has the same meaning as applied to both. Can it be, then, that the
term 'quick' has not the same meaning as applied to straight motion
and to circular motion respectively? If so, far less will it have
the same meaning as applied to alteration and to locomotion.
  Or shall we in the first place deny that things are always
commensurable if the same terms are applied to them without
equivocation? For the term 'much' has the same meaning whether applied
to water or to air, yet water and air are not commensurable in respect
of it: or, if this illustration is not considered satisfactory,
'double' at any rate would seem to have the same meaning as applied to
each (denoting in each case the proportion of two to one), yet water
and air are not commensurable in respect of it. But here again may
we not take up the same position and say that the term 'much' is
equivocal? In fact there are some terms of which even the
definitions are equivocal; e.g. if 'much' were defined as 'so much and
more','so much' would mean something different in different cases:
'equal' is similarly equivocal; and 'one' again is perhaps
inevitably an equivocal term; and if 'one' is equivocal, so is
'two'. Otherwise why is it that some things are commensurable while
others are not, if the nature of the attribute in the two cases is
really one and the same?
  Can it be that the incommensurability of two things in respect of
any attribute is due to a difference in that which is primarily
capable of carrying the attribute? Thus horse and dog are so
commensurable that we may say which is the whiter, since that which
primarily contains the whiteness is the same in both, viz. the
surface: and similarly they are commensurable in respect of size.
But water and speech are not commensurable in respect of clearness,
since that which primarily contains the attribute is different in
the two cases. It would seem, however that we must reject this
solution, since clearly we could thus make all equivocal attributes
univocal and say merely that that contains each of them is different
in different cases: thus 'equality', 'sweetness', and 'whiteness' will
severally always be the same, though that which contains them is
different in different cases. Moreover, it is not any casual thing
that is capable of carrying any attribute: each single attribute can
be carried primarily only by one single thing.
  Must we then say that, if two things are to be commensurable in
respect of any attribute, not only must the attribute in question be
applicable to both without equivocation, but there must also be no
specific differences either in the attribute itself or in that which
contains the attribute-that these, I mean, must not be divisible in
the way in which colour is divided into kinds? Thus in this respect
one thing will not be commensurable with another, i.e. we cannot say
that one is more coloured than the other where only colour in
general and not any particular colour is meant; but they are
commensurable in respect of whiteness.
  Similarly in the case of motion: two things are of the same velocity
if they occupy an equal time in accomplishing a certain equal amount
of motion. Suppose, then, that in a certain time an alteration is
undergone by one half of a body's length and a locomotion is
accomplished the other half: can be say that in this case the
alteration is equal to the locomotion and of the same velocity? That
would be absurd, and the reason is that there are different species of
motion. And if in consequence of this we must say that two things
are of equal velocity if they accomplish locomotion over an equal
distance in an equal time, we have to admit the equality of a straight
line and a circumference. What, then, is the reason of this? Is it
that locomotion is a genus or that line is a genus? (We may leave
the time out of account, since that is one and the same.) If the lines
are specifically different, the locomotions also differ specifically
from one another: for locomotion is specifically differentiated
according to the specific differentiation of that over which it
takes place. (It is also similarly differentiated, it would seem,
accordingly as the instrument of the locomotion is different: thus
if feet are the instrument, it is walking, if wings it is flying;
but perhaps we should rather say that this is not so, and that in this
case the differences in the locomotion are merely differences of
posture in that which is in motion.) We may say, therefore, that
things are of equal velocity in an equal time they traverse the same
magnitude: and when I call it 'the same' I mean that it contains no
specific difference and therefore no difference in the motion that
takes place over it. So we have now to consider how motion is
differentiated: and this discussion serves to show that the genus is
not a unity but contains a plurality latent in it and distinct from
it, and that in the case of equivocal terms sometimes the different
senses in which they are used are far removed from one another,
while sometimes there is a certain likeness between them, and
sometimes again they are nearly related either generically or
analogically, with the result that they seem not to be equivocal
though they really are.
  When, then, is there a difference of species? Is an attribute
specifically different if the subject is different while the attribute
is the same, or must the attribute itself be different as well? And
how are we to define the limits of a species? What will enable us to
decide that particular instances of whiteness or sweetness are the
same or different? Is it enough that it appears different in one
subject from what appears in another? Or must there be no sameness
at all? And further, where alteration is in question, how is one
alteration to be of equal velocity with another? One person may be
cured quickly and another slowly, and cures may also be
simultaneous: so that, recovery of health being an alteration, we have
here alterations of equal velocity, since each alteration occupies
an equal time. But what alteration? We cannot here speak of an 'equal'
alteration: what corresponds in the category of quality to equality in
the category of quantity is 'likeness'. However, let us say that there
is equal velocity where the same change is accomplished in an equal
time. Are we, then, to find the commensurability in the subject of the
affection or in the affection itself? In the case that we have just
been considering it is the fact that health is one and the same that
enables us to arrive at the conclusion that the one alteration is
neither more nor less than the other, but that both are alike. If on
the other hand the affection is different in the two cases, e.g.
when the alterations take the form of becoming white and becoming
healthy respectively, here there is no sameness or equality or
likeness inasmuch as the difference in the affections at once makes
the alterations specifically different, and there is no unity of
alteration any more than there would be unity of locomotion under like
conditions. So we must find out how many species there are of
alteration and of locomotion respectively. Now if the things that
are in motion-that is to say, the things to which the motions belong
essentially and not accidentally-differ specifically, then their
respective motions will also differ specifically: if on the other hand
they differ generically or numerically, the motions also will differ
generically or numerically as the case may be. But there still remains
the question whether, supposing that two alterations are of equal
velocity, we ought to look for this equality in the sameness (or
likeness) of the affections, or in the things altered, to see e.g.
whether a certain quantity of each has become white. Or ought we not
rather to look for it in both? That is to say, the alterations are the
same or different according as the affections are the same or
different, while they are equal or unequal according as the things
altered are equal or unequal.
  And now we must consider the same question in the case of becoming
and perishing: how is one becoming of equal velocity with another?
They are of equal velocity if in an equal time there are produced
two things that are the same and specifically inseparable, e.g. two
men (not merely generically inseparable as e.g. two animals).
Similarly one is quicker than the other if in an equal time the
product is different in the two cases. I state it thus because we have
no pair of terms that will convey this 'difference' in the way in
which unlikeness is conveyed. If we adopt the theory that it is number
that constitutes being, we may indeed speak of a 'greater number'
and a 'lesser number' within the same species, but there is no
common term that will include both relations, nor are there terms to
express each of them separately in the same way as we indicate a
higher degree or preponderance of an affection by 'more', of a
quantity by 'greater.'

  Now since wherever there is a movent, its motion always acts upon
something, is always in something, and always extends to something (by
'is always in something' I mean that it occupies a time: and by
'extends to something' I mean that it involves the traversing of a
certain amount of distance: for at any moment when a thing is
causing motion, it also has caused motion, so that there must always
be a certain amount of distance that has been traversed and a
certain amount of time that has been occupied). then, A the movement
have moved B a distance G in a time D, then in the same time the
same force A will move 1/2B twice the distance G, and in 1/2D it
will move 1/2B the whole distance for G: thus the rules of
proportion will be observed. Again if a given force move a given
weight a certain distance in a certain time and half the distance in
half the time, half the motive power will move half the weight the
same distance in the same time. Let E represent half the motive
power A and Z half the weight B: then the ratio between the motive
power and the weight in the one case is similar and proportionate to
the ratio in the other, so that each force will cause the same
distance to be traversed in the same time. But if E move Z a
distance G in a time D, it does not necessarily follow that E can move
twice Z half the distance G in the same time. If, then, A move B a
distance G in a time D, it does not follow that E, being half of A,
will in the time D or in any fraction of it cause B to traverse a part
of G the ratio between which and the whole of G is proportionate to
that between A and E (whatever fraction of AE may be): in fact it
might well be that it will cause no motion at all; for it does not
follow that, if a given motive power causes a certain amount of
motion, half that power will cause motion either of any particular
amount or in any length of time: otherwise one man might move a
ship, since both the motive power of the ship-haulers and the distance
that they all cause the ship to traverse are divisible into as many
parts as there are men. Hence Zeno's reasoning is false when he argues
that there is no part of the millet that does not make a sound: for
there is no reason why any such part should not in any length of
time fail to move the air that the whole bushel moves in falling. In
fact it does not of itself move even such a quantity of the air as
it would move if this part were by itself: for no part even exists
otherwise than potentially.
  If on the other hand we have two forces each of which separately
moves one of two weights a given distance in a given time, then the
forces in combination will move the combined weights an equal distance
in an equal time: for in this case the rules of proportion apply.
  Then does this hold good of alteration and of increase also?
Surely it does, for in any given case we have a definite thing that
cause increase and a definite thing that suffers increase, and the one
causes and the other suffers a certain amount of increase in a certain
amount of time. Similarly we have a definite thing that causes
alteration and a definite thing that undergoes alteration, and a
certain amount, or rather degree, of alteration is completed in a
certain amount of time: thus in twice as much time twice as much
alteration will be completed and conversely twice as much alteration
will occupy twice as much time: and the alteration of half of its
object will occupy half as much time and in half as much time half
of the object will be altered: or again, in the same amount of time it
will be altered twice as much.
  On the other hand if that which causes alteration or increase causes
a certain amount of increase or alteration respectively in a certain
amount of time, it does not necessarily follow that half the force
will occupy twice the time in altering or increasing the object, or
that in twice the time the alteration or increase will be completed by
it: it may happen that there will be no alteration or increase at all,
the case being the same as with the weight.
                              Book VIII

  IT remains to consider the following question. Was there ever a
becoming of motion before which it had no being, and is it perishing
again so as to leave nothing in motion? Or are we to say that it never
had any becoming and is not perishing, but always was and always
will be? Is it in fact an immortal never-failing property of things
that are, a sort of life as it were to all naturally constituted
  Now the existence of motion is asserted by all who have anything
to say about nature, because they all concern themselves with the
construction of the world and study the question of becoming and
perishing, which processes could not come about without the
existence of motion. But those who say that there is an infinite
number of worlds, some of which are in process of becoming while
others are in process of perishing, assert that there is always motion
(for these processes of becoming and perishing of the worlds
necessarily involve motion), whereas those who hold that there is only
one world, whether everlasting or not, make corresponding
assumptions in regard to motion. If then it is possible that at any
time nothing should be in motion, this must come about in one of two
ways: either in the manner described by Anaxagoras, who says that
all things were together and at rest for an infinite period of time,
and that then Mind introduced motion and separated them; or in the
manner described by Empedocles, according to whom the universe is
alternately in motion and at rest-in motion, when Love is making the
one out of many, or Strife is making many out of one, and at rest in
the intermediate periods of time-his account being as follows:

