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WE must now treat of youth and old age and life and death. We must

probably also at the same time state the causes of respiration as

well, since in some cases living and the reverse depend on this.

We have elsewhere given a precise account of the soul, and while

it is clear that its essential reality cannot be corporeal, yet

manifestly it must exist in some bodily part which must be one of

those possessing control over the members. Let us for the present

set aside the other divisions or faculties of the soul (whichever of

the two be the correct name). But as to being what is called an animal

and a living thing, we find that in all beings endowed with both

characteristics (viz. being an animal and being alive) there must be a

single identical part in virtue of which they live and are called

animals; for an animal qua animal cannot avoid being alive. But a

thing need not, though alive, be animal, for plants live without

having sensation, and it is by sensation that we distinguish animal

from what is not animal.

This organ, then, must be numerically one and the same and yet

possess multiple and disparate aspects, for being animal and living

are not identical. Since then the organs of special sensation have one

common organ in which the senses when functioning must meet, and

this must be situated midway between what is called before and

behind (we call 'before' the direction from which sensation comes,

'behind' the opposite), further, since in all living things the body

is divided into upper and lower (they all have upper and lower

parts, so that this is true of plants as well), clearly the

nutritive principle must be situated midway between these regions.

That part where food enters we call upper, considering it by itself

and not relatively to the surrounding universe, while downward is that

part by which the primary excrement is discharged.