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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 A BCD 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. And in a similar manner 

 with respect to that which is moved from C, if it 

 is moved with one motion to B, A is not contrary 

 to C, so that neither will the motions from these 

 be contrary. 



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 with- 

 out generation and corruption : for if it is gene- 

 rable and corruptible, it is generated from a con- 

 trary, and is corrupted into a contrary. But that 



