_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. THEOREM 3. Things which are naturally moved in a circle, neither participate of gravity nor levity. _Demonstration._—For if AB is either heavy or light, it is either naturally moved to the middle, or from the middle: for, from the definitions, that is heavy which is moved to the middle, and that is light which is moved from the middle. But that which is moved either from or to the middle, is the same with some one of the things moved in a right line. AB, therefore, is the same with something moved in a right line, though naturally moved in a circle, which is impossible. THEOREM 4. Nothing is contrary to a circular motion. _Demonstration._—For if this be possible, let the motion from A to B be a circular motion, and let the motion contrary to this be either some one of the motions in a right line, or some one of those in a circle. If, then, the motion upwards is contrary to that in a circle, the motion downwards and that in a circle will be one. But if the motion downwards is contrary to that in a circle, the motion upwards and that in a circle will be the same with each other; for one motion is contrary to one into opposite places. But if the motion from A is contrary to the motion from B, there will be infinite spaces between two contraries; for between the points A, B infinite circumferences may be described. But let AB be a semicircle, and let the motion from A to B be contrary to the motion from B to A. If, therefore, that which moves in the semicircle from A to B stops at B, it is by no means a motion in a circle: for a circular motion is continually from the same to the same point. But, if it does not stop at B, but continually moves in the other semicircle, A is not contrary to B. And if this be the case, neither is the motion from A to B contrary to the motion from B to A: for contrary motions are from contraries to contraries. But let ABCD be a circle, and let the motion from A to C be contrary to the motion from C to A. If therefore that which is moved from A passes through all the places similarly, and there is one motion from A to D, C is not contrary to A. But if these are not contrary, neither are the motions from them contrary.