Commentary Aristotle Physics
In regard to generation or alteration, the difficulty arises that there would also be an equal change of air into water and of water into air. For example, if air be generated from one cupful of water, it would be necessary that, from the same amount of air as was generated, an amount of water be generated somewhere else.
The reason is that there is now a greater amount of air than there previously was of water from which it was generated. The generated air, therefore, occupies a greater place than the water from which it was generated.
Consequently, either the whole body of the universe would have to occupy a greater place, or else as much air in some other place would have to be converted into water, or else it must finally be admitted that there is a void within bodies to allow them to be condensed, because these philosophers supposed that bodies could not become condensed and rarefied unless there was a void existing in them.
546. Then, at “now, if they mean” (216b30), he rejects this position:
first, according to one interpretation;
second, according to another interpretation, at “but, if they mean” (216b35; [547]).
He says therefore that those who posit a void within bodies can give this two interpretations.
The first is that, in each body, there are, as it were, many empty openings, each with a separate position from the other full parts, as can be seen in a sponge or in pumice or things of this sort.
The second interpretation is that the void is not separate in respect to position from the other parts of the body, as if we should say that the dimensions, which they said were the void, would penetrate all the parts of the body.
The refutation of their claim as to the first way of the void’s being in bodies is evident from what went before. For the very argument shows there is not a separate void outside of bodies, nor any place having such a space proper to itself over and above the dimensions of bodies. The same argument can be used to prove that there is no body so rarefied that it would have within itself any empty spaces distinct from the other parts of the body.
547. Then, at “but, if they mean” (216b35), he disproves the stated position as to the second interpretation and gives four reasons for rejecting it.
He says that, if the void is not in bodies in such a way as to be separable and distinct from the other parts but is nevertheless present in bodies, the situation is less impossible, because the difficulties mentioned above against a separate void do not arise; yet against this also certain discrepancies do arise.
First of all, the void will not be the cause of every local motion, as they maintained, but only of upward motion, for the void, according to them, is the cause of rarity, and the rare in turn is found to be light, as is evident in fire, and what is light travels upward. Consequently, the void will be the cause only of upward motion.
548. He gives the second reason at “second, the void turns out” (217a1), when he says that, according to those who posit a void in bodies, the void is the cause of motion not as that in which something is moved (in the way that those who held a separate empty space posited the void as a cause of motion), but as the cause of motion in such a way that the empty space within the bodies transports them.
It is analogous to the case of inflated wine skins, which, due to the fact that they are carried upward on account of their lightness, also carry upward whatever is attached to them. And, in this way, the void inherent in bodies carries with it the body in which it is.
But this seems impossible, because it would then follow that the void would have to be subject to motion and that there would exist a certain place for the void.
And, since the void and place are the same, it will follow that, of an interior void, there will be an exterior void into which it is transported—which is impossible.
549. He gives the third reason at “again, how will they” (217a5), when he says that, if the cause of upward motion is the void carrying a body upward, then there would be no explanation of why heavy bodies are carried downwards since there is nothing to carry the body down.
550. He then gives the fourth reason, at “and it is plain” (217a6), and says that, if the rare causes upward motion on account of emptiness, then the rarer and more empty a thing is, the faster it should be carried upward. And if it were completely empty, it should move with a maximum speed. But this is impossible, because what is completely empty cannot be moved, for the same reason by which it was shown above that motion is impossible in an empty space; for there is no way to compare the speeds of the empty and of the full (whether you consider the space or the mobile) according to some definite ratio, because there is no ratio between the full and the empty. Therefore, the void cannot be the cause of upward motion.
551. Then, at “since we deny that” (217a10), he answers a previous argument:
first, he repeats it, explaining it more extensively;
second, he solves it, at “our statement is based” (217a21; [552]).
He says first that, because we do not admit a void either in bodies or outside of them, we must answer the arguments of our opponents, because they present a real difficulty.
First of all, on the side of local motion, either local motion will not be if there is not rarity and density, which they believed could not be produced without the void, or we will be forced to say that, whenever a body is moved, the very heavens or some part of them are borne outward, which he calls the bulging of the heavens.
Second, from the viewpoint of generation and corruption, a transformation of water into air will always have to be balanced by an equal transformation of air into water somewhere else. For since more air is generated from water, it is required (unless condensation takes place, which they thought impossible without a void) either that the body that was held to be outermost according to common opinion—namely, the heavenly body—be pushed outward by the swelling of lower bodies or that there must be an equal amount of air converted into water somewhere else, such that the entire bulk of the universe remain always equal.
But, because one could in a certain way elude what he had said about local motion, he mentions this again in order to exclude it. Thus he repeats, or it follows that nothing is moved. Now, according to the foregoing, a disturbance of the heavens occurs whenever anything is changed. And this is true unless the motion is rotational.
For example, let A be in motion to place B, and B to place C, and C to place D, and again D to place A. In this case, on the assumption of rotational motion, it will not be necessary, if one thing moves, that the whole universe be disturbed. But we do not see every local motion of natural bodies to be rotational, but many are in a straight line. Hence, there will be still disturbance of the heavens unless condensation and the void be admitted. This, then, is the argument that prompted some to posit the void.
552. Then, at “our statement is based” (217a21), he answers this argument. Now, the entire force of this argument depends on rarefaction and condensation taking place by means of the void. Accordingly, Aristotle meets this by showing that rarefaction and condensation can take place without a void.
First, he reveals his proposition;
second, he introduces the conclusion he mainly intends, at “from what has been said” (217b20; [557]).
As to the first, he does three things:
first, he explains his proposition by an argument;
second, by examples, at “for as the same matter” (217a33; [555]);
third, by the effects of rarity and density, at “the dense is heavy” (217b11; [556]).
As to the first, he does two things: