MO\T.MENT OF ANIMALS, x. 



soul as the point in the joints, which moves and is 

 moved, bears to that which is unmoved. Now since 

 the origin is in some animals situated in the heart, in 

 others in what corresponds to the heart, it is therefore 

 clear that the innate spirit also is situated there. 

 Whether the spirit is always the same or is always 

 changing must be discussed elsewhere (for the same 

 question arises about the other parts of the body) ; 

 at any rate it is clearly well adapted by nature to be 

 a motive power and to exercise strength. Now the 

 functions of movement are thrusting and pulling, so 

 that the organ of movement must be able to increase 

 and contract. And the nature of spirit has these 

 quahties ; for when it contracts it is without force," 

 and one and the same cause ^ gives it force and en- 

 ables it to thrust, and it possesses weight as compared 

 with the fiery element, and lightness as compared 

 with the contrary elements,'' Now that which is to 

 create movement without causing alteration must be 

 of this kind ; for the natural bodies ^ overcome one 

 another according as one of them prevails, the light 

 being conquered and borne down by the heavier and 

 the heavy borne up by the lighter. 



We have now stated what is the part by the move- 

 ment of w^hich the soul creates movement and for 

 what reason. The constitution of an animal must be 

 regarded as resembling that of a well-governed city- 

 state. For when order is once established in a city 

 there is no need of a special ruler with arbitrary 

 powers to be present at every activity, but each indi- 

 vidual performs his own task as he is ordered, and one 

 act succeeds another because of custom. And in the 



" The contrary of fire is water, c/. Be gen. et corrupt. 331 a 1. 

 ^ i.e. the elements. 



Q 475 