        'Since One hath learned to spring from Manifold,
        And One disjoined makes manifold arise,
        Thus they Become, nor stable is their life:
        But since their motion must alternate be,
        Thus have they ever Rest upon their round':

for we must suppose that he means by this that they alternate from the
one motion to the other. We must consider, then, how this matter
stands, for the discovery of the truth about it is of importance,
not only for the study of nature, but also for the investigation of
the First Principle.
  Let us take our start from what we have already laid down in our
course on Physics. Motion, we say, is the fulfilment of the movable in
so far as it is movable. Each kind of motion, therefore, necessarily
involves the presence of the things that are capable of that motion.
In fact, even apart from the definition of motion, every one would
admit that in each kind of motion it is that which is capable of
that motion that is in motion: thus it is that which is capable of
alteration that is altered, and that which is capable of local
change that is in locomotion: and so there must be something capable
of being burned before there can be a process of being burned, and
something capable of burning before there can be a process of burning.
Moreover, these things also must either have a beginning before
which they had no being, or they must be eternal. Now if there was a
becoming of every movable thing, it follows that before the motion
in question another change or motion must have taken place in which
that which was capable of being moved or of causing motion had its
becoming. To suppose, on the other hand, that these things were in
being throughout all previous time without there being any motion
appears unreasonable on a moment's thought, and still more
unreasonable, we shall find, on further consideration. For if we are
to say that, while there are on the one hand things that are
movable, and on the other hand things that are motive, there is a time
when there is a first movent and a first moved, and another time
when there is no such thing but only something that is at rest, then
this thing that is at rest must previously have been in process of
change: for there must have been some cause of its rest, rest being
the privation of motion. Therefore, before this first change there
will be a previous change. For some things cause motion in only one
way, while others can produce either of two contrary motions: thus
fire causes heating but not cooling, whereas it would seem that
knowledge may be directed to two contrary ends while remaining one and
the same. Even in the former class, however, there seems to be
something similar, for a cold thing in a sense causes heating by
turning away and retiring, just as one possessed of knowledge
voluntarily makes an error when he uses his knowledge in the reverse
way. But at any rate all things that are capable respectively of
affecting and being affected, or of causing motion and being moved,
are capable of it not under all conditions, but only when they are
in a particular condition and approach one another: so it is on the
approach of one thing to another that the one causes motion and the
other is moved, and when they are present under such conditions as
rendered the one motive and the other movable. So if the motion was
not always in process, it is clear that they must have been in a
condition not such as to render them capable respectively of being
moved and of causing motion, and one or other of them must have been
in process of change: for in what is relative this is a necessary
consequence: e.g. if one thing is double another when before it was
not so, one or other of them, if not both, must have been in process
of change. It follows then, that there will be a process of change
previous to the first.
  (Further, how can there be any 'before' and 'after' without the
existence of time? Or how can there be any time without the
existence of motion? If, then, time is the number of motion or
itself a kind of motion, it follows that, if there is always time,
motion must also be eternal. But so far as time is concerned we see
that all with one exception are in agreement in saying that it is
uncreated: in fact, it is just this that enables Democritus to show
that all things cannot have had a becoming: for time, he says, is
uncreated. Plato alone asserts the creation of time, saying that it
had a becoming together with the universe, the universe according to
him having had a becoming. Now since time cannot exist and is
unthinkable apart from the moment, and the moment a kind of
middle-point, uniting as it does in itself both a beginning and an
end, a beginning of future time and an end of past time, it follows
that there must always be time: for the extremity of the last period
of time that we take must be found in some moment, since time contains
no point of contact for us except the moment. Therefore, since the
moment is both a beginning and an end, there must always be time on
both sides of it. But if this is true of time, it is evident that it
must also be true of motion, time being a kind of affection of
  The same reasoning will also serve to show the imperishability of
motion: just as a becoming of motion would involve, as we saw, the
existence of a process of change previous to the first, in the same
way a perishing of motion would involve the existence of a process
of change subsequent to the last: for when a thing ceases to be moved,
it does not therefore at the same time cease to be movable-e.g. the
cessation of the process of being burned does not involve the
cessation of the capacity of being burned, since a thing may be
capable of being burned without being in process of being
burned-nor, when a thing ceases to be movent, does it therefore at the
same time cease to a be motive. Again, the destructive agent will have
to be destroyed, after what it destroys has been destroyed, and then
that which has the capacity of destroying it will have to be destroyed
afterwards, (so that there will be a process of change subsequent to
the last,) for being destroyed also is a kind of change. If, then,
view which we are criticizing involves these impossible
consequences, it is clear that motion is eternal and cannot have
existed at one time and not at another: in fact such a view can hardly
be described as anythling else than fantastic.
  And much the same may be said of the view that such is the ordinance
of nature and that this must be regarded as a principle, as would seem
to be the view of Empedocles when he says that the constitution of the
world is of necessity such that Love and Strife alternately
predominate and cause motion, while in the intermediate period of time
there is a state of rest. Probably also those who like like
Anaxagoras, assert a single principle (of motion) would hold this
view. But that which is produced or directed by nature can never be
anything disorderly: for nature is everywhere the cause of order.
Moreover, there is no ratio in the relation of the infinite to the
infinite, whereas order always means ratio. But if we say that there
is first a state of rest for an infinite time, and then motion is
started at some moment, and that the fact that it is this rather
than a previous moment is of no importance, and involves no order,
then we can no longer say that it is nature's work: for if anything is
of a certain character naturally, it either is so invariably and is
not sometimes of this and sometimes of another character (e.g. fire,
which travels upwards naturally, does not sometimes do so and
sometimes not) or there is a ratio in the variation. It would be
better, therefore, to say with Empedocles and any one else who may
have maintained such a theory as his that the universe is
alternately at rest and in motion: for in a system of this kind we
have at once a certain order. But even here the holder of the theory
ought not only to assert the fact: he ought to explain the cause of
it: i.e. he should not make any mere assumption or lay down any
gratuitous axiom, but should employ either inductive or
demonstrative reasoning. The Love and Strife postulated by
Empedocles are not in themselves causes of the fact in question, nor
is it of the essence of either that it should be so, the essential
function of the former being to unite, of the latter to separate. If
he is to go on to explain this alternate predominance, he should
adduce cases where such a state of things exists, as he points to
the fact that among mankind we have something that unites men,
namely Love, while on the other hand enemies avoid one another: thus
from the observed fact that this occurs in certain cases comes the
assumption that it occurs also in the universe. Then, again, some
argument is needed to explain why the predominance of each of the
two forces lasts for an equal period of time. But it is a wrong
assumption to suppose universally that we have an adequate first
principle in virtue of the fact that something always is so or
always happens so. Thus Democritus reduces the causes that explain
nature to the fact that things happened in the past in the same way as
they happen now: but he does not think fit to seek for a first
principle to explain this 'always': so, while his theory is right in
so far as it is applied to certain individual cases, he is wrong in
making it of universal application. Thus, a triangle always has its
angles equal to two right angles, but there is nevertheless an
ulterior cause of the eternity of this truth, whereas first principles
are eternal and have no ulterior cause. Let this conclude what we have
to say in support of our contention that there never was a time when
there was not motion, and never will be a time when there will not
be motion.

  The arguments that may be advanced against this position are not
difficult to dispose of. The chief considerations that might be
thought to indicate that motion may exist though at one time it had
not existed at all are the following:
    First, it may be said that no process of change is eternal: for
the nature of all change is such that it proceeds from something to
something, so that every process of change must be bounded by the
contraries that mark its course, and no motion can go on to infinity.
  Secondly, we see that a thing that neither is in motion nor contains
any motion within itself can be set in motion; e.g. inanimate things
that are (whether the whole or some part is in question) not in motion
but at rest, are at some moment set in motion: whereas, if motion
cannot have a becoming before which it had no being, these things
ought to be either always or never in motion.
  Thirdly, the fact is evident above all in the case of animate
beings: for it sometimes happens that there is no motion in us and
we are quite still, and that nevertheless we are then at some moment
set in motion, that is to say it sometimes happens that we produce a
beginning of motion in ourselves spontaneously without anything having
set us in motion from without. We see nothing like this in the case of
inanimate things, which are always set in motion by something else
from without: the animal, on the other hand, we say, moves itself:
therefore, if an animal is ever in a state of absolute rest, we have a
motionless thing in which motion can be produced from the thing
itself, and not from without. Now if this can occur in an animal,
why should not the same be true also of the universe as a whole? If it
can occur in a small world it could also occur in a great one: and
if it can occur in the world, it could also occur in the infinite;
that is, if the infinite could as a whole possibly be in motion or
at rest.
  Of these objections, then, the first-mentioned motion to opposites
is not always the same and numerically one a correct statement; in
fact, this may be said to be a necessary conclusion, provided that
it is possible for the motion of that which is one and the same to
be not always one and the same. (I mean that e.g. we may question
whether the note given by a single string is one and the same, or is
different each time the string is struck, although the string is in
the same condition and is moved in the same way.) But still, however
this may be, there is nothing to prevent there being a motion that
is the same in virtue of being continuous and eternal: we shall have
something to say later that will make this point clearer.
  As regards the second objection, no absurdity is involved in the
fact that something not in motion may be set in motion, that which
caused the motion from without being at one time present, and at
another absent. Nevertheless, how this can be so remains matter for
inquiry; how it comes about, I mean, that the same motive force at one
time causes a thing to be in motion, and at another does not do so:
for the difficulty raised by our objector really amounts to this-why
is it that some things are not always at rest, and the rest always
in motion?
  The third objection may be thought to present more difficulty than
the others, namely, that which alleges that motion arises in things in
which it did not exist before, and adduces in proof the case of
animate things: thus an animal is first at rest and afterwards
walks, not having been set in motion apparently by anything from
without. This, however, is false: for we observe that there is
always some part of the animal's organism in motion, and the cause
of the motion of this part is not the animal itself, but, it may be,
its environment. Moreover, we say that the animal itself originates
not all of its motions but its locomotion. So it may well be the
case-or rather we may perhaps say that it must necessarily be the
case-that many motions are produced in the body by its environment,
and some of these set in motion the intellect or the appetite, and
this again then sets the whole animal in motion: this is what
happens when animals are asleep: though there is then no perceptive
motion in them, there is some motion that causes them to wake up
again. But we will leave this point also to be elucidated at a later
stage in our discussion.

