Various Works

by Aristotle

  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

&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