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