by Aristotle
This same argument may also be stated in another way as follows. Every mover 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 [25] impossible for that with which a thing is moved to move it without being moved by that which imparts motion by its own agency; but 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 mover while being itself moved, the series must stop somewhere and not be infinite. Thus, if the stick [30] 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 is moving and [256b1] 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 mover of this kind.
And if we consider the matter in yet another way we shall get this same result. If everything that is in motion is moved by something that is in motion, either this is [5] an accidental attribute of the things (so that each of them moves something while being itself in motion, but not because it is itself in motion) or it belongs to them in their own right. If, then, it is an accidental attribute, it is not necessary that that which causes motion should be in motion; and if this is so it is clear that there may [10] be a time when nothing that exists is in motion, since the accidental is not necessary but contingent. Now if we assume something possible, nothing impossible will follow (though something false may). But the non-existence 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 mover, and the instrument of motion. [15] 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 mover—that is to say, that which causes motion in such a manner that it is not merely the instrument of [20] motion—must be unmoved. Now we see the last things, which have the capacity of being in motion, but do not contain a motive principle, and also things which are in motion but are moved by themselves 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 [25] says that Mind is impassive and unmixed, since he makes it the principle of motion; for it could cause motion in this way only by being itself unmoved, and have control only by being unmixed.
Now if the mover is not accidentally but necessarily in motion—so that, if it were not in motion, it would not move anything—then the mover, in so far as it is in motion, must be moved either with the same kind of motion, or with a different [30] kind—either that which is heating, I mean, is itself becoming hot, that which is making healthy becoming healthy, and that which is causing locomotion in process of locomotion, or else that which is making healthy is in process of locomotion, and that which is causing locomotion in process of increase. But it is evident that this is impossible. For we must apply this to the very lowest species into which motion can [257a1] be divided: e.g. we must say that if someone is teaching some lesson in geometry, he is also being taught that same lesson in geometry, and that if he is throwing he is 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 being [5] altered by something else, and that which is causing this alteration is 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 series bends back, i.e. 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 being taught; for it is clear that everything that is moved is [10] also moved by the mover that is further back in the series—in fact the earlier mover is that which more strictly moves it. But this is of course impossible; for it involves the consequence that one who is teaching is learning whereas teaching necessarily implies possessing knowledge, and learning not possessing it. Still more unreasonable is the consequence that, since everything that is moved is moved by something [15] that is itself moved, everything that has a capacity for causing motion is capable of 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 has a capacity for being made healthy, and that which has a capacity for building has a capacity for being built, either immediately or through one or more links (as it will if, while everything that has a capacity for causing motion has a capacity for being moved by something else, the motion that it has the capacity for suffering is not that with [20] 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 have a capacity for learning: the series, however, could be traced back, as we said before, until at some time or other we arrive 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 [25] should necessarily have a capacity for increase. It is not necessary, therefore, that that which is moved should always be moved by something else that is itself moved: 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, everyone would [30] decide for the former; for that which is in itself a cause is always prior to that which is so in virtue of something else.
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 [257a1] course on Physics, that everything that is in motion in its own right 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 [5] be both teaching and being taught, 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 moves potentially, not in fulfilment, and the potential is in process to fulfilment, and motion is an incomplete fulfilment of the movable. The mover on the other hand is already in actuality: e.g. it is that which is hot that produces heat, [10] and in general that which produces the form possesses it. Consequently, the same thing in respect of the same thing will be at the same time both hot and not hot. So, too, in every other case where the mover must have the synonymous property. Therefore when a thing moves itself it is one part of it that is the mover and another part that is moved. But it is not self-moving in the sense that each of the two parts is [15] moved by the other part: the following considerations make this evident. If each of the two
parts is to move the other, there will be no first mover; for 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 mover; for we found that there are two kinds of mover, 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 [20] principle of motion than that which is intermediate. Again, there is no necessity for the mover 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 mover. Again, there is no necessity for the mover to be moved in return: on the contrary the necessity [25] that there should always be motion makes it necessary that there should be some mover that is either unmoved or moved by itself. Again, 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 [30] 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-mover, 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 [258a1] 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 [5] 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 C, C something that is moved [10] by B but moves nothing (granted that we eventually arrive at C we may take it that there is only one intermediate term, though there may be more). Then the whole ABC moves itself. But if I take away C, AB will move itself, A imparting motion and B being moved, whereas C will not move itself or in fact be moved at all. Nor [15] again will BC 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 [20] them, must be in contact with the other. If, then, that which imparts motion is continuous—that which is moved must of course be so—the one will be in contact with the other. 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 [25] nor is it moved as a whole: it is A that imparts motion and B alone that is moved.
Here a difficulty arises: if something is taken away from A (supposing that that which imparts motion but is unmoved is continuous), 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, [30] when something is taken away from AB, the remainder of AB will continue to move itself. Perhaps there is nothing to prevent each of the two parts, or at any rate one of [258b1] them, that which is moved, being potentially divided though actually undivided, so that it if is divided it will not continue in the possession of the same nature; 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 [5] motion is unmoved; for, whether that which is in motion but moved by something leads straight to the first unmoved, or whether it leads to 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.
[10] 6 · Since there must always be motion without intermission, there must necessarily be something eternal, whether one or many, that first imparts motion, and this first mover 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 [15] unmoved and exempt from all change, both unqualified and accidental. Let us suppose, if you will, 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 change at [20] 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 [25] motion, everything which moves itself must have magnitude, though nothing that we have said makes this necessarily true of every mover. 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 some be caused by some and others by others. The eternity and continuity of [30] 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 movers is infinite and they do not all exist together. It is clear, then, that though [259a1] there may be countless instances of the perishing of movers unmoved, 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 [5] that some things are and others are not and of the continuous process of change; and this causes the motion of the other movers, while they are the causes of the motion of other things. Motion, then, being eternal, the first mover, if there is but one, will be eternal also; if there are more than one, there will be a plurality of such eternal movers. 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 [10] 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 mover, 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 mover must be something that is one and eternal. We have shown that there must always be [15] motion. That being so, motion must be continuous, because what is al
ways is continuous, whereas what is in succession is not continuous. But further, if motion is continuous, it is one; and it is one only if the mover and the moved 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 [20] not only from the foregoing arguments, but also by considering again the principles operative in movers.50 Now it is evident that among existing things there are some that are sometimes in motion and sometimes at rest. This fact has served 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 [25] 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 [30] something, and that the mover is either unmoved or in motion, and that, if it is in motion, it is moved at each stage either by itself or by something else; and so we proceeded to the position that of things that are moved, the principle of things that are in motion is that which moves itself, and the principle of the whole series is the [259b1] 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 [5] 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: there are 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 [10] respect of the motion set up by its own agency; here the motion is caused by the environment 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 they awake and move themselves, the first principle of this motion being thus originally derived from outside. Therefore animals are not always in continuous [15] 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 things the first mover 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 moves itself by [20] leverage.) Hence we may be sure that if a thing belongs to the class of unmoved things which move themselves 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 mover that is unmoved even accidentally, if, as [25] we have said, there is to be in the world of things an unceasing and undying motion, and the world is to remain self-contained and within the same limits; for if the principle is permanent, the universe must also be permanent, since it is continuous with the principle. (We must distinguish, however, between accidental motion of a thing by itself and such motion by something else, the former being confined to [30] perishable things, whereas the latter belongs also to certain principles of heavenly bodies, of all those, that is to say, that experience more than one locomotion.)