Commentary Aristotle Physics
Lecture 11
It is shown from motion that there is no separated void
Let us explain again that there is no void existing separately, as some maintain. If each of the simple bodies has a natural locomotion—such as fire upward and earth downward and towards the middle of the universe—it is clear that it cannot be the void that is the cause of locomotion. Of what, then, will the void be the cause? It is thought to be the cause of motion in respect of place, and it is not the cause of this.
Again, if void is a sort of place deprived of body, then, when there is a void, where will a body placed in it move to? It certainly cannot move into the whole of the void.
The same argument applies as against those who think that place is something separate into which things are carried: how will what is placed in it move or rest?
Much the same argument will apply to the void as to up and down in place, as is natural enough, since those who maintain the existence of the void make it a place. And in what way will things be present either in place or in the void?
For the expected result does not take place when a body is placed as a whole in a place conceived of as separate and permanent; for a part of it, unless it be placed apart, will not be in a place, but in the whole.
Further, if separate place does not exist, neither will void.
If people say that the void must exist, as being necessary if there is to be motion, what rather turns out to be the case is the opposite, if one is mindful: not a single thing can be moved if there is a void.
For as with those who for a like reason say the earth is at rest, so, too, in the void, things must be at rest, since there is no place to which things can move more or less than to another, since the void, insofar as it is void, admits no difference.
The second reason is this. All motion is either by force or according to nature, and if there is motion by force, there must also be natural (for motion by force is contrary to nature, and motion contrary to nature is posterior to that according to nature, such that, if each of the natural bodies has not a natural motion, none of the other motions can exist).
But how can there be natural motion if there is no difference throughout the void or the infinite? For insofar as it is infinite, there will be no up or down or middle, and insofar as it is a void, up does not differ from down; for as there is no difference in what is nothing, there is none in the void (for the void seems to be a non-existent and a privation of being), but natural locomotion seems to be differentiated, such that the things that exist by nature must be differentiated. So, either nothing has a natural locomotion or else there is no void.
Further, in point of fact, things that are thrown move, although that which gave them their impulse is not touching them—either by reason of antiperistasis, as some maintain, or because the air that has been pushed pushes them with a motion quicker than the natural locomotion of the projectile wherewith it moves to its proper place.
But, in a void, none of these things can take place, nor can anything be moved save as that which is carried is moved.
Further, no one could say why a thing once set in motion should stop anywhere—for why should it stop here rather than there?—such that a thing will either be at rest or must be moved to infinity, unless something more powerful get in its way.
Further, things are now thought to move into the void because it yields; but, in a void, this quality is present equally everywhere, such that things should move in all directions.
520. Having gone over others’ opinions about the void and having indicated what is meant by the word “void,” he now begins to search for the truth.
First, he shows that the void does not have a separate existence;
second, that there is no void in bodies, at “there are some who think” (216b22; [544]).
Concerning the first, he does two things:
first, he uses motion to show that a separate void does not exist;
second, by considering the void in itself, at “but, even if we consider” (216a26; [541]).
As to the first, he does two things:
first, from the fact of motion, he shows there is no void;
second, from the fact of faster and slower motions, at “further, the truth” (215a24; [527]).
521. In regard to the first point, he gives six reasons.
In regard to the first of these, he says (214b12) that we must repeat that there is no separated void as some assert. He says again because this was already somewhat proved from the notion of place: for if place is not space, it follows that the void is nothing, as was said above. But now he proves the same point again from motion: for void was posited, as we said, on account of motion. But motion does not necessarily require a void. For it would seem especially to be the cause of local motion; but it is not necessary to posit a void in order to explain local motion, because all simple bodies have natural local motions, as the natural motion of fire is upward and that of earth downward toward the center. Thus, it is clear that it is the nature of each body that causes its local motion and not the void. The latter would be the cause if any natural bodies were moved due to the necessity of a void. But, if it is not posited as the cause of local motion, it cannot be considered the cause of any other motion or of any other thing. The void, therefore, would exist without a purpose.
522. He then gives the second reason at “again, if void” (214b17). If a void be postulated, no reason can be assigned for natural motion and rest. For it is clear that a natural body is moved toward its own natural place and rests there naturally on account of the affinity it has with its place and because it has no affinity to the place from which it departs. But the void has no nature by which it could be akin or hostile to a natural body. Therefore, if there were a void, considered as a certain place without a body in it, one could not assign any part to which the body would be naturally moved. For we cannot say that it would be borne to just any part, because observation shows that this is wrong, for a body naturally goes from one place and naturally approaches another.
