Commentary Aristotle Physics
second, he gives an argument that shows the proposition, at “if, then, everything” (256a13; [1040]).
1038. Now, he sets down two things, of which the first (256a4) is a division of movers.
For since it has been said that whatever is moved is moved by something, a thing might be a mover in two senses.
In one sense, it moves not on its own account, that is, not by its own power, but because it has been moved by some other mover. This is a second mover.
In another sense, something moves on its own account, that is, by its own power and not because it has been moved by another.
Now, such a mover can cause motion in two ways.
First, it can do so in such a way that the first mover moves the one next to last, that is, the one that is nearest to it after the second mover. This happens when the first mover moves a mobile through just one intermediate.
Second, it can cause motion in such a way that the mover moves a mobile through a number of intermediates, as when a stick moves a stone and the stick is moved by a hand, which is moved by a man who does not move as being moved by something else.
In this way, the man is a first mover on his own account and he moves the stone through a number of intermediates; however, if he moved the stone with his hand, he would be moving the stone through one intermediate only.
1039. Second, at “now, we say” (256a8), he compares the first mover with the second. For when we say that the first mover moves as much as does the ultimate mover, we say that the first mover is more a mover than the ultimate mover.
This is clear for two reasons:
first, because the first mover moves the second mover but not vice versa;
second, because the second mover cannot cause motion independently of the first, but the first can cause it independently of the second. For example, the stick cannot move the stone unless it is moved by the man, but the man can move the stone without using the stick.
1040. Then, at “if, then, everything” (256a13), he proves his proposition in the light of the foregoing. For it has been shown that whatever is being moved is being moved by another. But that by which it is moved is itself either moved or not moved; and if it is moved, it is either moved by another or not. Now, these two—namely, being moved by another and not being moved by another—are such that, if one is posited, the other must be, but not vice versa: that is, if there is something that is moved by another, it is necessary to come to a first that is not moved by another; but if such a first is posited (namely, a first that is not moved by another), it is not necessary further to posit another (namely, one that is moved by another).
This, indeed, is self-evident, but there could be some doubt about the first one, namely, that, if there be found something moved by another, there will be found a first that is not moved by another. For this reason, he proves this in the following manner.
If something is moved by another, and this in turn by another, and if something not moved by another is never reached, it follows that there is a process to infinity in movers and moved things. But this is impossible, as was proved in book 7. However, he here proves it in a more certain way from the fact that there is no first in an infinite series. Therefore, if movers and moved things go on to infinity, there will be no first mover. But it has already been said that, if the first mover does not act, the last mover does not act and, consequently, there will be no mover, which is clearly false. Therefore, the process of something being moved by another cannot go on to infinity. If, therefore, it be conceded that whatever is being moved is being moved by another, as has been proved, and again, if it be supposed that the first mover is itself being moved but not by something else, it is necessarily being moved by itself.
It should be noted that this argument is not proving that the first mover is being moved, but he is supposing this according to the common opinion of the Platonists. As to the force of the argument, it does not prove that the first mover moves itself more than it proves that it is immobile. Hence, he later presents this same conclusion under a disjunction, as will be clear below.
1041. Then, at “this same argument” (256a21), he proves his proposition by descending. And it is the same argument as the preceding so far as its inferential value is concerned, but differs with respect to the order of the process; he repeats it, however, for the sake of greater clarity.
He says therefore that the previous argument might be presented in another way. And he sets down propositions that have the same account of truth as the previous ones, but in a different order. For above he had argued that whatever is being moved is being moved by another, and that that by which it is moved acts either on its own account or on account of something else previously moving it; and this was an ascending process.
But he now uses a descending process, saying that every mover moves something and moves by means of something, that is, either by itself or by means of some lesser mover, as a man moves a stone either by himself or by means of a stick and the wind casts something to the earth either by its own impulse or by means of a stone that it moves.
