On The Heavens   

from the portions in another world or from those in the same world,

but similarly to them, since in form no portion differs from

another. The result is that we must either abandon our present

assumption or assert that the centre and the extremity are each

numerically one. But this being so, the heaven, by the same evidence

and the same necessary inferences, must be one only and no more.

A consideration of the other kinds of movement also makes it plain

that there is some point to which earth and fire move naturally. For

in general that which is moved changes from something into

something, the starting-point and the goal being different in form,

and always it is a finite change. For instance, to recover health is

to change from disease to health, to increase is to change from

smallness to greatness. Locomotion must be similar: for it also has

its goal and starting-point--and therefore the starting-point and

the goal of the natural movement must differ in form-just as the

movement of coming to health does not take any direction which

chance or the wishes of the mover may select. Thus, too, fire and

earth move not to infinity but to opposite points; and since the

opposition in place is between above and below, these will be the

limits of their movement. (Even in circular movement there is a sort

of opposition between the ends of the diameter, though the movement as

a whole has no contrary: so that here too the movement has in a

sense an opposed and finite goal.) There must therefore be some end to

locomotion: it cannot continue to infinity.

This conclusion that local movement is not continued to infinity

is corroborated by the fact that earth moves more quickly the nearer

it is to the centre, and fire the nearer it is to the upper place. But

if movement were infinite speed would be infinite also; and if speed

then weight and lightness. For as superior speed in downward

movement implies superior weight, so infinite increase of weight

necessitates infinite increase of speed.

Further, it is not the action of another body that makes one of