Commentary Aristotle Physics
It can arise second from the nature of the mobile, as is evident in forced motions, as when a heavy object is thrown upward.
It can arise third from the medium.
All three are taken together as one resistance to constitute one cause of slowing up in the motion. Therefore, when the mobile, considered in isolation as different from the mover, is a being in act, the resistance of the mobile to the mover can be traced either to the mobile only, as happens in the heavenly bodies, or to the mobile and medium together, as happens in the case of animate bodies on this earth.
But, in heavy and light objects, if you take away what the mobile receives from the mover—namely, the form that is the principle of motion given by the generator (the mover)—nothing remains but the matter that can offer no resistance to the mover. Hence, in light and heavy objects, the only source of resistance is the medium.
Consequently, in heavenly bodies, differences in velocity arise only on account of the ratio between mover and mobile; in animate bodies, they arise from the proportion of the mover to the mobile and to the resisting medium—both together.
And it is in these latter cases that the given objection would have effect—that, if you remove the slowing up caused by the impeding medium, there still remains a definite amount of time in the motion, according to the proportion of the mover to the mobile. But, in heavy and light bodies, there can be no slowing up of speed except what the resistance of the medium causes—and in such cases, Aristotle’s argument applies.
536. But all this seems quite frivolous.
First, although the quantity of slowing up is not parallel to the mode of continuous quantity (such that motion is added to motion), but rather parallel to the mode of intensive quantity, as when something is whiter than something else, still the quantity of time from which Aristotle argues is parallel to the manner of continuous quantity—and time becomes greater by the addition of time to time. Hence, if you subtract the time that was added on account of the obstacle, the time of the natural velocity remains.
Next, if you remove the form that the generator gives to light and heavy bodies, there still remains in the understanding “quantified body,” which, from the very fact that it is a quantified body existing in an opposite position, offers resistance to the mover. For we cannot suppose in heavenly bodies any other resistance to their movers. Hence, as Averroes presents the case, even in the case of heavy and light bodies, the reasoning of Aristotle would not follow.
Therefore, it is better and briefer to say that the argument brought forward by Aristotle is an argument aimed at contradicting his opponent’s position and not a demonstrative argument in the absolute sense. For those who posited a void did so in order that motion be not prevented.
Thus, according to them, the cause of motion was on the part of a medium that did not impede motion. And therefore Aristotle argued against them as though the total cause of fastness and slowness derived from the medium, as he clearly shows above when he says that, if nature is the cause of the motion of simple bodies, it is not necessary to posit the void as the cause of their motion.
In this way, he gives us to understand that they supposed the total cause of the motion to depend on the medium and not on the nature of the mobile.
537. The second difficulty against his argument is that, if the medium that is full impedes, as he says it does, then it follows that there will not be any pure unimpeded motion in this lower medium, and this seems unfitting.
To this, the Commentator replies that the impediment that arises from the medium is required by the natural motion of heavy and light bodies, such that there can be resistance of the mobile to the mover, at least on the side of the medium. But it is better to say that every natural motion begins from a place that is not natural and tends to a natural place. Hence, until it reaches its natural place, it is not unfitting if something unnatural be attached to it. For it gradually departs from what is against its nature and tends to what is in keeping with its nature. And, for this reason, a natural motion is intended by its end.
538. The third objection is that, since there is a fixed limit of rarity in natural bodies, it does not seem that one can keep supposing a rarer and rarer body according to any given proportion of time to time.
In reply, it should be said that a fixed rarity in natural things is not due to the nature of the mobile body insofar as it is mobile, but to the nature of specific forms that require specific rareness and density. But, in this book, we are dealing with mobile body in general, and therefore Aristotle frequently uses in his arguments things that are false if the specific natures of bodies are considered, but possible if the nature of body in general is considered.
Or it can be replied that he is here also proceeding according to the opinion of the earlier philosophers who posited the rare and the dense as the first formal principles. According to them, rarity and density could be increased to infinity, since these did not depend on other previous forms according to whose exigencies they would be determined.
539. Then, at “the following depend upon” (216a12), he shows that there is no separated void, arguing from the speed and slowness of motion, insofar as the cause is taken entirely from the viewpoint of the mobile. And he says that what he is about to say will follow logically if we attend to the difference of speed and slowness insofar as bodies in motion exceed one another. For we see that, over a given equal space, greater speed is shown by bodies having a greater inclination due either to heaviness or lightness, whether they are greater in quantity but equally heavy or light, or whether they are equal in quantity but unequal in heaviness or lightness.
And I say this if they are similar in shape. For a wide body, if it be deficient in heaviness or size, is moved more slowly than a body with a pointed shape. And the ratio of the velocity corresponds to the ratio that the moving magnitudes have to one another in respect to their weight or in respect to their magnitude. And this will have to be true also if the motion occurs in the void, namely, that a heavier body or a lighter body or a more pointed body will be moved faster through an empty medium.
But this cannot be, since it is impossible to explain why one body would be moved faster than another. For if the motion takes place within a space filled with some body, an explanation for the greater or lesser speed can be given—it will be due to any of the named causes. The explanation is that a greater body will, on account of its strength, divide the medium more quickly; or on account of its shape, because what is sharp has greater penetrating power; or on account of a greater inclination, due either to the heaviness or lightness of the body, or even to the force imparted by that which projects it. But the void cannot be cleaved more quickly or more slowly. Hence, it will be moved through a void with equal speed. But this clearly appears as impossible. And so, from a consideration of the velocity of motion, it is evident that the void does not exist.
It should be observed that, in this reasoning process, there exists the same difficulty as in the first one. For he seems to suppose that difference in velocity in motions is due only to the different ways in which the medium can be cleaved, whereas the fact is that there are differences of velocity among the heavenly bodies in which there is no full medium resisting that has to be cleaved by the motion of the heavenly body. But this difficulty should be solved as the above one was.
540. Finally (216a21), he summarizes, and concludes that, from the foregoing, it is clear that, in regard to the philosophers who posit a void, it is the contrary of what they supposed as a reason for proving it. For they proceeded on the assumption that motion could not take place unless there was a void. But the contrary has been proved: if there be a void, there is no motion. Thus, those philosophers believe that the void is some distinct and separate thing—a space having separate dimensions—and they believed it was such a space that had to exist if local motion were to be possible. However, to posit such a separated void is the same as saying that place is a kind of space distinct from bodies—which is impossible, as was shown in the treatise on place.