A motion which is naturally circular is perpetual.
_Demonstration._—Let the circular motion be that of the circle A B, I say
that it is perpetual: for, since time is perpetual, it is also necessary
that motion should be perpetual. And since time is continued, (for
there is the same _now_ in the past and present time,) it is necessary
that there should be some one continued motion: for time is the number
of motion. However, all other motions are not perpetual: for they are
generated from contraries into contraries. A circular motion, therefore,
is alone perpetual: for to this, as we have demonstrated, nothing is
contrary. But that all the motions which subsist between contraries,
are bounded, and are not perpetual, we thus demonstrate. Let A B be a
motion between the two contraries A and B. The motion, therefore, of A B
is bounded by A and B, and is not infinite. But the motion from A is not
continued with that from B. But, when that which is moved returns, it
will stand still in B: for, if the motion from A is one continued motion,
and also that from B, that which is moved from B will be moved into the
same. It will therefore be moved in vain, being now in A. But nature
does nothing in vain: and hence, there is not one motion. The motions,
therefore, between contraries are not perpetual. Nor is it possible for
a thing to be moved to infinity in a right line: for contraries are the
boundaries. Nor when it returns will it make one motion.