Commentary Aristotle Physics
About the first, he does two things:
first, he shows that, in a thing that moves itself, there are just two parts, one of which causes motion and is not moved and the other of which is moved and does not cause any motion;
second, he shows how these two parts are joined to one another, at “and each of these two” (258a20; [1064]).
He explains the first part in this way (258a9). If it be said that the moved part of a thing that moves itself does in turn move something else that is part of the very thing that moves itself, then let A be the first immobile part of this self-moving thing. Let B be the second part, and let it be both the one moved by A and the mover of a third part C, which is so moved by B as to move nothing else that is a part of this self-moving thing. For it cannot be said that there is an infinite descent in the parts of a thing that moves itself such that a moved part in turn moves another, for it would then be moving itself to infinity, which is impossible, as was shown above. There will be, therefore, in that self-moving thing, a part that is moved but is not a mover, that is, the part C.
And, although it might be that it is through many intermediate moved movers that the last moved part C is reached, we can accept B as the one intermediate taken in place of all these intermediates. Thus, therefore, does this whole, which is ABC, move itself. If, from this whole, there be taken away the part C, then AB will still move itself, because one of its parts is a mover (A) and the other moved (B), which was required for a thing to be able to move itself, as was shown above. But C will not move itself or move any other part, as we have assumed.
Likewise, even BC does not move itself without A, because B does not cause motion except inasmuch as it is moved by something else, which is A, which is not a part of BC. It remains, therefore, that only AB moves itself first and per se. Hence, a thing that moves itself must have two parts, one of which is an immobile mover and the other of which is moved and necessarily does not move anything that is part of the whole thing that moves itself, for this was concluded by the foregoing argument.
Or else it moves nothing of necessity—since it is not a necessity of a self-mover that the moved part move anything else, even anything extrinsic.
1064. Then, at “and each of these two” (258a20), he shows how these two parts are mutually related.
Here it must be considered that Aristotle has not yet proved that the first mover has no magnitude, as will be proved later. But some of the earlier philosophers posited that no substance can exist without magnitude. Hence, Aristotle is in keeping with his custom when he leaves this matter doubtful until it is proved; and he says that the two parts of a self-mover, of which one is a mover and the other is moved, must be somehow conjoined if they are to be parts of one whole. But this cannot be by continuation, because he has said above that a self-mover and a moved thing cannot form a continuum, but are necessarily divided. Hence, it remains that these two parts must be joined by contact: either by both parts touching one another, if they have magnitude; or by just one of the parts touching the other and not vice versa, which will be the case if the mover has no magnitude. For what is incorporeal can indeed touch a body by means of its power, and so move it, but it is not touched in turn by the body; two bodies, however, touch each other.
1065. Then, at “if, then, that which imparts” (258a21), he shows by what reason a whole is said to move itself with one part causing motion and the other part being moved.
And let us suppose at first that each part is continuous, that is, having a magnitude, because, in book 6, it has been proved of anything that is moved that it is a continuum, and let the same thing be supposed at the present time for the mover, before the truth is proved.
Therefore, using this supposition, three things are attributed to this whole composed of two parts:
it is moved,
it causes motion,
and it moves itself.
But self-motion is attributed to it not because a part moves itself, but because the entire whole moves itself, while to cause motion and to be moved are attributed to the whole by reason of the part. For the whole neither moves nor is moved, but one part, A, moves and the other part, B, is moved only; and it has already been shown that there is no third part C that is moved by B. For this is impossible, if we are dealing with a thing that moves itself primarily, as has been shown above.
1066. Then, at “here a difficulty” (258a27), he raises a doubt about the foregoing.
First, he raises it;
second, he solves it, at “perhaps we may state” (258a32; [1067]).
This doubt springs from what he had previously proved: in a thing that moves itself in a primary sense, there are but two parts, of which one moves and the other is moved; for if there were a third, even if this third were removed, the composite of the first two would still move itself, and thus the latter is the primary self-mover.
From this, therefore, the following doubt follows. Let us suppose that the immobile but motion-causing part A of a self-moving whole is a continuum. Now, it is clear that its part B, which is the moved part, is a continuum, according to what has been previously proved. But every continuum is divisible. Therefore, the doubt is this: if, through division, a part be removed from A or B, would the remaining part be a mover or a moved part? For if it is either, the part of AB that remains will move itself and, accordingly, AB will not be something that moves itself in a primary sense. Thus it further follows that nothing will be a self-mover in a primary sense.
1067. Then, at “perhaps we may state” (258a32), he resolves this doubt. Now it should be remembered here that, in book 6, Aristotle has proved that there is no first in motion on the part either of the mobile or of the time or of that in which there is motion, and that this is especially true in growth and local motion. The reason is that he was then speaking of motion in common and of the mobile as it is a certain continuum, without yet making application to particular natures. And according to this, it would follow that there would not be anything that is first moved and, consequently, no first mover, if the mover were a continuum. Likewise, there would also not be anything that is a first mover.
But now Aristotle is speaking of motion and applying his doctrine to definite natures, and for that reason, he posits that there is a first self-mover. And he resolves the doubt in the following manner, stating that there is nothing to prevent the mover and moved from being divisible in potency, due to the fact that they are continua—that is, if both are continua, or at least one of them, namely, the one that is moved, which necessarily is a continuum.
But yet it is possible that some continuum, whether it be a mover or something moved, have such a nature that it cannot be actually divided, as is evident of the body of the sun. And, if it happens that some continuum is divided, it will not retain the same potency for causing motion or being moved as it had before, because such a potency follows upon the form, and a natural form requires a determinate quantity. Hence, if it is an incorruptible body, it cannot be actually divided. But if it is a corruptible one, then if it be divided, it will not retain the same potency, as is evident with respect to the heart. Hence, there is nothing to prevent there being one first thing in things potentially divisible.
1068. Then, at “from what has been said” (258b4), he infers the conclusion mainly intended from all this. And he says that, from the foregoing, it is clear that it is necessary to posit a first mover that is immobile. For since there is not an infinite process in movers and moved things, but a halt must be made at a first that is immobile or self-moving, then, whether the movers and moved stop at some first immobile or at some first that moves itself, in either case, it turns out that the first mover is immobile, because one part even of a thing that moves itself is an immobile mover, as has just been proved.