  Our enquiry will resolve itself at the outset into a consideration
of the above-mentioned problem-what can be the reason why some
things in the world at one time are in motion and at another are at
rest again? Now one of three things must be true: either all things
are always at rest, or all things are always in motion, or some things
are in motion and others at rest: and in this last case again either
the things that are in motion are always in motion and the things that
are at rest are always at rest, or they are all constituted so as to
be capable alike of motion and of rest; or there is yet a third
possibility remaining-it may be that some things in the world are
always motionless, others always in motion, while others again admit
of both conditions. This last is the account of the matter that we
must give: for herein lies the solution of all the difficulties raised
and the conclusion of the investigation upon which we are engaged.
  To maintain that all things are at rest, and to disregard
sense-perception in an attempt to show the theory to be reasonable,
would be an instance of intellectual weakness: it would call in
question a whole system, not a particular detail: moreover, it would
be an attack not only on the physicist but on almost all sciences
and all received opinions, since motion plays a part in all of them.
Further, just as in arguments about mathematics objections that
involve first principles do not affect the mathematician-and the other
sciences are in similar case-so, too, objections involving the point
that we have just raised do not affect the physicist: for it is a
fundamental assumption with him that motion is ultimately referable to
nature herself.
  The assertion that all things are in motion we may fairly regard
as equally false, though it is less subversive of physical science:
for though in our course on physics it was laid down that rest no less
than motion is ultimately referable to nature herself, nevertheless
motion is the characteristic fact of nature: moreover, the view is
actually held by some that not merely some things but all things in
the world are in motion and always in motion, though we cannot
apprehend the fact by sense-perception. Although the supporters of
this theory do not state clearly what kind of motion they mean, or
whether they mean all kinds, it is no hard matter to reply to them:
thus we may point out that there cannot be a continuous process either
of increase or of decrease: that which comes between the two has to be
included. The theory resembles that about the stone being worn away by
the drop of water or split by plants growing out of it: if so much has
been extruded or removed by the drop, it does not follow that half the
amount has previously been extruded or removed in half the time: the
case of the hauled ship is exactly comparable: here we have so many
drops setting so much in motion, but a part of them will not set as
much in motion in any period of time. The amount removed is, it is
true, divisible into a number of parts, but no one of these was set in
motion separately: they were all set in motion together. It is
evident, then, that from the fact that the decrease is divisible
into an infinite number of parts it does not follow that some part
must always be passing away: it all passes away at a particular
moment. Similarly, too, in the case of any alteration whatever if that
which suffers alteration is infinitely divisible it does not follow
from this that the same is true of the alteration itself, which
often occurs all at once, as in freezing. Again, when any one has
fallen ill, there must follow a period of time in which his
restoration to health is in the future: the process of change cannot
take place in an instant: yet the change cannot be a change to
anything else but health. The assertion. therefore, that alteration is
continuous is an extravagant calling into question of the obvious: for
alteration is a change from one contrary to another. Moreover, we
notice that a stone becomes neither harder nor softer. Again, in the
matter of locomotion, it would be a strange thing if a stone could
be falling or resting on the ground without our being able to perceive
the fact. Further, it is a law of nature that earth and all other
bodies should remain in their proper places and be moved from them
only by violence: from the fact then that some of them are in their
proper places it follows that in respect of place also all things
cannot be in motion. These and other similar arguments, then, should
convince us that it is impossible either that all things are always in
motion or that all things are always at rest.
  Nor again can it be that some things are always at rest, others
always in motion, and nothing sometimes at rest and sometimes in
motion. This theory must be pronounced impossible on the same
grounds as those previously mentioned: viz. that we see the
above-mentioned changes occurring in the case of the same things. We
may further point out that the defender of this position is fighting
against the obvious, for on this theory there can be no such thing
as increase: nor can there be any such thing as compulsory motion,
if it is impossible that a thing can be at rest before being set in
motion unnaturally. This theory, then, does away with becoming and
perishing. Moreover, motion, it would seem, is generally thought to be
a sort of becoming and perishing, for that to which a thing changes
comes to be, or occupancy of it comes to be, and that from which a
thing changes ceases to be, or there ceases to be occupancy of it.
It is clear, therefore, that there are cases of occasional motion
and occasional rest.
  We have now to take the assertion that all things are sometimes at
rest and sometimes in motion and to confront it with the arguments
previously advanced. We must take our start as before from the
possibilities that we distinguished just above. Either all things
are at rest, or all things are in motion, or some things are at rest
and others in motion. And if some things are at rest and others in
motion, then it must be that either all things are sometimes at rest
and sometimes in motion, or some things are always at rest and the
remainder always in motion, or some of the things are always at rest
and others always in motion while others again are sometimes at rest
and sometimes in motion. Now we have said before that it is impossible
that all things should be at rest: nevertheless we may now repeat that
assertion. We may point out that, even if it is really the case, as
certain persons assert, that the existent is infinite and
motionless, it certainly does not appear to be so if we follow
sense-perception: many things that exist appear to be in motion. Now
if there is such a thing as false opinion or opinion at all, there
is also motion; and similarly if there is such a thing as imagination,
or if it is the case that anything seems to be different at
different times: for imagination and opinion are thought to be motions
of a kind. But to investigate this question at all-to seek a
reasoned justification of a belief with regard to which we are too
well off to require reasoned justification-implies bad judgement of
what is better and what is worse, what commends itself to belief and
what does not, what is ultimate and what is not. It is likewise
impossible that all things should be in motion or that some things
should be always in motion and the remainder always at rest. We have
sufficient ground for rejecting all these theories in the single
fact that we see some things that are sometimes in motion and
sometimes at rest. It is evident, therefore, that it is no less
impossible that some things should be always in motion and the
remainder always at rest than that all things should be at rest or
that all things should be in motion continuously. It remains, then, to
consider whether all things are so constituted as to be capable both
of being in motion and of being at rest, or whether, while some things
are so constituted, some are always at rest and some are always in
motion: for it is this last view that we have to show to be true.

  Now of things that cause motion or suffer motion, to some the motion
is accidental, to others essential: thus it is accidental to what
merely belongs to or contains as a part a thing that causes motion
or suffers motion, essential to a thing that causes motion or
suffers motion not merely by belonging to such a thing or containing
it as a part.
  Of things to which the motion is essential some derive their
motion from themselves, others from something else: and in some
cases their motion is natural, in others violent and unnatural. Thus
in things that derive their motion from themselves, e.g. all
animals, the motion is natural (for when an animal is in motion its
motion is derived from itself): and whenever the source of the
motion of a thing is in the thing itself we say that the motion of
that thing is natural. Therefore the animal as a whole moves itself
naturally: but the body of the animal may be in motion unnaturally
as well as naturally: it depends upon the kind of motion that it may
chance to be suffering and the kind of element of which it is
composed. And the motion of things that derive their motion from
something else is in some cases natural, in other unnatural: e.g.
upward motion of earthy things and downward motion of fire are
unnatural. Moreover the parts of animals are often in motion in an
unnatural way, their positions and the character of the motion being
abnormal. The fact that a thing that is in motion derives its motion
from something is most evident in things that are in motion
unnaturally, because in such cases it is clear that the motion is
derived from something other than the thing itself. Next to things
that are in motion unnaturally those whose motion while natural is
derived from themselves-e.g. animals-make this fact clear: for here
the uncertainty is not as to whether the motion is derived from
something but as to how we ought to distinguish in the thing between
the movent and the moved. It would seem that in animals, just as in
ships and things not naturally organized, that which causes motion
is separate from that which suffers motion, and that it is only in
this sense that the animal as a whole causes its own motion.
  The greatest difficulty, however, is presented by the remaining case
of those that we last distinguished. Where things derive their
motion from something else we distinguished the cases in which the
motion is unnatural: we are left with those that are to be
contrasted with the others by reason of the fact that the motion is
natural. It is in these cases that difficulty would be experienced
in deciding whence the motion is derived, e.g. in the case of light
and heavy things. When these things are in motion to positions the
reverse of those they would properly occupy, their motion is
violent: when they are in motion to their proper positions-the light
thing up and the heavy thing down-their motion is natural; but in this
latter case it is no longer evident, as it is when the motion is
unnatural, whence their motion is derived. It is impossible to say
that their motion is derived from themselves: this is a characteristic
of life and peculiar to living things. Further, if it were, it would
have been in their power to stop themselves (I mean that if e.g. a
thing can cause itself to walk it can also cause itself not to
walk), and so, since on this supposition fire itself possesses the
power of upward locomotion, it is clear that it should also possess
the power of downward locomotion. Moreover if things move
themselves, it would be unreasonable to suppose that in only one
kind of motion is their motion derived from themselves. Again, how can
anything of continuous and naturally connected substance move
itself? In so far as a thing is one and continuous not merely in
virtue of contact, it is impassive: it is only in so far as a thing is
divided that one part of it is by nature active and another passive.
Therefore none of the things that we are now considering move
themselves (for they are of naturally connected substance), nor does
anything else that is continuous: in each case the movent must be
separate from the moved, as we see to be the case with inanimate
things when an animate thing moves them. It is the fact that these
things also always derive their motion from something: what it is
would become evident if we were to distinguish the different kinds
of cause.
  The above-mentioned distinctions can also be made in the case of
things that cause motion: some of them are capable of causing motion
unnaturally (e.g. the lever is not naturally capable of moving the
weight), others naturally (e.g. what is actually hot is naturally
capable of moving what is potentially hot): and similarly in the
case of all other things of this kind.
  In the same way, too, what is potentially of a certain quality or of
a certain quantity in a certain place is naturally movable when it
contains the corresponding principle in itself and not accidentally
(for the same thing may be both of a certain quality and of a
certain quantity, but the one is an accidental, not an essential
property of the other). So when fire or earth is moved by something
the motion is violent when it is unnatural, and natural when it brings
to actuality the proper activities that they potentially possess.
But the fact that the term 'potentially' is used in more than one
sense is the reason why it is not evident whence such motions as the
upward motion of fire and the downward motion of earth are derived.
One who is learning a science potentially knows it in a different
sense from one who while already possessing the knowledge is not
actually exercising it. Wherever we have something capable of acting
and something capable of being correspondingly acted on, in the
event of any such pair being in contact what is potential becomes at
times actual: e.g. the learner becomes from one potential something
another potential something: for one who possesses knowledge of a
science but is not actually exercising it knows the science
potentially in a sense, though not in the same sense as he knew it
potentially before he learnt it. And when he is in this condition,
if something does not prevent him, he actively exercises his
knowledge: otherwise he would be in the contradictory state of not
knowing. In regard to natural bodies also the case is similar. Thus
what is cold is potentially hot: then a change takes place and it is
fire, and it burns, unless something prevents and hinders it. So, too,
with heavy and light: light is generated from heavy, e.g. air from
water (for water is the first thing that is potentially light), and
air is actually light, and will at once realize its proper activity as
such unless something prevents it. The activity of lightness
consists in the light thing being in a certain situation, namely
high up: when it is in the contrary situation, it is being prevented
from rising. The case is similar also in regard to quantity and
quality. But, be it noted, this is the question we are trying to
answer-how can we account for the motion of light things and heavy
things to their proper situations? The reason for it is that they have
a natural tendency respectively towards a certain position: and this
constitutes the essence of lightness and heaviness, the former being
determined by an upward, the latter by a downward, tendency. As we
have said, a thing may be potentially light or heavy in more senses
than one. Thus not only when a thing is water is it in a sense
potentially light, but when it has become air it may be still
potentially light: for it may be that through some hindrance it does
not occupy an upper position, whereas, if what hinders it is
removed, it realizes its activity and continues to rise higher. The
process whereby what is of a certain quality changes to a condition of
active existence is similar: thus the exercise of knowledge follows at
once upon the possession of it unless something prevents it. So,
too, what is of a certain quantity extends itself over a certain space
unless something prevents it. The thing in a sense is and in a sense
is not moved by one who moves what is obstructing and preventing its
motion (e.g. one who pulls away a pillar from under a roof or one
who removes a stone from a wineskin in the water is the accidental
cause of motion): and in the same way the real cause of the motion
of a ball rebounding from a wall is not the wall but the thrower. So
it is clear that in all these cases the thing does not move itself,
but it contains within itself the source of motion-not of moving
something or of causing motion, but of suffering it.
  If then the motion of all things that are in motion is either
natural or unnatural and violent, and all things whose motion is
violent and unnatural are moved by something, and something other than
themselves, and again all things whose motion is natural are moved
by something-both those that are moved by themselves and those that
are not moved by themselves (e.g. light things and heavy things, which
are moved either by that which brought the thing into existence as
such and made it light and heavy, or by that which released what was
hindering and preventing it); then all things that are in motion
must be moved by something.

