On The Heavens
if the natural motion is upward, it will be fire or air, and if
downward, water or earth. Further, this circular motion is necessarily
primary. For the perfect is naturally prior to the imperfect, and
the circle is a perfect thing. This cannot be said of any straight
line:-not of an infinite line; for, if it were perfect, it would
have a limit and an end: nor of any finite line; for in every case
there is something beyond it, since any finite line can be extended.
And so, since the prior movement belongs to the body which naturally
prior, and circular movement is prior to straight, and movement in a
straight line belongs to simple bodies-fire moving straight upward and
earthy bodies straight downward towards the centre-since this is so,
it follows that circular movement also must be the movement of some
simple body. For the movement of composite bodies is, as we said,
determined by that simple body which preponderates in the composition.
These premises clearly give the conclusion that there is in nature
some bodily substance other than the formations we know, prior to them
all and more divine than they. But it may also be proved as follows.
We may take it that all movement is either natural or unnatural, and
that the movement which is unnatural to one body is natural to
another-as, for instance, is the case with the upward and downward
movements, which are natural and unnatural to fire and earth
respectively. It necessarily follows that circular movement, being
unnatural to these bodies, is the natural movement of some other.
Further, if, on the one hand, circular movement is natural to
something, it must surely be some simple and primary body which is
ordained to move with a natural circular motion, as fire is ordained
to fly up and earth down. If, on the other hand, the movement of the
rotating bodies about the centre is unnatural, it would be
remarkable and indeed quite inconceivable that this movement alone
should be continuous and eternal, being nevertheless contrary to
nature. At any rate the evidence of all other cases goes to show
that it is the unnatural which quickest passes away. And so, if, as