Things naturally moved in a circle, are neither the same with those moved
in a right line, nor with those which are composed from things moved in a
right line.
_Demonstration._—Let AB be that which is naturally moved in a circle. I
say that it is not the same with those things which are moved in a right
line. For, if it is the same with any one of these, it must either be
naturally moved upwards or downwards. But every simple body is moved with
one simple motion according to nature. Hence, that which is naturally
moved in a circle, is not the same with anything moved in a right line.
But neither is it the same with anything compounded. For it has been
shown that everything which naturally moves in a circle is simple; but
that which consists from things moved in a right line is a composite. AB
therefore, which is naturally moved in a circle, is neither the same with
things moved in a right line, nor with those composed from these.