  Now this may come about in either of two ways. Either the movent
is not itself responsible for the motion, which is to be referred to
something else which moves the movent, or the movent is itself
responsible for the motion. Further, in the latter case, either the
movent immediately precedes the last thing in the series, or there may
be one or more intermediate links: e.g. the stick moves the stone
and is moved by the hand, which again is moved by the man: in the man,
however, we have reached a movent that is not so in virtue of being
moved by something else. Now we say that the thing is moved both by
the last and by the first movent in the series, but more strictly by
the first, since the first movent moves the last, whereas the last
does not move the first, and the first will move the thing without the
last, but the last will not move it without the first: e.g. the
stick will not move anything unless it is itself moved by the man.
If then everything that is in motion must be moved by something, and
the movent must either itself be moved by something else or not, and
in the former case there must be some first movent that is not
itself moved by anything else, while in the case of the immediate
movent being of this kind there is no need of an intermediate movent
that is also moved (for it is impossible that there should be an
infinite series of movents, each of which is itself moved by something
else, since in an infinite series there is no first term)-if then
everything that is in motion is moved by something, and the first
movent is moved but not by anything else, it much be moved by itself.
  This same argument may also be stated in another way as follows.
Every movent moves something and moves it with something, either
with itself or with something else: e.g. a man moves a thing either
himself or with a stick, and a thing is knocked down either by the
wind itself or by a stone propelled by the wind. But it is
impossible for that with which a thing is moved to move it without
being moved by that which imparts motion by its own agency: on the
other hand, if a thing imparts motion by its own agency, it is not
necessary that there should be anything else with which it imparts
motion, whereas if there is a different thing with which it imparts
motion, there must be something that imparts motion not with something
else but with itself, or else there will be an infinite series. If,
then, anything is a movent while being itself moved, the series must
stop somewhere and not be infinite. Thus, if the stick moves something
in virtue of being moved by the hand, the hand moves the stick: and if
something else moves with the hand, the hand also is moved by
something different from itself. So when motion by means of an
instrument is at each stage caused by something different from the
instrument, this must always be preceded by something else which
imparts motion with itself. Therefore, if this last movent is in
motion and there is nothing else that moves it, it must move itself.
So this reasoning also shows that when a thing is moved, if it is
not moved immediately by something that moves itself, the series
brings us at some time or other to a movent of this kind.
  And if we consider the matter in yet a third wa Ly we shall get this
same result as follows. If everything that is in motion is moved by
something that is in motion, ether this being in motion is an
accidental attribute of the movents in question, so that each of
them moves something while being itself in motion, but not always
because it is itself in motion, or it is not accidental but an
essential attribute. Let us consider the former alternative. If then
it is an accidental attribute, it is not necessary that that is in
motion should be in motion: and if this is so it is clear that there
may be a time when nothing that exists is in motion, since the
accidental is not necessary but contingent. Now if we assume the
existence of a possibility, any conclusion that we thereby reach
will not be an impossibility though it may be contrary to fact. But
the nonexistence of motion is an impossibility: for we have shown
above that there must always be motion.
  Moreover, the conclusion to which we have been led is a reasonable
one. For there must be three things-the moved, the movent, and the
instrument of motion. Now the moved must be in motion, but it need not
move anything else: the instrument of motion must both move
something else and be itself in motion (for it changes together with
the moved, with which it is in contact and continuous, as is clear
in the case of things that move other things locally, in which case
the two things must up to a certain point be in contact): and the
movent-that is to say, that which causes motion in such a manner
that it is not merely the instrument of motion-must be unmoved. Now we
have visual experience of the last term in this series, namely that
which has the capacity of being in motion, but does not contain a
motive principle, and also of that which is in motion but is moved
by itself and not by anything else: it is reasonable, therefore, not
to say necessary, to suppose the existence of the third term also,
that which causes motion but is itself unmoved. So, too, Anaxagoras is
right when he says that Mind is impassive and unmixed, since he
makes it the principle of motion: for it could cause motion in this
sense only by being itself unmoved, and have supreme control only by
being unmixed.
  We will now take the second alternative. If the movement is not
accidentally but necessarily in motion-so that, if it were not in
motion, it would not move anything-then the movent, in so far as it is
in motion, must be in motion in one of two ways: it is moved either as
that is which is moved with the same kind of motion, or with a
different kind-either that which is heating, I mean, is itself in
process of becoming hot, that which is making healthy in process of
becoming healthy, and that which is causing locomotion in process of
locomotion, or else that which is making healthy is, let us say, in
process of locomotion, and that which is causing locomotion in process
of, say, increase. But it is evident that this is impossible. For if
we adopt the first assumption we have to make it apply within each
of the very lowest species into which motion can be divided: e.g. we
must say that if some one is teaching some lesson in geometry, he is
also in process of being taught that same lesson in geometry, and that
if he is throwing he is in process of being thrown in just the same
manner. Or if we reject this assumption we must say that one kind of
motion is derived from another; e.g. that that which is causing
locomotion is in process of increase, that which is causing this
increase is in process of being altered by something else, and that
which is causing this alteration is in process of suffering some
different kind of motion. But the series must stop somewhere, since
the kinds of motion are limited; and if we say that the process is
reversible, and that that which is causing alteration is in process of
locomotion, we do no more than if we had said at the outset that
that which is causing locomotion is in process of locomotion, and that
one who is teaching is in process of being taught: for it is clear
that everything that is moved is moved by the movent that is further
back in the series as well as by that which immediately moves it: in
fact the earlier movent is that which more strictly moves it. But this
is of course impossible: for it involves the consequence that one
who is teaching is in process of learning what he is teaching, whereas
teaching necessarily implies possessing knowledge, and learning not
possessing it. Still more unreasonable is the consequence involved
that, since everything that is moved is moved by something that is
itself moved by something else, everything that has a capacity for
causing motion has as such a corresponding capacity for being moved:
i.e. it will have a capacity for being moved in the sense in which one
might say that everything that has a capacity for making healthy,
and exercises that capacity, has as such a capacity for being made
healthy, and that which has a capacity for building has as such a
capacity for being built. It will have the capacity for being thus
moved either immediately or through one or more links (as it will
if, while everything that has a capacity for causing motion has as
such a capacity for being moved by something else, the motion that
it has the capacity for suffering is not that with which it affects
what is next to it, but a motion of a different kind; e.g. that
which has a capacity for making healthy might as such have a
capacity for learn. the series, however, could be traced back, as we
said before, until at some time or other we arrived at the same kind
of motion). Now the first alternative is impossible, and the second is
fantastic: it is absurd that that which has a capacity for causing
alteration should as such necessarily have a capacity, let us say, for
increase. It is not necessary, therefore, that that which is moved
should always be moved by something else that is itself moved by
something else: so there will be an end to the series. Consequently
the first thing that is in motion will derive its motion either from
something that is at rest or from itself. But if there were any need
to consider which of the two, that which moves itself or that which is
moved by something else, is the cause and principle of motion, every
one would decide the former: for that which is itself independently
a cause is always prior as a cause to that which is so only in
virtue of being itself dependent upon something else that makes it so.
  We must therefore make a fresh start and consider the question; if a
thing moves itself, in what sense and in what manner does it do so?
Now everything that is in motion must be infinitely divisible, for
it has been shown already in our general course on Physics, that
everything that is essentially in motion is continuous. Now it is
impossible that that which moves itself should in its entirety move
itself: for then, while being specifically one and indivisible, it
would as a Whole both undergo and cause the same locomotion or
alteration: thus it would at the same time be both teaching and
being taught (the same thing), or both restoring to and being restored
to the same health. Moreover, we have established the fact that it
is the movable that is moved; and this is potentially, not actually,
in motion, but the potential is in process to actuality, and motion is
an incomplete actuality of the movable. The movent on the other hand
is already in activity: e.g. it is that which is hot that produces
heat: in fact, that which produces the form is always something that
possesses it. Consequently (if a thing can move itself as a whole),
the same thing in respect of the same thing may be at the same time
both hot and not hot. So, too, in every other case where the movent
must be described by the same name in the same sense as the moved.
Therefore when a thing moves itself it is one part of it that is the
movent and another part that is moved. But it is not self-moving in
the sense that each of the two parts is moved by the other part: the
following considerations make this evident. In the first place, if
each of the two parts is to move the other, there will be no first
movent. If a thing is moved by a series of movents, that which is
earlier in the series is more the cause of its being moved than that
which comes next, and will be more truly the movent: for we found that
there are two kinds of movent, that which is itself moved by something
else and that which derives its motion from itself: and that which
is further from the thing that is moved is nearer to the principle
of motion than that which is intermediate. In the second place,
there is no necessity for the movent part to be moved by anything
but itself: so it can only be accidentally that the other part moves
it in return. I take then the possible case of its not moving it: then
there will be a part that is moved and a part that is an unmoved
movent. In the third place, there is no necessity for the movent to be
moved in return: on the contrary the necessity that there should
always be motion makes it necessary that there should be some movent
that is either unmoved or moved by itself. In the fourth place we
should then have a thing undergoing the same motion that it is
causing-that which is producing heat, therefore, being heated. But
as a matter of fact that which primarily moves itself cannot contain
either a single part that moves itself or a number of parts each of
which moves itself. For, if the whole is moved by itself, it must be
moved either by some part of itself or as a whole by itself as a
whole. If, then, it is moved in virtue of some part of it being
moved by that part itself, it is this part that will be the primary
self-movent, since, if this part is separated from the whole, the part
will still move itself, but the whole will do so no longer. If on
the other hand the whole is moved by itself as a whole, it must be
accidentally that the parts move themselves: and therefore, their
self-motion not being necessary, we may take the case of their not
being moved by themselves. Therefore in the whole of the thing we
may distinguish that which imparts motion without itself being moved
and that which is moved: for only in this way is it possible for a
thing to be self-moved. Further, if the whole moves itself we may
distinguish in it that which imparts the motion and that which is
moved: so while we say that AB is moved by itself, we may also say
that it is moved by A. And since that which imparts motion may be
either a thing that is moved by something else or a thing that is
unmoved, and that which is moved may be either a thing that imparts
motion to something else or a thing that does not, that which moves
itself must be composed of something that is unmoved but imparts
motion and also of something that is moved but does not necessarily
impart motion but may or may not do so. Thus let A be something that
imparts motion but is unmoved, B something that is moved by A and
moves G, G something that is moved by B but moves nothing (granted
that we eventually arrive at G we may take it that there is only one
intermediate term, though there may be more). Then the whole ABG moves
itself. But if I take away G, AB will move itself, A imparting
motion and B being moved, whereas G will not move itself or in fact be
moved at all. Nor again will BG move itself apart from A: for B
imparts motion only through being moved by something else, not through
being moved by any part of itself. So only AB moves itself. That which
moves itself, therefore, must comprise something that imparts motion
but is unmoved and something that is moved but does not necessarily
move anything else: and each of these two things, or at any rate one
of them, must be in contact with the other. If, then, that which
imparts motion is a continuous substance-that which is moved must of
course be so-it is clear that it is not through some part of the whole
being of such a nature as to be capable of moving itself that the
whole moves itself: it moves itself as a whole, both being moved and
imparting motion through containing a part that imparts motion and a
part that is moved. It does not impart motion as a whole nor is it
moved as a whole: it is A alone that imparts motion and B alone that
is moved. It is not true, further, that G is moved by A, which is
  Here a difficulty arises: if something is taken away from A
(supposing that that which imparts motion but is unmoved is a
continuous substance), or from B the part that is moved, will the
remainder of A continue to impart motion or the remainder of B
continue to be moved? If so, it will not be AB primarily that is moved
by itself, since, when something is taken away from AB, the
remainder of AB will still continue to move itself. Perhaps we may
state the case thus: there is nothing to prevent each of the two
parts, or at any rate one of them, that which is moved, being
divisible though actually undivided, so that if it is divided it
will not continue in the possession of the same capacity: and so there
is nothing to prevent self-motion residing primarily in things that
are potentially divisible.
  From what has been said, then, it is evident that that which
primarily imparts motion is unmoved: for, whether the series is closed
at once by that which is in motion but moved by something else
deriving its motion directly from the first unmoved, or whether the
motion is derived from what is in motion but moves itself and stops
its own motion, on both suppositions we have the result that in all
cases of things being in motion that which primarily imparts motion is

