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
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
&n
bsp; 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.
5
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