ON THE MOVEMENT OF ANIMALS 



I. We have inquired elsewhere ° into the details of 

 the movement of the various kinds of animals, the 

 differences between these movements, and the causes 

 of the characteristics which each exhibit ; we must 

 now inquire generally into the common cause of 

 animal movement of whatever kind — for some animals 

 move by flight, some by swimming, some by walking, 

 and others by other such methods. 



Now that the origin of all the other movements is 

 that which moves itself, and that the origin of this is 

 the immovable, and that the prime mover must neces- 

 sarily be immovable, has already been determined 

 when we were investigating ^ whether or not eternal 

 movement exists, and if it does exist what it is. And 

 this we must apprehend not merely in theory as a 

 general principle but also in its individual manifesta- 

 tions and in the objects of sense-perception, on the 

 basis of which we search for general theories and 

 with which we hold that these theories ought to 

 agree. For it is clear also in the objects of sense- 

 perception that movement is impossible if there 

 is nothing in a state of rest, and above all in the 

 animals themselves. For if any one of their parts 

 moves, another part must necessarily be at rest ; and 



* In the De partibus animalium. 

 ^ Physics viii. 258 b 4-9. 



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