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On The Heavens   

excess of solid or by its defect of void. On the former view there

could be an amount of earth so small as to contain less solid than a

large mass of fire. And similarly, if the distinction rests on the

amount of void, there will be a body, lighter than the absolutely

light, which nevertheless moves downward as constantly as the other

moves upward. But that cannot be so, since the absolutely light is

always lighter than bodies which have weight and move downward, while,

on the other hand, that which is lighter need not be light, because in

common speech we distinguish a lighter and a heavier (viz. water and

earth) among bodies endowed with weight. Again, the suggestion of a

certain ratio between the void and the solid in a body is no more

equal to solving the problem before us. The manner of speaking will

issue in a similar impossibility. For any two portions of fire,

small or great, will exhibit the same ratio of solid to void, but

the upward movement of the greater is quicker than that of the less,

just as the downward movement of a mass of gold or lead, or of any

other body endowed with weight, is quicker in proportion to its

size. This, however, should not be the case if the ratio is the ground

of distinction between heavy things and light. There is also an

absurdity in attributing the upward movement of bodies to a void which

does not itself move. If, however, it is the nature of a void to

move upward and of a plenum to move downward, and therefore each

causes a like movement in other things, there was no need to raise the

question why composite bodies are some light and some heavy; they

had only to explain why these two things are themselves light and

heavy respectively, and to give, further, the reason why the plenum

and the void are not eternally separated. It is also unreasonable to

imagine a place for the void, as if the void were not itself a kind of

place. But if the void is to move, it must have a place out of which

and into which the change carries it. Also what is the cause of its

movement? Not, surely, its voidness: for it is not the void only which

is moved, but also the solid.

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