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