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