  Since there must always be motion without intermission, there must
necessarily be something, one thing or it may be a plurality, that
first imparts motion, and this first movent must be unmoved. Now the
question whether each of the things that are unmoved but impart motion
is eternal is irrelevant to our present argument: but the following
considerations will make it clear that there must necessarily be
some such thing, which, while it has the capacity of moving
something else, is itself unmoved and exempt from all change, which
can affect it neither in an unqualified nor in an accidental sense.
Let us suppose, if any one likes, that in the case of certain things
it is possible for them at different times to be and not to be,
without any process of becoming and perishing (in fact it would seem
to be necessary, if a thing that has not parts at one time is and at
another time is not, that any such thing should without undergoing any
process of change at one time be and at another time not be). And
let us further suppose it possible that some principles that are
unmoved but capable of imparting motion at one time are and at another
time are not. Even so, this cannot be true of all such principles,
since there must clearly be something that causes things that move
themselves at one time to be and at another not to be. For, since
nothing that has not parts can be in motion, that which moves itself
must as a whole have magnitude, though nothing that we have said makes
this necessarily true of every movent. So the fact that some things
become and others perish, and that this is so continuously, cannot
be caused by any one of those things that, though they are unmoved, do
not always exist: nor again can it be caused by any of those which
move certain particular things, while others move other things. The
eternity and continuity of the process cannot be caused either by
any one of them singly or by the sum of them, because this causal
relation must be eternal and necessary, whereas the sum of these
movents is infinite and they do not all exist together. It is clear,
then, that though there may be countless instances of the perishing of
some principles that are unmoved but impart motion, and though many
things that move themselves perish and are succeeded by others that
come into being, and though one thing that is unmoved moves one
thing while another moves another, nevertheless there is something
that comprehends them all, and that as something apart from each one
of them, and this it is that is the cause of the fact that some things
are and others are not and of the continuous process of change: and
this causes the motion of the other movents, while they are the causes
of the motion of other things. Motion, then, being eternal, the
first movent, if there is but one, will be eternal also: if there
are more than one, there will be a plurality of such eternal
movents. We ought, however, to suppose that there is one rather than
many, and a finite rather than an infinite number. When the
consequences of either assumption are the same, we should always
assume that things are finite rather than infinite in number, since in
things constituted by nature that which is finite and that which is
better ought, if possible, to be present rather than the reverse:
and here it is sufficient to assume only one movent, the first of
unmoved things, which being eternal will be the principle of motion to
everything else.
  The following argument also makes it evident that the first movent
must be something that is one and eternal. We have shown that there
must always be motion. That being so, motion must also be
continuous, because what is always is continuous, whereas what is
merely in succession is not continuous. But further, if motion is
continuous, it is one: and it is one only if the movent and the
moved that constitute it are each of them one, since in the event of a
thing's being moved now by one thing and now by another the whole
motion will not be continuous but successive.
  Moreover a conviction that there is a first unmoved something may be
reached not only from the foregoing arguments, but also by considering
again the principles operative in movents. Now it is evident that
among existing things there are some that are sometimes in motion
and sometimes at rest. This fact has served above to make it clear
that it is not true either that all things are in motion or that all
things are at rest or that some things are always at rest and the
remainder always in motion: on this matter proof is supplied by things
that fluctuate between the two and have the capacity of being
sometimes in motion and sometimes at rest. The existence of things
of this kind is clear to all: but we wish to explain also the nature
of each of the other two kinds and show that there are some things
that are always unmoved and some things that are always in motion.
In the course of our argument directed to this end we established
the fact that everything that is in motion is moved by something,
and that the movent is either unmoved or in motion, and that, if it is
in motion, it is moved either by itself or by something else and so on
throughout the series: and so we proceeded to the position that the
first principle that directly causes things that are in motion to be
moved is that which moves itself, and the first principle of the whole
series is the unmoved. Further it is evident from actual observation
that there are things that have the characteristic of moving
themselves, e.g. the animal kingdom and the whole class of living
things. This being so, then, the view was suggested that perhaps it
may be possible for motion to come to be in a thing without having
been in existence at all before, because we see this actually
occurring in animals: they are unmoved at one time and then again they
are in motion, as it seems. We must grasp the fact, therefore, that
animals move themselves only with one kind of motion, and that this is
not strictly originated by them. The cause of it is not derived from
the animal itself: it is connected with other natural motions in
animals, which they do not experience through their own
instrumentality, e.g. increase, decrease, and respiration: these are
experienced by every animal while it is at rest and not in motion in
respect of the motion set up by its own agency: here the motion is
caused by the atmosphere and by many things that enter into the
animal: thus in some cases the cause is nourishment: when it is
being digested animals sleep, and when it is being distributed through
the system they awake and move themselves, the first principle of this
motion being thus originally derived from outside. Therefore animals
are not always in continuous motion by their own agency: it is
something else that moves them, itself being in motion and changing as
it comes into relation with each several thing that moves itself.
(Moreover in all these self-moving things the first movent and cause
of their self-motion is itself moved by itself, though in an
accidental sense: that is to say, the body changes its place, so
that that which is in the body changes its place also and is a
self-movent through its exercise of leverage.) Hence we may
confidently conclude that if a thing belongs to the class of unmoved
movents that are also themselves moved accidentally, it is
impossible that it should cause continuous motion. So the necessity
that there should be motion continuously requires that there should be
a first movent that is unmoved even accidentally, if, as we have said,
there is to be in the world of things an unceasing and undying motion,
and the world is to remain permanently self-contained and within the
same limits: for if the first principle is permanent, the universe
must also be permanent, since it is continuous with the first
principle. (We must distinguish, however, between accidental motion of
a thing by itself and such motion by something else, the former
being confined to perishable things, whereas the latter belongs also
to certain first principles of heavenly bodies, of all those, that
is to say, that experience more than one locomotion.)
  And further, if there is always something of this nature, a movent
that is itself unmoved and eternal, then that which is first moved
by it must be eternal. Indeed this is clear also from the
consideration that there would otherwise be no becoming and
perishing and no change of any kind in other things, which require
something that is in motion to move them: for the motion imparted by
the unmoved will always be imparted in the same way and be one and the
same, since the unmoved does not itself change in relation to that
which is moved by it. But that which is moved by something that,
though it is in motion, is moved directly by the unmoved stands in
varying relations to the things that it moves, so that the motion that
it causes will not be always the same: by reason of the fact that it
occupies contrary positions or assumes contrary forms at different
times it will produce contrary motions in each several thing that it
moves and will cause it to be at one time at rest and at another
time in motion.
  The foregoing argument, then, has served to clear up the point about
which we raised a difficulty at the outset-why is it that instead of
all things being either in motion or at rest, or some things being
always in motion and the remainder always at rest, there are things
that are sometimes in motion and sometimes not? The cause of this is
now plain: it is because, while some things are moved by an eternal
unmoved movent and are therefore always in motion, other things are
moved by a movent that is in motion and changing, so that they too
must change. But the unmoved movent, as has been said, since it
remains permanently simple and unvarying and in the same state, will
cause motion that is one and simple.

