Place, however, is not the heavens, but its external quiescent boundary which touches the body moved. Hence the earth is in water, water in air, air in ether, but ether in the heavens.” From this point Aristotle goes on (Phys. IV. 6, 7) to empty space, in which an old question is involved which physicists even now cannot explain: they could do so if they studied Aristotle, but as far as they are concerned there might have been no thought nor Aristotle in the world. “Vacuum, according to ordinary ideas, is a space in which there is no body, and, fancying that all Being is body, they say that vacuum is that in which there is nothing at all. The conception of a vacuum has its justification for one thing in the fact that a vacuum,” the negative to an existent form, “is essential to motion; for a body could not move in a plenum,” and in the place to which it does move there must be nothing. “The other argument in favour of a vacuum is found in the compression of bodies, in which the parts press into the empty spaces.” This is the conception of varying density and the alteration of the same, in accordance with which an equal weight might consist of an equal number of parts, but these, as being separated by vacuum, might present a greater volume. Aristotle confutes these reasonings most adroitly, and first of all in this way; “The plenum could be changed, and bodies could yield to one another even if no interval of vacuum separated them. Liquids as well as solids are not condensed into a vacuum; something that they contained is expelled, just as air is expelled if water is compressed.”