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On The Soul   

in the soul; and lastly (4) the appetitive, which would seem to be

distinct both in definition and in power from all hitherto enumerated.

It is absurd to break up the last-mentioned faculty: as these

thinkers do, for wish is found in the calculative part and desire

and passion in the irrational; and if the soul is tripartite

appetite will be found in all three parts. Turning our attention to

the present object of discussion, let us ask what that is which

originates local movement of the animal.

The movement of growth and decay, being found in all living

things, must be attributed to the faculty of reproduction and

nutrition, which is common to all: inspiration and expiration, sleep

and waking, we must consider later: these too present much difficulty:

at present we must consider local movement, asking what it is that

originates forward movement in the animal.

That it is not the nutritive faculty is obvious; for this kind of

movement is always for an end and is accompanied either by imagination

or by appetite; for no animal moves except by compulsion unless it has

an impulse towards or away from an object. Further, if it were the

nutritive faculty, even plants would have been capable of

originating such movement and would have possessed the organs

necessary to carry it out. Similarly it cannot be the sensitive

faculty either; for there are many animals which have sensibility

but remain fast and immovable throughout their lives.

If then Nature never makes anything without a purpose and never

leaves out what is necessary (except in the case of mutilated or

imperfect growths; and that here we have neither mutilation nor

imperfection may be argued from the facts that such animals (a) can

reproduce their species and (b) rise to completeness of nature and

decay to an end), it follows that, had they been capable of

originating forward movement, they would have possessed the organs

necessary for that purpose. Further, neither can the calculative

faculty or what is called 'mind' be the cause of such movement; for

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