On The Heavens   

proof of this is given in our discussion of movement, where it is

shown that no finite thing possesses infinite power, and no infinite

thing finite power.) If then that which moves naturally can also

move unnaturally, there will be two infinites, one which causes, and

another which exhibits the latter motion. Again, what is it that moves

the infinite? If it moves itself, it must be animate. But how can it

possibly be conceived as an infinite animal? And if there is something

else that moves it, there will be two infinites, that which moves

and that which is moved, differing in their form and power.

If the whole is not continuous, but exists, as Democritus and

Leucippus think, in the form of parts separated by void, there must

necessarily be one movement of all the multitude. They are

distinguished, we are told, from one another by their figures; but

their nature is one, like many pieces of gold separated from one

another. But each piece must, as we assert, have the same motion.

For a single clod moves to the same place as the whole mass of

earth, and a spark to the same place as the whole mass of fire. So

that if it be weight that all possess, no body is, strictly

speaking, light: and if lightness be universal, none is heavy.

Moreover, whatever possesses weight or lightness will have its place

either at one of the extremes or in the middle region. But this is

impossible while the world is conceived as infinite. And, generally,

that which has no centre or extreme limit, no up or down, gives the

bodies no place for their motion; and without that movement is

impossible. A thing must move either naturally or unnaturally, and the

two movements are determined by the proper and alien places. Again,

a place in which a thing rests or to which it moves unnaturally,

must be the natural place for some other body, as experience shows.

Necessarily, therefore, not everything possesses weight or

lightness, but some things do and some do not. From these arguments

then it is clear that the body of the universe is not infinite.