Aristotle deals more thoroughly, in the first place (Phys. IV. 8), with the erroneous conception that the vacuum is the cause of movement. For, on the one hand, he shows that the vacuum really abolishes motion, and consequently in vacuum a universal rest would reign. He calls it perfect indifference as to the greater or less distance to which a thing is moved; in vacuum there are no distinctions. It is pure negation without object or difference; there is no reason for standing still or going on. But body is in movement, and that, indeed, as distinguished; it has a positive relation, and not one merely to nothing. On the other hand, Aristotle refutes the idea that movement is in vacuum because compression is possible. But this does not happen in a vacuum; there would be established in it not one movement, but a movement towards all sides, a general annihilation, an absolute yielding, where no cohesion would remain in the body. “Again, a weight or a body is borne along more swiftly or more slowly from two causes; either because there is a difference in that through which it is borne along, as when it moves through air or water or earth, or because that which is borne along differs through excess of weight or lightness.” As regards difference of movement on account of the first difference—that in the density of the medium—Aristotle says: “The medium through which the body is borne along is the cause of the resistance encountered, which is greater if the medium is moving in a contrary direction (and less if it is at rest); resistance is increased also if the medium is not easily divided. The difference in velocity is in inverse ratio to the specific gravity of the medium, air and water, so that if the medium has only half the density, the rate of progress will be double as quick. But vacuum has to body no such relation of differences of specific gravity. Body can no more contain a vacuum within its dimensions than a line can contain a point, unless the line were composed of points. The vacuum has no ratio to the plenum.” But as to the other case, the difference in weight and lightness, which has to be considered as being in bodies themselves, whereby one moves more quickly than another through the same space: “this distinction exists only in the plenum, for the heavy body, by reason of its power, divides the plenum more quickly.” This point of view is quite correct, and it is mainly directed against a number of conceptions that prevail in our physics. The conception of equal movement of the heavy and the light, as that of pure weight, pure matter, is an abstraction, being taken as though they were inherently like, only differing through the accidental resistance of the air.