Again, he had argued above that the last mover does not cause motion independently of the first mover, but vice versa. In place of that, he here says that what a mover uses as an instrument in causing motion cannot itself cause motion without a principal mover moving it, as a stick cannot cause motion independently of the hand. But, if something moves by itself as a principal mover, the addition of an instrument is not required.
And this is more evident in instruments than in an ordered array of mobiles, although the same truth is present in both cases, because not everyone would consider the second mover an instrument of the first. But, just as he deduced above that, if there is something that is being moved by another, there must be something that is not being moved, but not vice versa, so here in a descending process he says that, if that by which the mover causes motion is another thing, as an instrument, there has to be something that causes motion not by an instrument, but by itself. Otherwise, there is an infinite process with respect to instruments, which is the same as proceeding to infinity with respect to movers, and that is impossible, as has been proved above.
If, therefore, there exists a mover of that which is being moved, a halt must be made and the process cannot go to infinity. For if the stick causes motion because it is moved by the hand, it follows that the hand moves the stick; if, however, something else is moving the hand, it also follows conversely that a mover is moving the hand. Consequently, the same process that was valid with respect to moved instruments is valid for movers of instruments. But, with respect to movers, as was shown, an infinite process must be avoided; therefore, it must be avoided with respect to instruments. Therefore, since it is always such that a thing being moved is moved by another that moves, and an infinite process must be avoided, it is necessary that there be a first mover that moves by itself and not through an instrument.
If, therefore, it be granted that this first that moves itself is indeed moved but there is no other moving it (because then it would be an instrument), it follows of necessity that it is moving itself—following the supposition of the Platonists that every mover is moved. Hence, also according to this argument, either what is being moved will be immediately moved by a mover that moves itself or, at some time, such a mover that moves itself must be reached.
1042. Then, at “and, if we consider” (256b3), he shows that not every mover is being moved, as was supposed in the preceding arguments.
About this, he does two things:
first, he proves that not every mover is being moved;
second, from this and from the previous arguments, he argues for his main proposition, at “it is not necessary” (257a25; [1049]).
He therefore says first that to the above-mentioned things may be added the following in order to show our proposition.
About this, he does three things:
first, he gives a division;
second, he rejects one member, at “let us consider” (256b7; [1043]);
third, he rejects another, at “we will now take” (256b27; [1046]).
He says first (256b3) that, if whatever is being moved is being moved by another—which is tantamount to saying that every mover is moved—this can be in two ways:
in one way, that it is per accidens in things that a mover is moved, that is, the mover does not act in virtue of being moved (as if we should say that a musician is a builder not because he is a musician, but per accidens); or, in a second way, that it is not per accidens but per se that a mover is moved.
1043. Then, at “let us consider” (256b7), he rejects the first member in three ways.
First, with this argument. Nothing per accidens is necessary, for what is in a thing per accidens is not present of necessity, but may happen not to be present, as musician in a builder. If, therefore, it is per accidens that movers are moved, it follows that it can happen that they not be moved. But once you posit that every mover is moved, it is a consequence, if movers are not moved, that they do not cause motion. It follows, therefore, that, at some time, nothing is being moved.
But this is impossible, for it has been proved above that it is necessary that motion always exist. This impossibility, however, does not follow from the supposition that movers are not moved; for if it is per accidens that a mover is moved, it will be possible for movers not to be moved, and if a possibility is posited, no impossibility follows. It remains, therefore, that the other statement from which it followed is impossible, namely, the statement that every mover is moved.
1044. Second, at “moreover, the conclusion” (256b13), he proves the same with another probable argument, which is this. Three things are found in motion:
one is the mobile that is being moved;
another is the mover;
and the third is the instrument by which the mover causes motion.
Now, among these three, it is clear that the thing that is moved has to be moved, but it does not have to cause motion. But the instrument by which the mover causes motion must both move and be moved: it is moved by the principal mover and it moves the last thing moved. For this reason, whatever moves and is moved has the character of an instrument.