MOVEMENT OF ANIMALS, ix.-x. 



body is potentially one, but actually must necessarily 

 become more than one ; for the limbs are set in 

 motion simultaneously from the origin of movement, 

 and when one is at rest the other is in motion. For 

 example, in ABC, B is moved and A moves it ; there 

 must, however, be something at rest . 



if one thing is to be moved and 

 another is to move it. So A, though 

 potentially one, will be actually two, 

 so that it must be not a point but a 

 magnitude. Again, C may be moved 

 simultaneously with B, so that both 

 the origins in A must cause movement by being 

 moved ; there must, therefore, be something other 

 than these origins which causes movement without 

 being itself moved. Otherwise, when movement took 

 place, the extremities, or origins, in A would rest 

 upon one another, like men standing back to back and 

 mo\dng their limbs. There must be something which 

 moves them both, namely the soul, other than such 

 a magnitude as we have described but situated in it. 

 X. In accordance with the definition which defines 

 the cause of motion, desire is the central origin, which 

 moves by being itself moved ; but in animate bodies 

 there must be some bodily substance which has these 

 characteristics. That, then, which is moved but does 

 not possess the natural quality of setting up move- 

 ment may be affected by a power external to it, and 

 that which causes movement must possess some 

 power and strength. Now all animals clearly both 

 possess an innate spirit and exercise their strength 

 in virtue of it. (WTiat it is that conserves the innate 

 spirit has been explained elsewhere. <*) This spirit 

 seems to bear the same relation to the origin in the 



473 



