On The Heavens   

these bodies move up and the other down; nor is it constraint, like

the 'extrusion' of some writers. For in that case the larger the

mass of fire or earth the slower would be the upward or downward

movement; but the fact is the reverse: the greater the mass of fire or

earth the quicker always is its movement towards its own place. Again,

the speed of the movement would not increase towards the end if it

were due to constraint or extrusion; for a constrained movement always

diminishes in speed as the source of constraint becomes more

distant, and a body moves without constraint to the place whence it

was moved by constraint.

A consideration of these points, then, gives adequate assurance of

the truth of our contentions. The same could also be shown with the

aid of the discussions which fall under First Philosophy, as well as

from the nature of the circular movement, which must be eternal both

here and in the other worlds. It is plain, too, from the following

considerations that the universe must be one.

The bodily elements are three, and therefore the places of the

elements will be three also; the place, first, of the body which sinks

to the bottom, namely the region about the centre; the place,

secondly, of the revolving body, namely the outermost place, and

thirdly, the intermediate place, belonging to the intermediate body.

Here in this third place will be the body which rises to the

surface; since, if not here, it will be elsewhere, and it cannot be

elsewhere: for we have two bodies, one weightless, one endowed with

weight, and below is place of the body endowed with weight, since

the region about the centre has been given to the heavy body. And

its position cannot be unnatural to it, for it would have to be

natural to something else, and there is nothing else. It must then

occupy the intermediate place. What distinctions there are within

the intermediate itself we will explain later on.

We have now said enough to make plain the character and number of

the bodily elements, the place of each, and further, in general, how