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.