On The Heavens   

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.

To postulate a difference of nature in the simple bodies according

as they are more or less distant from their proper places is

unreasonable. For what difference can it make whether we say that a

thing is this distance away or that? One would have to suppose a

difference proportionate to the distance and increasing with it, but

the form is in fact the same. Moreover, the bodies must have some

movement, since the fact that they move is quite evident. Are we to

say then that all their movements, even those which are mutually

contrary, are due to constraint? No, for a body which has no natural

movement at all cannot be moved by constraint. If then the bodies have

a natural movement, the movement of the particular instances of each

form must necessarily have for goal a place numerically one, i.e. a

particular centre or a particular extremity. If it be suggested that

the goal in each case is one in form but numerically more than one, on

the analogy of particulars which are many though each undifferentiated

in form, we reply that the variety of goal cannot be limited to this

portion or that but must extend to all alike. For all are equally

undifferentiated in form, but any one is different numerically from

any other. What I mean is this: if the portions in this world behave

similarly both to one another and to those in another world, then

the portion which is taken hence will not behave differently either