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
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.