Commentary Aristotle Physics
Lecture 10. How a thing moves itself
Now, everything that is in motion must be infinitely divisible, for it has been shown already about universals of nature 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, and thus it would at the same time be both teaching and being taught, or both restoring to health 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 in motion, not actually, but the potential is in process to act, and motion is an imperfect act of the movable.
The mover, on the other hand, is already in activity; for instance, 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, and this is so, too, in every other case where the mover must be univocal with the moved.
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 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 mover.
If a thing is moved by a series of movers, 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 principle of motion than that which is intermediate.
In the second place, it is not necessary for a mover to be moved except by itself according to accident, such that the other part can move it in return only accidentally.
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.
In the third place, there is no necessity for the mover to be moved in return: on the contrary, the necessity that there should always be motion makes it necessary that there should be some mover 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 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-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; 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.
1050. After showing that, in mobiles and movers, there is no going on to infinity, but that a first is reached that is either immobile or self-moving, the Philosopher now shows that, even if a first that moves itself is reached, it is nevertheless necessary to come to a first that is immobile.
This treatment is divided into three parts.
In the first, he shows that what moves itself is divided into two parts, one of which is mover and the other moved;
in the second, he shows how these parts are mutually related, at “and, since what imparts” (258a5; [1062]);
in the third, he concludes that it is necessary to come to a first that is immobile, at “from what has been said” (258b4; [1068]).
About the first, he does two things:
first, he shows that, in a thing that moves itself, one part is mover and the other is moved, because a whole cannot move its whole self;
second, he rejects other ways in which a thing that moves itself might be thought to do so, at “but it is not” (257b13; [1054]).
About the first, he does three things:
first, he proposes that what moves itself does not totally move itself as a whole;
second, he proves the proposition, at “for then, while being” (257b2; [1052]);
third, he argues for the main conclusion intended, at “therefore, when a thing” (257b12; [1053]).
1051. Because whole and part have no place except in things that are divisible, Aristotle concludes from what he had proved in book 6 (240b8), first, that whatever is moved is necessarily divisible into parts that are always further divisible—for this pertains to the very account of a continuum. Now, whatever is being moved is a continuum if it is being moved per se (for it is not impossible for an indivisible, such as a point or whiteness, to be moved per accidens).
And this was shown previously in book 6; all the statements made prior to book 8 he calls universals of nature, for in book 8, he begins to apply to things the statements he previously made about motion in common. Accordingly, since what is moved is divisible, a whole and a part can be found in everything that is being moved. If, therefore, there is anything that moves itself, we shall be able to take a whole and a part in it; but a whole cannot move its whole self, that is, in its entirety move itself.
1052. Then, at “for then, while being” (257b2), he proves his proposition with two arguments.
The first is this. The motion of a thing that moves itself at one time and in one motion is numerically one; if, therefore, a thing should move itself in such a way that the whole moves the whole, it will follow that one and the same will be mover and moved with respect to one and the same motion, whether it be local motion or alteration. But this seems unsuitable, for mover and moved are mutually opposite, and opposites cannot exist in the same thing with respect to the same. It is, therefore, not possible that one thing be mover and moved with respect to the same motion.
For when something is at once moving and being moved, the motion according to which it moves is different from the one according to which it is being moved, as when a stick, moved by the hand, moves a stone, the motion of the stick is numerically different from the motion of the stone.
Accordingly, it will follow further that someone will be both teaching and taught at the same time with respect to one and the same knowable thing, and, similarly, that someone will heal and be healed with respect to one and the same numerical health.
1053. He gives the second argument at “moreover, we have established” (257b6), which is this. It has been determined in book 3 that what is being moved is a mobile (that is, something existing in a state of potency), since what is being moved is being moved precisely because it is in potency and not in act, for a thing is considered to be in motion when, being in potency, it is tending toward act.
However, that which is being moved is not in potency in such a way that it is in no way in act, because the very motion is a kind of act of the mobile precisely as being moved; but it is an imperfect act, being the act of the mobile inasmuch as it is still in potency.