MOVEMENT OF ANIMALS, iv.-v. 



move themselves. For while one part of the animal 

 must be in motion, another part must be at rest, 

 supported upon which that will be moved which 

 is moved, if, for example, it moves one of its parts ; 

 for one part rests on another part in virtue of the 

 fact that the latter is at rest. 



But regarding inanimate things which are moved, 

 one might raise the question whether they all possess 

 in themselves both that wliich is at rest and that which 

 creates movement, and whether they too must be 

 supported by something external which is at rest. 

 Or is this impossible — for example, in the case of 

 fire or earth or any inanimate thing — but motion is 

 due to the primary causes by which these are moved ? 

 For all inanimate things are moved by something else, 

 and the origin of all the things that are thus moved is 

 the things that move themselves. Among things of 

 this class we have already dealt \\'ith animals ; for all 

 such things must necessarily have within themselves 

 that which is at rest and something outside them on 

 which they are to support themselves. But whether 

 there is something higher and primary which moves 

 them is uncertain, and the question of such an origin 

 of movement is a matter for separate discussion. But 

 animals which move all do so supported upon things 

 outside themselves, as also when they draw their 

 breath in and out. For it makes no difference whether 

 the}" propel a great or a small weight, as those do who 

 spit and cough, and breathe in and out. 



V. But is it only in that which moves itself in 

 respect of place that something must remain at rest, 

 or is this also true of that in which alteration is 

 caused by its own agency and in that which grows ? 

 The question of original coming into being and 



455 



