Commentary Aristotle Physics
But what causes motion is already in act, for what is in potency is not reduced to act except by something in act, and this is the mover. For example, the hot causes heat, and what generates has the form to be generated, as one who has the human form generates a man, and so on for other things. If, therefore, the whole moves its whole self, it follows that the same thing is, with respect to the same, at once hot and not hot, for insofar as it moves, it will be hot in act; insofar as it is moved, it will be hot in potency.
The same is true in all other cases in which the mover is univocal, that is, agreeing in name and account with the thing moved, as when the hot makes the hot and a man generates a man.
And he says this because there are some agents that are not univocal and that do not agree in name and account with their effects, as the sun generates a man. In such agents, nevertheless, even though they do not possess the form of the effect according to the same account, they do so in a higher and more universal sense. Consequently, it is universally true that the mover is somehow actually what the mobile is potentially. If, therefore, the whole moves its whole self, it follows that the same thing is at once in potency and in act—which is impossible.
From this, he concludes (257b12) the main proposition that, with respect to a thing that moves itself, one part is mover and the other part moved.
1054. Then, when he says, “but it is not” (257b13), he rejects certain ways in which someone might suppose that a thing moves itself.
First, he shows that, with respect to a thing that moves itself, both parts are not moved by each other;
second, he shows that, with respect to a thing that moves itself, one part does not move itself, at “but, as a matter” (257b26; [1059]).
About the first, he does two things:
first, he proposes what he intends;
second, he proves his proposition, at in the first place (257b15; [1055]).
He says therefore, first (257b13), that it is clear from what follows that a thing cannot move itself in such a way that each part is moved by the other—for example, if AB moves itself, that A move B, and B move A.
1055. Then, at “in the first place” (257b15), he proves the proposition with four arguments. And it should be noted that, for this conclusion, he reuses the reasons previously given to show that not every mover is being moved by another.
Hence, from the foregoing, he here collects four abridged arguments.
The first of these he takes from the first argument presented above in a double order (ascending and descending) to show that the process of one thing being moved by another does not go on forever to infinity, because then there would be no first mover, from whose non-existence would follow the non-existence of all coming after it. Hence, in this place, too, the Philosopher argues that the outcome is likewise unacceptable.
For he says that, if both parts in the first thing moved, which moves itself per our supposition, are reciprocally being moved by each other, it will follow that there is no first mover. This follows because, as was said above, the prior mover is more the cause of motion and moves more than the subsequent mover.
And this was proved above on the ground that something causes motion in two ways.
In one way, something moves by being moved by another, as a stick moves a stone because it is being moved by the hand, and this is a second mover.
In another way, something moves by being moved of itself, as a man moves, and this is a condition of a first mover. Now, what causes motion independently of being moved by another is farther removed from the last thing moved and nearer to the first mover than an intermediate that causes motion because it is moved by another.
This argument should be formulated in the following way. If both parts of a thing that moves itself move each other reciprocally, one is no more the cause of motion than the other. But the first mover is more a cause of motion than a second mover; therefore, neither of the parts will be a first mover. Now, this is unacceptable, since it would then follow that what is moved of itself would be no nearer to the first principle of motion (whose existence would thereby be rejected) than what is moved by another, whereas it was proved above that a mover that moves itself is first in the genus of mobiles. Therefore, it is not true that both parts of a thing that moves itself are moved by each other.
1056. Then, at “in the second place” (257b20), he presents two arguments for the same, taken from one he used above when he showed that not every mover is being moved, in the sense that being moved is found per accidens in the mover.
In this argument, he drew two conclusions above:
first, that a mover can happen not to be moved;
second, that motion is not eternal.
In the light of these two conclusions, he now forms two arguments.
For he says first of all that it is not necessary for a mover to be moved except by itself according to accident, the sense of this being that, unless the first mover be taken as being moved by itself, it will not also be necessary that the first mover be moved according to an accident, as some posited that every mover is being moved, but that its being moved is in it per accidens.
When, therefore, it is supposed that, of a thing that moves itself, the part causing motion is equally being moved by the other, this will be only per accidens. But, as we conceded above, whatever is per accidens is able not to be; therefore, it is possible for the part which causes motion not to be moved. Thus, therefore, it remains that, of a thing that moves itself, one part is moved and the other causes motion and is not moved.
1057. Then, at “in the third place” (257b23), he gives another argument corresponding to the second conclusion that he inferred above, namely, that it follows that motion does not always exist. Here, however, he argues in reverse order. If it is necessary that motion always exist, it is not necessary that a mover, when it causes motion, be moved, but it is necessary either that the mover be immobile or that it be moved by itself.
The reason for this conditional is apparent from an argument given above. If a mover does not cause motion unless it is being moved, and if being moved is only in it per accidens, it follows that it can happen not to be moved.
Consequently, it can happen also not to cause motion, and as a result, there will be no motion. But motion was proved to be eternal. Therefore, it is not necessary for a mover to be moved when it is causing motion. Consequently, it is not true that each part of a thing that moves itself is moved by the other.
1058. Then, at “in the fourth place” (257b25), he presents the fourth argument, which is taken from the argument previously given to prove that it is not essential to a mover that it be moved, because it would follow that we must come to this: a mover would be being moved by the same motion that it is causing, as explained above.
And now, abridging this argument, he says that, if each part is being moved by the other, it will follow that it causes motion and is being moved with respect to the same motion. Hence, it follows that the heater is heated—which is impossible.
Now, the reason that it follows that the same thing is causing motion and being moved with respect to the same motion when it is posited that each part of a thing that moves itself is moved by the other is that there is just one motion in the thing that moves itself, and it is according to that motion that the part causing motion will itself have to be moved.
1059. Then, at “but, as a matter of fact” (257b26), he excludes another way, namely, the supposition that the part of a thing that moves itself does not move itself.
First, he proposes what he intends;
second, he proves his proposition, at “for if the whole” (257b28; [1060]).
He thus says first that, if something that is first moving itself be assumed, it cannot be said either that one part of it moves itself or that a number of parts do so in such a way that each of them moves itself.
1060. Then, at “for if the whole” (257b28), he proves this with two arguments.
The first is that, if the whole is being moved by itself, this belongs to it either by reason of a part that is being moved by itself or by reason of the whole.
If it belongs to it by reason of its part, then that part will be a first mover that moves itself, because that part separated from the whole will move itself, but then the whole will no longer be a first mover of itself, as was supposed.