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.