  This matter will be made clearer, however, if we start afresh from
another point. We must consider whether it is or is not possible
that there should be a continuous motion, and, if it is possible,
which this motion is, and which is the primary motion: for it is plain
that if there must always be motion, and a particular motion is
primary and continuous, then it is this motion that is imparted by the
first movent, and so it is necessarily one and the same and continuous
and primary.
  Now of the three kinds of motion that there are-motion in respect of
magnitude, motion in respect of affection, and motion in respect of
place-it is this last, which we call locomotion, that must be primary.
This may be shown as follows. It is impossible that there should be
increase without the previous occurrence of alteration: for that which
is increased, although in a sense it is increased by what is like
itself, is in a sense increased by what is unlike itself: thus it is
said that contrary is nourishment to contrary: but growth is
effected only by things becoming like to like. There must be
alteration, then, in that there is this change from contrary to
contrary. But the fact that a thing is altered requires that there
should be something that alters it, something e.g. that makes the
potentially hot into the actually hot: so it is plain that the
movent does not maintain a uniform relation to it but is at one time
nearer to and at another farther from that which is altered: and we
cannot have this without locomotion. If, therefore, there must
always be motion, there must also always be locomotion as the
primary motion, and, if there is a primary as distinguished from a
secondary form of locomotion, it must be the primary form. Again,
all affections have their origin in condensation and rarefaction: thus
heavy and light, soft and hard, hot and cold, are considered to be
forms of density and rarity. But condensation and rarefaction are
nothing more than combination and separation, processes in
accordance with which substances are said to become and perish: and in
being combined and separated things must change in respect of place.
And further, when a thing is increased or decreased its magnitude
changes in respect of place.
  Again, there is another point of view from which it will be
clearly seen that locomotion is primary. As in the case of other
things so too in the case of motion the word 'primary' may be used
in several senses. A thing is said to be prior to other things when,
if it does not exist, the others will not exist, whereas it can
exist without the others: and there is also priority in time and
priority in perfection of existence. Let us begin, then, with the
first sense. Now there must be motion continuously, and there may be
continuously either continuous motion or successive motion, the
former, however, in a higher degree than the latter: moreover it is
better that it should be continuous rather than successive motion, and
we always assume the presence in nature of the better, if it be
possible: since, then, continuous motion is possible (this will be
proved later: for the present let us take it for granted), and no
other motion can be continuous except locomotion, locomotion must be
primary. For there is no necessity for the subject of locomotion to be
the subject either of increase or of alteration, nor need it become or
perish: on the other hand there cannot be any one of these processes
without the existence of the continuous motion imparted by the first
  Secondly, locomotion must be primary in time: for this is the only
motion possible for things. It is true indeed that, in the case of any
individual thing that has a becoming, locomotion must be the last of
its motions: for after its becoming it first experiences alteration
and increase, and locomotion is a motion that belongs to such things
only when they are perfected. But there must previously be something
else that is in process of locomotion to be the cause even of the
becoming of things that become, without itself being in process of
becoming, as e.g. the begotten is preceded by what begot it: otherwise
becoming might be thought to be the primary motion on the ground
that the thing must first become. But though this is so in the case of
any individual thing that becomes, nevertheless before anything
becomes, something else must be in motion, not itself becoming but
being, and before this there must again be something else. And since
becoming cannot be primary-for, if it were, everything that is in
motion would be perishable-it is plain that no one of the motions next
in order can be prior to locomotion. By the motions next in order I
mean increase and then alteration, decrease, and perishing. All
these are posterior to becoming: consequently, if not even becoming is
prior to locomotion, then no one of the other processes of change is
so either.
  Thirdly, that which is in process of becoming appears universally as
something imperfect and proceeding to a first principle: and so what
is posterior in the order of becoming is prior in the order of nature.
Now all things that go through the process of becoming acquire
locomotion last. It is this that accounts for the fact that some
living things, e.g. plants and many kinds of animals, owing to lack of
the requisite organ, are entirely without motion, whereas others
acquire it in the course of their being perfected. Therefore, if the
degree in which things possess locomotion corresponds to the degree in
which they have realized their natural development, then this motion
must be prior to all others in respect of perfection of existence: and
not only for this reason but also because a thing that is in motion
loses its essential character less in the process of locomotion than
in any other kind of motion: it is the only motion that does not
involve a change of being in the sense in which there is a change in
quality when a thing is altered and a change in quantity when a
thing is increased or decreased. Above all it is plain that this
motion, motion in respect of place, is what is in the strictest
sense produced by that which moves itself; but it is the self-movent
that we declare to be the first principle of things that are moved and
impart motion and the primary source to which things that are in
motion are to be referred.
  It is clear, then, from the foregoing arguments that locomotion is
the primary motion. We have now to show which kind of locomotion is
primary. The same process of reasoning will also make clear at the
same time the truth of the assumption we have made both now and at a
previous stage that it is possible that there should be a motion
that is continuous and eternal. Now it is clear from the following
considerations that no other than locomotion can be continuous.
Every other motion and change is from an opposite to an opposite: thus
for the processes of becoming and perishing the limits are the
existent and the non-existent, for alteration the various pairs of
contrary affections, and for increase and decrease either greatness
and smallness or perfection and imperfection of magnitude: and changes
to the respective contraries are contrary changes. Now a thing that is
undergoing any particular kind of motion, but though previously
existent has not always undergone it, must previously have been at
rest so far as that motion is concerned. It is clear, then, that for
the changing thing the contraries will be states of rest. And we
have a similar result in the case of changes that are not motions: for
becoming and perishing, whether regarded simply as such without
qualification or as affecting something in particular, are
opposites: therefore provided it is impossible for a thing to
undergo opposite changes at the same time, the change will not be
continuous, but a period of time will intervene between the opposite
processes. The question whether these contradictory changes are
contraries or not makes no difference, provided only it is
impossible for them both to be present to the same thing at the same
time: the point is of no importance to the argument. Nor does it
matter if the thing need not rest in the contradictory state, or if
there is no state of rest as a contrary to the process of change: it
may be true that the non-existent is not at rest, and that perishing
is a process to the non-existent. All that matters is the intervention
of a time: it is this that prevents the change from being
continuous: so, too, in our previous instances the important thing was
not the relation of contrariety but the impossibility of the two
processes being present to a thing at the same time. And there is no
need to be disturbed by the fact that on this showing there may be
more than one contrary to the same thing, that a particular motion
will be contrary both to rest and to motion in the contrary direction.
We have only to grasp the fact that a particular motion is in a
sense the opposite both of a state of rest and of the contrary motion,
in the same way as that which is of equal or standard measure is the
opposite both of that which surpasses it and of that which it
surpasses, and that it is impossible for the opposite motions or
changes to be present to a thing at the same time. Furthermore, in the
case of becoming and perishing it would seem to be an utterly absurd
thing if as soon as anything has become it must necessarily perish and
cannot continue to exist for any time: and, if this is true of
becoming and perishing, we have fair grounds for inferring the same to
be true of the other kinds of change, since it would be in the natural
order of things that they should be uniform in this respect.

