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.