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.
The particles of earth, then, in another world move naturally also to our centre and its fire to our circumference.This, however, is impossible, since, if it were true, earth must, in its own world, move upwards, and fire to the centre; in the same way the earth of our world must move naturally away from the centre when it moves towards the centre of another universe.This follows from the supposed juxtaposition of the worlds.For either we must refuse to admit the identical nature of the simple bodies in the various universes, or, admitting this, we must make the centre and the extremity one as suggested.This being so, it follows that there cannot be more worlds than one.