  Let us now proceed to maintain that it is possible that there should
be an infinite motion that is single and continuous, and that this
motion is rotatory motion. The motion of everything that is in process
of locomotion is either rotatory or rectilinear or a compound of the
two: consequently, if one of the former two is not continuous, that
which is composed of them both cannot be continuous either. Now it
is plain that if the locomotion of a thing is rectilinear and finite
it is not continuous locomotion: for the thing must turn back, and
that which turns back in a straight line undergoes two contrary
locomotions, since, so far as motion in respect of place is concerned,
upward motion is the contrary of downward motion, forward motion of
backward motion, and motion to the left of motion to the right,
these being the pairs of contraries in the sphere of place. But we
have already defined single and continuous motion to be motion of a
single thing in a single period of time and operating within a
sphere admitting of no further specific differentiation (for we have
three things to consider, first that which is in motion, e.g. a man or
a god, secondly the 'when' of the motion, that is to say, the time,
and thirdly the sphere within which it operates, which may be either
place or affection or essential form or magnitude): and contraries are
specifically not one and the same but distinct: and within the
sphere of place we have the above-mentioned distinctions. Moreover
we have an indication that motion from A to B is the contrary of
motion from B to A in the fact that, if they occur at the same time,
they arrest and stop each other. And the same is true in the case of a
circle: the motion from A towards B is the contrary of the motion from
A towards G: for even if they are continuous and there is no turning
back they arrest each other, because contraries annihilate or obstruct
one another. On the other hand lateral motion is not the contrary of
upward motion. But what shows most clearly that rectilinear motion
cannot be continuous is the fact that turning back necessarily implies
coming to a stand, not only when it is a straight line that is
traversed, but also in the case of locomotion in a circle (which is
not the same thing as rotatory locomotion: for, when a thing merely
traverses a circle, it may either proceed on its course without a
break or turn back again when it has reached the same point from which
it started). We may assure ourselves of the necessity of this coming
to a stand not only on the strength of observation, but also on
theoretical grounds. We may start as follows: we have three points,
starting-point, middle-point, and finishing-point, of which the
middle-point in virtue of the relations in which it stands severally
to the other two is both a starting-point and a finishing-point, and
though numerically one is theoretically two. We have further the
distinction between the potential and the actual. So in the straight
line in question any one of the points lying between the two
extremes is potentially a middle-point: but it is not actually so
unless that which is in motion divides the line by coming to a stand
at that point and beginning its motion again: thus the middle-point
becomes both a starting-point and a goal, the starting-point of the
latter part and the finishing-point of the first part of the motion.
This is the case e.g. when A in the course of its locomotion comes
to a stand at B and starts again towards G: but when its motion is
continuous A cannot either have come to be or have ceased to be at the
point B: it can only have been there at the moment of passing, its
passage not being contained within any period of time except the whole
of which the particular moment is a dividing-point. To maintain that
it has come to be and ceased to be there will involve the
consequence that A in the course of its locomotion will always be
coming to a stand: for it is impossible that A should simultaneously
have come to be at B and ceased to be there, so that the two things
must have happened at different points of time, and therefore there
will be the intervening period of time: consequently A will be in a
state of rest at B, and similarly at all other points, since the
same reasoning holds good in every case. When to A, that which is in
process of locomotion, B, the middle-point, serves both as a
finishing-point and as a starting-point for its motion, A must come to
a stand at B, because it makes it two just as one might do in thought.
However, the point A is the real starting-point at which the moving
body has ceased to be, and it is at G that it has really come to be
when its course is finished and it comes to a stand. So this is how we
must meet the difficulty that then arises, which is as follows.
Suppose the line E is equal to the line Z, that A proceeds in
continuous locomotion from the extreme point of E to G, and that, at
the moment when A is at the point B, D is proceeding in uniform
locomotion and with the same velocity as A from the extremity of Z
to H: then, says the argument, D will have reached H before A has
reached G for that which makes an earlier start and departure must
make an earlier arrival: the reason, then, for the late arrival of A
is that it has not simultaneously come to be and ceased to be at B:
otherwise it will not arrive later: for this to happen it will be
necessary that it should come to a stand there. Therefore we must
not hold that there was a moment when A came to be at B and that at
the same moment D was in motion from the extremity of Z: for the
fact of A's having come to be at B will involve the fact of its also
ceasing to be there, and the two events will not be simultaneous,
whereas the truth is that A is at B at a sectional point of time and
does not occupy time there. In this case, therefore, where the
motion of a thing is continuous, it is impossible to use this form
of expression. On the other hand in the case of a thing that turns
back in its course we must do so. For suppose H in the course of its
locomotion proceeds to D and then turns back and proceeds downwards
again: then the extreme point D has served as finishing-point and as
starting-point for it, one point thus serving as two: therefore H must
have come to a stand there: it cannot have come to be at D and
departed from D simultaneously, for in that case it would
simultaneously be there and not be there at the same moment. And
here we cannot apply the argument used to solve the difficulty
stated above: we cannot argue that H is at D at a sectional point of
time and has not come to be or ceased to be there. For here the goal
that is reached is necessarily one that is actually, not
potentially, existent. Now the point in the middle is potential: but
this one is actual, and regarded from below it is a finishing-point,
while regarded from above it is a starting-point, so that it stands in
these same two respective relations to the two motions. Therefore that
which turns back in traversing a rectilinear course must in so doing
come to a stand. Consequently there cannot be a continuous rectilinear
motion that is eternal.
  The same method should also be adopted in replying to those who ask,
in the terms of Zeno's argument, whether we admit that before any
distance can be traversed half the distance must be traversed, that
these half-distances are infinite in number, and that it is impossible
to traverse distances infinite in number-or some on the lines of
this same argument put the questions in another form, and would have
us grant that in the time during which a motion is in progress it
should be possible to reckon a half-motion before the whole for
every half-distance that we get, so that we have the result that
when the whole distance is traversed we have reckoned an infinite
number, which is admittedly impossible. Now when we first discussed
the question of motion we put forward a solution of this difficulty
turning on the fact that the period of time occupied in traversing the
distance contains within itself an infinite number of units: there
is no absurdity, we said, in supposing the traversing of infinite
distances in infinite time, and the element of infinity is present
in the time no less than in the distance. But, although this
solution is adequate as a reply to the questioner (the question
asked being whether it is possible in a finite time to traverse or
reckon an infinite number of units), nevertheless as an account of the
fact and explanation of its true nature it is inadequate. For
suppose the distance to be left out of account and the question
asked to be no longer whether it is possible in a finite time to
traverse an infinite number of distances, and suppose that the inquiry
is made to refer to the time taken by itself (for the time contains an
infinite number of divisions): then this solution will no longer be
adequate, and we must apply the truth that we enunciated in our recent
discussion, stating it in the following way. In the act of dividing
the continuous distance into two halves one point is treated as two,
since we make it a starting-point and a finishing-point: and this same
result is also produced by the act of reckoning halves as well as by
the act of dividing into halves. But if divisions are made in this
way, neither the distance nor the motion will be continuous: for
motion if it is to be continuous must relate to what is continuous:
and though what is continuous contains an infinite number of halves,
they are not actual but potential halves. If the halves are made
actual, we shall get not a continuous but an intermittent motion. In
the case of reckoning the halves, it is clear that this result
follows: for then one point must be reckoned as two: it will be the
finishing-point of the one half and the starting-point of the other,
if we reckon not the one continuous whole but the two halves.
Therefore to the question whether it is possible to pass through an
infinite number of units either of time or of distance we must reply
that in a sense it is and in a sense it is not. If the units are
actual, it is not possible: if they are potential, it is possible. For
in the course of a continuous motion the traveller has traversed an
infinite number of units in an accidental sense but not in an
unqualified sense: for though it is an accidental characteristic of
the distance to be an infinite number of half-distances, this is not
its real and essential character. It is also plain that unless we hold
that the point of time that divides earlier from later always
belongs only to the later so far as the thing is concerned, we shall
be involved in the consequence that the same thing is at the same
moment existent and not existent, and that a thing is not existent
at the moment when it has become. It is true that the point is
common to both times, the earlier as well as the later, and that,
while numerically one and the same, it is theoretically not so,
being the finishing-point of the one and the starting-point of the
other: but so far as the thing is concerned it belongs to the later
stage of what happens to it. Let us suppose a time ABG and a thing
D, D being white in the time A and not-white in the time B. Then D
is at the moment G white and not-white: for if we were right in saying
that it is white during the whole time A, it is true to call it
white at any moment of A, and not-white in B, and G is in both A and
B. We must not allow, therefore, that it is white in the whole of A,
but must say that it is so in all of it except the last moment G. G
belongs already to the later period, and if in the whole of A
not-white was in process of becoming and white of perishing, at G
the process is complete. And so G is the first moment at which it is
true to call the thing white or not white respectively. Otherwise a
thing may be non-existent at the moment when it has become and
existent at the moment when it has perished: or else it must be
possible for a thing at the same time to be white and not white and in
fact to be existent and non-existent. Further, if anything that exists
after having been previously non-existent must become existent and
does not exist when it is becoming, time cannot be divisible into
time-atoms. For suppose that D was becoming white in the time A and
that at another time B, a time-atom consecutive with the last atom
of A, D has already become white and so is white at that moment: then,
inasmuch as in the time A it was becoming white and so was not white
and at the moment B it is white, there must have been a becoming
between A and B and therefore also a time in which the becoming took
place. On the other hand, those who deny atoms of time (as we do)
are not affected by this argument: according to them D has become
and so is white at the last point of the actual time in which it was
becoming white: and this point has no other point consecutive with
or in succession to it, whereas time-atoms are conceived as
successive. Moreover it is clear that if D was becoming white in the
whole time A, the time occupied by it in having become white in
addition to having been in process of becoming white is no more than
all that it occupied in the mere process of becoming white.
  These and such-like, then, are the arguments for our conclusion that
derive cogency from the fact that they have a special bearing on the
point at issue. If we look at the question from the point of view of
general theory, the same result would also appear to be indicated by
the following arguments. Everything whose motion is continuous must,
on arriving at any point in the course of its locomotion, have been
previously also in process of locomotion to that point, if it is not
forced out of its path by anything: e.g. on arriving at B a thing must
also have been in process of locomotion to B, and that not merely when
it was near to B, but from the moment of its starting on its course,
since there can be, no reason for its being so at any particular stage
rather than at an earlier one. So, too, in the case of the other kinds
of motion. Now we are to suppose that a thing proceeds in locomotion
from A to G and that at the moment of its arrival at G the
continuity of its motion is unbroken and will remain so until it has
arrived back at A. Then when it is undergoing locomotion from A to G
it is at the same time undergoing also its locomotion to A from G:
consequently it is simultaneously undergoing two contrary motions,
since the two motions that follow the same straight line are
contrary to each other. With this consequence there also follows
another: we have a thing that is in process of change from a
position in which it has not yet been: so, inasmuch as this is
impossible, the thing must come to a stand at G. Therefore the
motion is not a single motion, since motion that is interrupted by
stationariness is not single.
  Further, the following argument will serve better to make this point
clear universally in respect of every kind of motion. If the motion
undergone by that which is in motion is always one of those already
enumerated, and the state of rest that it undergoes is one of those
that are the opposites of the motions (for we found that there could
be no other besides these), and moreover that which is undergoing
but does not always undergo a particular motion (by this I mean one of
the various specifically distinct motions, not some particular part of
the whole motion) must have been previously undergoing the state of
rest that is the opposite of the motion, the state of rest being
privation of motion; then, inasmuch as the two motions that follow the
same straight line are contrary motions, and it is impossible for a
thing to undergo simultaneously two contrary motions, that which is
undergoing locomotion from A to G cannot also simultaneously be
undergoing locomotion from G to A: and since the latter locomotion
is not simultaneous with the former but is still to be undergone,
before it is undergone there must occur a state of rest at G: for
this, as we found, is the state of rest that is the opposite of the
motion from G. The foregoing argument, then, makes it plain that the
motion in question is not continuous.
  Our next argument has a more special bearing than the foregoing on
the point at issue. We will suppose that there has occurred in
something simultaneously a perishing of not-white and a becoming of
white. Then if the alteration to white and from white is a
continuous process and the white does not remain any time, there
must have occurred simultaneously a perishing of not-white, a becoming
of white, and a becoming of not-white: for the time of the three
will be the same.
  Again, from the continuity of the time in which the motion takes
place we cannot infer continuity in the motion, but only
successiveness: in fact, how could contraries, e.g. whiteness and
blackness, meet in the same extreme point?
  On the other hand, in motion on a circular line we shall find
singleness and continuity: for here we are met by no impossible
consequence: that which is in motion from A will in virtue of the same
direction of energy be simultaneously in motion to A (since it is in
motion to the point at which it will finally arrive), and yet will not
be undergoing two contrary or opposite motions: for a motion to a
point and a motion from that point are not always contraries or
opposites: they are contraries only if they are on the same straight
line (for then they are contrary to one another in respect of place,
as e.g. the two motions along the diameter of the circle, since the
ends of this are at the greatest possible distance from one
another), and they are opposites only if they are along the same line.
Therefore in the case we are now considering there is nothing to
prevent the motion being continuous and free from all intermission:
for rotatory motion is motion of a thing from its place to its
place, whereas rectilinear motion is motion from its place to
another place.
  Moreover the progress of rotatory motion is never localized within
certain fixed limits, whereas that of rectilinear motion repeatedly is
so. Now a motion that is always shifting its ground from moment to
moment can be continuous: but a motion that is repeatedly localized
within certain fixed limits cannot be so, since then the same thing
would have to undergo simultaneously two opposite motions. So, too,
there cannot be continuous motion in a semicircle or in any other
arc of a circle, since here also the same ground must be traversed
repeatedly and two contrary processes of change must occur. The reason
is that in these motions the starting-point and the termination do not
coincide, whereas in motion over a circle they do coincide, and so
this is the only perfect motion.
  This differentiation also provides another means of showing that the
other kinds of motion cannot be continuous either: for in all of
them we find that there is the same ground to be traversed repeatedly;
thus in alteration there are the intermediate stages of the process,
and in quantitative change there are the intervening degrees of
magnitude: and in becoming and perishing the same thing is true. It
makes no difference whether we take the intermediate stages of the
process to be few or many, or whether we add or subtract one: for in
either case we find that there is still the same ground to be
traversed repeatedly. Moreover it is plain from what has been said
that those physicists who assert that all sensible things are always
in motion are wrong: for their motion must be one or other of the
motions just mentioned: in fact they mostly conceive it as
alteration (things are always in flux and decay, they say), and they
go so far as to speak even of becoming and perishing as a process of
alteration. On the other hand, our argument has enabled us to assert
the fact, applying universally to all motions, that no motion admits
of continuity except rotatory motion: consequently neither
alteration nor increase admits of continuity. We need now say no
more in support of the position that there is no process of change
that admits of infinity or continuity except rotatory locomotion.

