Theorem 5
Things which are naturally moved in a circle, neither receive generation nor corruption.
Demonstration.—For let AB be that which is naturally moved in a circle, I say that AB is without generation and corruption: for if it is generable and corruptible, it is generated from a contrary, and is corrupted into a contrary. But that which is moved in a circle has not any contrary. It is therefore without generation and corruption. But that there is nothing contrary to things naturally moving in a circle, is evident from what has been previously demonstrated: for the motions of things contrary according to nature are contrary. But, as we have demonstrated, there is nothing contrary to the motion in a circle. Neither, therefore, has that which is moved in a circle any contrary.