On The Heavens   

8



We must now proceed to explain why there cannot be more than one

heaven-the further question mentioned above. For it may be thought

that we have not proved universal of bodies that none whatever can

exist outside our universe, and that our argument applied only to

those of indeterminate extent.

Now all things rest and move naturally and by constraint. A thing

moves naturally to a place in which it rests without constraint, and

rests naturally in a place to which it moves without constraint. On

the other hand, a thing moves by constraint to a place in which it

rests by constraint, and rests by constraint in a place to which it

moves by constraint. Further, if a given movement is due to

constraint, its contrary is natural. If, then, it is by constraint

that earth moves from a certain place to the centre here, its movement

from here to there will be natural, and if earth from there rests here

without constraint, its movement hither will be natural. And the

natural movement in each case is one. Further, these worlds, being

similar in nature to ours, must all be composed of the same bodies

as it. Moreover each of the bodies, fire, I mean, and earth and

their intermediates, must have the same power as in our world. For

if these names are used equivocally, if the identity of name does

not rest upon an identity of form in these elements and ours, then the

whole to which they belong can only be called a world by equivocation.

Clearly, then, one of the bodies will move naturally away from the

centre and another towards the centre, since fire must be identical

with fire, earth with earth, and so on, as the fragments of each are

identical in this world. That this must be the case is evident from

the principles laid down in our discussion of the movements, for these

are limited in number, and the distinction of the elements depends

upon the distinction of the movements. Therefore, since the

movements are the same, the elements must also be the same everywhere.