This same reason is valid against those who posit place as a separate space into which a mobile body is borne. For it would be impossible to explain how a body in such a place could either be moved or be at rest, since the dimensions of space have no nature to which a natural body could be similar or dissimilar. Deservedly, then, the same argument applies to the void as to up and down—namely, to place, whose parts are up and down—for those who posit a void call it place.
Moreover, those who posit a void and those who posit place to be space are unable to explain not only how something is moved and at rest in respect to place but also how something exists in place or in the void. For if place is supposed to be space, then the whole body would have to be enclosed inside that space, and not as happens with those who agree that place is the boundary of the containing body and that a thing is in place as in something separate and as in a body that contains and sustains it. Indeed, it seems to be of the very account of place that something be in place as in something separated and existing apart from it, for if any part of a body is not posited as separated from the body, it will exist in that body not as in a place, but as in the whole. Therefore, it pertains to the very account of place and of the thing in place that one be separated from the other. But this does not happen if place is space into whose entirety the entire body is immersed. Therefore, space is not place. And if space is not place, it is clear that no void is place.
523. He gives the third reason at “if people say that” (214b28), saying that, whereas the early philosophers claimed that the void had to be if motion existed, the very opposite is the case, for if there were a void, there would be no motion. And this he proves by a simile. For some have said that earth comes to rest at the center on account of the likeness of the parts on the whole circumference: earth, having no reason to be moved toward one part of the circumference more than another, rests. The same reason would cause rest in a void. For there would be no reason for earth to be moved to one part rather than to another, since the void, as such, does not have differences among its parts, as non-being does not possess differences.
524. He gives the fourth reason at “the second reason is this” (215a1). Natural motion is prior to motion by force, since motion by force is only a departure from natural motion.
Therefore, remove natural motion, and all other motion is removed; for when the prior is removed, all that follows is removed.
But, if the void is posited, natural motion is removed, because the differences among the parts of place would be taken away, and it is toward such parts that natural motions tend. The same holds if the infinite be posited, as we said above.
There is, however, this difference between the void and the infinite: after granting the infinite, there is no way of positing “up” or “down” or “center,” as we pointed out in book 3; but, after granting a void, these places could be posited, though it would not be because they were mutually different.
For no difference can be assigned in the realm of nothing and non-being, nor consequently in that of the void, which is a non-being and a privation. Yet natural changes of place do require difference of place, because diverse bodies are moved to diverse places.
Consequently, natural places must be different one from the other. Therefore, if a void be posited, nothing could undergo a natural change of place; and, if there is no natural change of place, there will be no change of place of any sort. Hence, if there is any change of place, there can be no void.
525. The fifth reason is then given at “further, in point of fact” (215a14). In regard to this, it should be considered that some question exists about projectiles, for the mover and the thing moved must be always together, as will be proved below in book 7. Yet a projectile is found to be in motion even after it is separated from the projector, as is evident in the case of a stone that is thrown, or of an arrow shot from a bow. Now, on the supposition that there is no void, this difficulty is solved by attending to the air with which the medium is filled.
And it is solved in two ways.
For some assert that projectiles remain in motion even after they are no longer in contact with what gave them impulse on account of antiperistasis, that is, repercussion or counter-resistance.
For the air that has been pushed pushes against other air, and that against other air, and so on, and it is on account of this impact of air against air that the stone is moved.
The other explanation is that the continuum of air that received the impact from the projector pushes the impelled body with more speed than the speed of the motion by which the projectile is naturally borne to its proper place. Hence, the speed of the air’s motion prevents the projectile, such as a stone or something similar, from falling downward; but it is carried along by the impulse of the air.
Now, neither of these explanations could be given if there were a void; consequently, a projectile could be moved only as long as it was carried—for example—in the hand of the one casting it, but as soon as it was released from the hand, it would fall. But it is the opposite that happens. Therefore, there is not a void.
526. He then gives the sixth reason at “further, no one could say” (215a19). If motion were in a void, no one could give a reason why the moving object should stop anywhere. For there is no reason why it should stop at one part of the void rather than another. This is true both in the case of objects that are moved naturally—because there is no difference among the parts of the void, as we have said—and in the case of objects moved by a forced motion.
For now we say that a forced motion ceases when repercussions or impulsions of the air are lacking on account of the two reasons already given.
Therefore, it will have to be admitted either that every body is at rest and nothing in motion or that, if anything be in motion, it will remain in motion to infinity unless it runs into a more powerful body that could impede its forced motion.
In support of this reasoning, he gives the reason why some posit motion in the void. It is because the void yields to and does not resist the mobile. Hence, since the void yields in the same way, in all directions, a mobile thing should be moved in all directions to infinity.