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bounded, and are not perpetual, we thus demon- 

 strate. 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 in- 

 finity in a right line : for contraries are the boun- 

 daries. Nor when it returns will it make one 

 motion. 



THEOREM 10. 



That which moves a perpetual motion is per- 

 petual. 



Demonstration. For let A be that which moves 

 a perpetual motion. I say that A also is perpe- 

 tual : for, if it is not, it will not then move when 

 it is not. But this not moving, neither does the 

 motion subsist, which it moved before. It is how- 

 ever supposed to be perpetual. But, nothing else 

 moving, that will be immoveable which is perpe- 



