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
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 necessaril
y
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
impossible.
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
unmoved.
6
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 establis
hed
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