1. I. ' 7 



the soul cannot exist; as would seem to be the case, seeing at 

 any rate that when the soul departs, what is left is no longer a 

 living animal, and that none of the parts remain what they were 

 before, excepting in mere configuration, like the animals that in 

 the fable are turned into stone ; if, I say, this be so, then it will 

 come within the province of the natural philosopher to inform 

 himself concerning the soul, and to treat of it, either in its entirety, 

 or, at any rate, of that part of it which constitutes the essential 

 character of an animal ; and it will be his duty to say what this 

 soul or this part of a soul is ; and to discuss the attributes that 

 attach to this essential character. Again, nature is spoken of in 

 two senses, and the nature of a thing is either its matter or its 

 essence ; nature as essence including both the motor cause and 

 the final cause.i'^ Now it is in the latter of these two senses that 

 either the whole soul or some part of it constitutes the nature of 

 an animal ; and inasmuch as it is the presence of the soul that 

 enables matter to constitute the animal nature, much more than 

 it is the presence of matter which so enables the soul, the enquirer 

 into nature is bound on every ground to treat of the soul rather 

 than of the matter. For though the wood of which they are made 

 constitutes the couch and the tripod, it only does so because it is 

 capable of receiving such and such a form. 



So far as has yet been said it remains a debatable question, 

 whether it is the whole soul or only some part of it, the considera- 

 tion of which comes within the province of natural science. Now 

 if it be of the whole soul that this should treat, then there is no 

 place for any other philosophy beside it. For as it belongs in all 

 cases to one and the same science to deal with correlated subjects 

 — one and the same science, for instance, deals with sensation and 

 with the objects of sense — and as therefore the intelligent soul 

 and the objects of intellect, being correlated, must belong to one 

 and the same science, it follows that natural science will have to 

 include the whole universe in its province. But perhaps it is not 

 the whole soul, nor all its parts collectively, that constitutes the 

 source of motion ; but there may be one part, identical with that 

 in plants, which is the source of [change of quantity or] growth, 

 another which is the source of feeling or change of quality, while 

 still another, and this not the intellectual part, is the source of 

 [change of place or] locomotion.^^ I say not the intellectual part ; 

 for other animals than man have the power of locomotion, but 

 641b. 



