Commentary Aristotle Physics
Lecture 14. There is no void within bodies
There are some who think that the existence of rarity and density shows that there is a void. If rarity and density do not exist, they say, neither can things go in and harden.
But, if this were not to take place, either there would be no motion at all or the universe would bulge, as Xuthus said, or air and water must always change into equal amounts (for instance, if air has been made out of a cupful of water, at the same time, out of an equal amount of air, a cupful of water must have been made), or void must necessarily exist.
For compression and expansion cannot take place otherwise.
Now, if they mean by “the rare” that which has many voids existing separately, it is plain that, if void cannot exist separately any more than a place can exist with an extension all to itself, neither can the rare exist in this sense.
But, if they mean that there is void not separately existent but still present in the rare, this is less impossible. But, first, the void turns out not to be a cause of all motion, but only of motion upward, for the rare is light, which is the reason they say fire is rare.
Second, the void turns out to be a cause of motion not as that in which it takes place, but as what carries things up, just as skins, by being carried up themselves, carry up what is continuous with them. Yet how can void have a local motion or a place? For thus, that into which void moves is till then void of a void.
Again, how will they explain, in the case of what is heavy as to its motion downward?
And it is plain that, if a thing will move upward more quickly the rarer and more void it is, then, if it were completely void, it would move with a maximum speed. But perhaps it is impossible that it should even move at all; the same reason that showed that, in the void, all things are incapable of moving shows that the void cannot move, which is the fact that the speeds are incomparable.
Since we deny that a void exists, but as for the rest, there are real doubts—either there will be no motion, if there is not to be condensation and rarefaction, or there will be a bulging of the heavens, or a transformation of water into air will always be balanced by an equal transformation of air into water (for it is clear that the air produced from water is bulkier than the water)—it is necessary, therefore, if compression does not exist, either that the next portion will be pushed outward and make the outermost part bulge, or that there must be an equal amount of water produced somewhere else out of air, such that the entire bulk of the whole may be equal, or it follows that nothing is moved.
For when anything is displaced, this will always happen, unless it comes round in a circle; but locomotion is not always circular, but sometimes in a straight line. These, then, are the reasons for which they might say that there is a void.
Our statement is based on the assumption that (1) there is a single matter for contraries—hot and cold and the other natural contrarieties—and that (2) what exists actually is produced from a potential existent, and that (3) matter is not separable from the contraries, but its being is different, and that (4) a single matter may serve for color and heat and cold.
The same matter also serves for both a large and a small body. This is evident, for when air is produced from water, the same matter has not become something different by acquiring an addition to it, but has become actually what it was potentially; again, water is produced from air in the same way, the change being sometimes from smallness to greatness and sometimes from greatness to smallness.
Similarly, therefore, if air that is large in extent comes to have a smaller volume, or if air becomes greater from being smaller, it is the matter that is potentially both that comes to be each of the two.
For as the same matter becomes hot from being cold and cold from being hot because it was potentially both, so too, from hot, it can become more hot, though nothing in the matter has become hot that was not hot when the thing was less hot—just as if the ambit or curve of a greater circle becomes that of a smaller, whether it remains the same or becomes a different curve, convexity has not come to exist in anything that was not convex but straight (for difference of degree do not depend on an intermission of the quality); nor can we get any portion of a flame in which both heat and whiteness are not present.
So too, then, is the earlier heat related to the later. Thus, the greatness and smallness also of the sensible volume are extended not by the matter’s acquiring anything new, but because the matter is potentially matter for both states; in this way, the same thing is dense and rare and the two qualities have one matter.
The dense is heavy, and the rare is light. Again, just as the arc of a circle does not acquire a new part that is convex when it is contracted into a smaller space, but rather what was there has been contracted, and just as any part of fire that one takes will be hot, so too, it is all a question of contraction and expansion of the same matter.
There are two types in each case, both in the dense and in the rare, for both the heavy and the hard are thought to be dense and, contrariwise, both the light and the soft are rare; and weight and hardness fail to coincide in the case of lead and iron.
From what has been said, it is evident, then, that void does not exist either separate (either absolutely separate or as a separate element in the rare) or potentially, unless one is willing to call the cause of motion void, whatever it may be.
At that rate, the matter of the heavy and the light, qua matter, would be the void, for the dense and the rare are productive of locomotion in virtue of this contrariety, and in virtue of their hardness and softness, productive of passivity and impassivity—that is, not of locomotion, but rather of qualitative change. So much, then, for the discussion of the void and of the sense in which it exists and the sense in which it does not exist.
544. Having shown that there is no separated void, the Philosopher here shows that there is no void inherent in bodies.
As to this, he does three things:
first, he gives the reason proposed by those who posit such a void;
second, he disproves their position, at “now, if they mean” (216b30; [546]);
third, he dissolves their argument, at “since we deny” (217a10; [551]).
545. He first says, therefore (216b22), that there have been some philosophers who believed that there is a void in bodies, basing their argument on the existence of rarity and density. For they believed that rarefaction and condensation took place on account of a void inhering in bodies. If rarity and density did not exist, they say, the parts of bodies could not contract (enter each other) and harden (be compressed by condensation). But, if this does not take place, they deduced certain difficulties both in respect to local motion and in respect to the motions of generation and corruption, or alteration.
Such a difficulty in respect to local motion is that it would be necessary to admit either that motion does not exist at all or that the whole universe is moved with one motion, as says Xuthus, a philosopher. This would be because, if a body were moved locally, when it approached a place full of another body, this body would have to be expelled and tend toward another place, and the body found there would have to go to yet another place, such that, unless there were condensation of bodies, all bodies would have to be in motion.