Aristotle

by Various Works [lit]

  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 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