  It can now be shown plainly that rotation is the primary locomotion.
Every locomotion, as we said before, is either rotatory or rectilinear
or a compound of the two: and the two former must be prior to the
last, since they are the elements of which the latter consists.
Moreover rotatory locomotion is prior to rectilinear locomotion,
because it is more simple and complete, which may be shown as follows.
The straight line traversed in rectilinear motion cannot be
infinite: for there is no such thing as an infinite straight line; and
even if there were, it would not be traversed by anything in motion:
for the impossible does not happen and it is impossible to traverse an
infinite distance. On the other hand rectilinear motion on a finite
straight line is if it turns back a composite motion, in fact two
motions, while if it does not turn back it is incomplete and
perishable: and in the order of nature, of definition, and of time
alike the complete is prior to the incomplete and the imperishable
to the perishable. Again, a motion that admits of being eternal is
prior to one that does not. Now rotatory motion can be eternal: but no
other motion, whether locomotion or motion of any other kind, can be
so, since in all of them rest must occur and with the occurrence of
rest the motion has perished. Moreover the result at which we have
arrived, that rotatory motion is single and continuous, and
rectilinear motion is not, is a reasonable one. In rectilinear
motion we have a definite starting-point, finishing-point,
middle-point, which all have their place in it in such a way that
there is a point from which that which is in motion can be said to
start and a point at which it can be said to finish its course (for
when anything is at the limits of its course, whether at the
starting-point or at the finishing-point, it must be in a state of
rest). On the other hand in circular motion there are no such definite
points: for why should any one point on the line be a limit rather
than any other? Any one point as much as any other is alike
starting-point, middle-point, and finishing-point, so that we can
say of certain things both that they are always and that they never
are at a starting-point and at a finishing-point (so that a
revolving sphere, while it is in motion, is also in a sense at rest,
for it continues to occupy the same place). The reason of this is that
in this case all these characteristics belong to the centre: that is
to say, the centre is alike starting-point, middle-point, and
finishing-point of the space traversed; consequently since this
point is not a point on the circular line, there is no point at
which that which is in process of locomotion can be in a state of rest
as having traversed its course, because in its locomotion it is
proceeding always about a central point and not to an extreme point:
therefore it remains still, and the whole is in a sense always at rest
as well as continuously in motion. Our next point gives a
convertible result: on the one hand, because rotation is the measure
of motions it must be the primary motion (for all things are
measured by what is primary): on the other hand, because rotation is
the primary motion it is the measure of all other motions. Again,
rotatory motion is also the only motion that admits of being
regular. In rectilinear locomotion the motion of things in leaving the
starting-point is not uniform with their motion in approaching the
finishing-point, since the velocity of a thing always increases
proportionately as it removes itself farther from its position of
rest: on the other hand rotatory motion is the only motion whose
course is naturally such that it has no starting-point or
finishing-point in itself but is determined from elsewhere.
  As to locomotion being the primary motion, this is a truth that is
attested by all who have ever made mention of motion in their
theories: they all assign their first principles of motion to things
that impart motion of this kind. Thus 'separation' and 'combination'
are motions in respect of place, and the motion imparted by 'Love' and
'Strife' takes these forms, the latter 'separating' and the former
'combining'. Anaxagoras, too, says that 'Mind', his first movent,
'separates'. Similarly those who assert no cause of this kind but
say that 'void' accounts for motion-they also hold that the motion
of natural substance is motion in respect of place: for their motion
that is accounted for by 'void' is locomotion, and its sphere of
operation may be said to be place. Moreover they are of opinion that
the primary substances are not subject to any of the other motions,
though the things that are compounds of these substances are so
subject: the processes of increase and decrease and alteration, they
say, are effects of the 'combination' and 'separation' of atoms. It is
the same, too, with those who make out that the becoming or
perishing of a thing is accounted for by 'density' or 'rarity': for it
is by 'combination' and 'separation' that the place of these things in
their systems is determined. Moreover to these we may add those who
make Soul the cause of motion: for they say that things that undergo
motion have as their first principle 'that which moves itself': and
when animals and all living things move themselves, the motion is
motion in respect of place. Finally it is to be noted that we say that
a thing 'is in motion' in the strict sense of the term only when its
motion is motion in respect of place: if a thing is in process of
increase or decrease or is undergoing some alteration while
remaining at rest in the same place, we say that it is in motion in
some particular respect: we do not say that it 'is in motion'
without qualification.
  Our present position, then, is this: We have argued that there
always was motion and always will be motion throughout all time, and
we have explained what is the first principle of this eternal
motion: we have explained further which is the primary motion and
which is the only motion that can be eternal: and we have pronounced
the first movent to be unmoved.

  We have now to assert that the first movent must be without parts
and without magnitude, beginning with the establishment of the
premisses on which this conclusion depends.
  One of these premisses is that nothing finite can cause motion
during an infinite time. We have three things, the movent, the
moved, and thirdly that in which the motion takes place, namely the
time: and these are either all infinite or all finite or partly-that
is to say two of them or one of them-finite and partly infinite. Let A
be the movement, B the moved, and G the infinite time. Now let us
suppose that D moves E, a part of B. Then the time occupied by this
motion cannot be equal to G: for the greater the amount moved, the
longer the time occupied. It follows that the time Z is not
infinite. Now we see that by continuing to add to D, I shall use up
A and by continuing to add to E, I shall use up B: but I shall not use
up the time by continually subtracting a corresponding amount from it,
because it is infinite. Consequently the duration of the part of G
which is occupied by all A in moving the whole of B, will be finite.
Therefore a finite thing cannot impart to anything an infinite motion.
It is clear, then, that it is impossible for the finite to cause
motion during an infinite time.
  It has now to be shown that in no case is it possible for an
infinite force to reside in a finite magnitude. This can be shown as
follows: we take it for granted that the greater force is always
that which in less time than another does an equal amount of work when
engaged in any activity-in heating, for example, or sweetening or
throwing; in fact, in causing any kind of motion. Then that on which
the forces act must be affected to some extent by our supposed
finite magnitude possessing an infinite force as well as by anything
else, in fact to a greater extent than by anything else, since the
infinite force is greater than any other. But then there cannot be any
time in which its action could take place. Suppose that A is the
time occupied by the infinite power in the performance of an act of
heating or pushing, and that AB is the time occupied by a finite power
in the performance of the same act: then by adding to the latter
another finite power and continually increasing the magnitude of the
power so added I shall at some time or other reach a point at which
the finite power has completed the motive act in the time A: for by
continual addition to a finite magnitude I must arrive at a
magnitude that exceeds any assigned limit, and in the same way by
continual subtraction I must arrive at one that falls short of any
assigned limit. So we get the result that the finite force will occupy
the same amount of time in performing the motive act as the infinite
force. But this is impossible. Therefore nothing finite can possess an
infinite force. So it is also impossible for a finite force to
reside in an infinite magnitude. It is true that a greater force can
reside in a lesser magnitude: but the superiority of any such
greater force can be still greater if the magnitude in which it
resides is greater. Now let AB be an infinite magnitude. Then BG
possesses a certain force that occupies a certain time, let us say the
time Z in moving D. Now if I take a magnitude twice as great at BG,
the time occupied by this magnitude in moving D will be half of EZ
(assuming this to be the proportion): so we may call this time ZH.
That being so, by continually taking a greater magnitude in this way I
shall never arrive at the full AB, whereas I shall always be getting a
lesser fraction of the time given. Therefore the force must be
infinite, since it exceeds any finite force. Moreover the time
occupied by the action of any finite force must also be finite: for if
a given force moves something in a certain time, a greater force
will do so in a lesser time, but still a definite time, in inverse
proportion. But a force must always be infinite-just as a number or
a magnitude is-if it exceeds all definite limits. This point may
also be proved in another way-by taking a finite magnitude in which
there resides a force the same in kind as that which resides in the
infinite magnitude, so that this force will be a measure of the finite
force residing in the infinite magnitude.
  It is plain, then, from the foregoing arguments that it is
impossible for an infinite force to reside in a finite magnitude or
for a finite force to reside in an infinite magnitude. But before
proceeding to our conclusion it will be well to discuss a difficulty
that arises in connexion with locomotion. If everything that is in
motion with the exception of things that move themselves is moved by
something else, how is it that some things, e.g. things thrown,
continue to be in motion when their movent is no longer in contact
with them? If we say that the movent in such cases moves something
else at the same time, that the thrower e.g. also moves the air, and
that this in being moved is also a movent, then it would be no more
possible for this second thing than for the original thing to be in
motion when the original movent is not in contact with it or moving
it: all the things moved would have to be in motion simultaneously and
also to have ceased simultaneously to be in motion when the original
movent ceases to move them, even if, like the magnet, it makes that
which it has moved capable of being a movent. Therefore, while we must
accept this explanation to the extent of saying that the original
movent gives the power of being a movent either to air or to water
or to something else of the kind, naturally adapted for imparting
and undergoing motion, we must say further that this thing does not
cease simultaneously to impart motion and to undergo motion: it ceases
to be in motion at the moment when its movent ceases to move it, but
it still remains a movent, and so it causes something else consecutive
with it to be in motion, and of this again the same may be said. The
motion begins to cease when the motive force produced in one member of
the consecutive series is at each stage less than that possessed by
the preceding member, and it finally ceases when one member no
longer causes the next member to be a movent but only causes it to
be in motion. The motion of these last two-of the one as movent and of
the other as moved-must cease simultaneously, and with this the
whole motion ceases. Now the things in which this motion is produced
are things that admit of being sometimes in motion and sometimes at
rest, and the motion is not continuous but only appears so: for it
is motion of things that are either successive or in contact, there
being not one movent but a number of movents consecutive with one
another: and so motion of this kind takes place in air and water. Some
say that it is 'mutual replacement': but we must recognize that the
difficulty raised cannot be solved otherwise than in the way we have
described. So far as they are affected by 'mutual replacement', all
the members of the series are moved and impart motion
simultaneously, so that their motions also cease simultaneously: but
our present problem concerns the appearance of continuous motion in
a single thing, and therefore, since it cannot be moved throughout its
motion by the same movent, the question is, what moves it?
  Resuming our main argument, we proceed from the positions that there
must be continuous motion in the world of things, that this is a
single motion, that a single motion must be a motion of a magnitude
(for that which is without magnitude cannot be in motion), and that
the magnitude must be a single magnitude moved by a single movent (for
otherwise there will not be continuous motion but a consecutive series
of separate motions), and that if the movement is a single thing, it
is either itself in motion or itself unmoved: if, then, it is in
motion, it will have to be subject to the same conditions as that
which it moves, that is to say it will itself be in process of
change and in being so will also have to be moved by something: so
we have a series that must come to an end, and a point will be reached
at which motion is imparted by something that is unmoved. Thus we have
a movent that has no need to change along with that which it moves but
will be able to cause motion always (for the causing of motion under
these conditions involves no effort): and this motion alone is
regular, or at least it is so in a higher degree than any other, since
the movent is never subject to any change. So, too, in order that
the motion may continue to be of the same character, the moved must
not be subject to change in respect of its relation to the movent.
Moreover the movent must occupy either the centre or the
circumference, since these are the first principles from which a
sphere is derived. But the things nearest the movent are those whose
motion is quickest, and in this case it is the motion of the
circumference that is the quickest: therefore the movent occupies
the circumference.
  There is a further difficulty in supposing it to be possible for
anything that is in motion to cause motion continuously and not merely
in the way in which it is caused by something repeatedly pushing (in
which case the continuity amounts to no more than successiveness).
Such a movent must either itself continue to push or pull or perform
both these actions, or else the action must be taken up by something
else and be passed on from one movent to another (the process that
we described before as occurring in the case of things thrown, since
the air or the water, being divisible, is a movent only in virtue of
the fact that different parts of the air are moved one after another):
and in either case the motion cannot be a single motion, but only a
consecutive series of motions. The only continuous motion, then, is
that which is caused by the unmoved movent: and this motion is
continuous because the movent remains always invariable, so that its
relation to that which it moves remains also invariable and
  Now that these points are settled, it is clear that the first
unmoved movent cannot have any magnitude. For if it has magnitude,
this must be either a finite or an infinite magnitude. Now we have
already'proved in our course on Physics that there cannot be an
infinite magnitude: and we have now proved that it is impossible for a
finite magnitude to have an infinite force, and also that it is
impossible for a thing to be moved by a finite magnitude during an
infinite time. But the first movent causes a motion that is eternal
and does cause it during an infinite time. It is clear, therefore,
that the first movent is indivisible and is without parts and
without magnitude.

                                   -THE END-


This text is part of the Internet Ancient History Sourcebook. The Sourcebook is a collection of public domain and copy-permitted texts related to ancient history. Unless otherwise indicated the specific electronic form of the document is copyright. Permission is granted for electronic copying, distribution in print form for educational purposes and personal use. If you do reduplicate the document, indicate the source. No permission is granted for commercial use.

Paul Halsall, February 2023

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