Further, we must ask what is the force that holds together
the earth and the fire which tend to travel in contrary directions; if
there is no counteracting force, they will be torn asunder; if there
is, this must be the soul and the cause of nutrition and growth. By
some the element of fire is held to be the cause of nutrition and
growth, for it alone of the primary bodies or elements is observed
to feed and increase itself. Hence the suggestion that in both
plants and animals it is it which is the operative force. A concurrent
cause in a sense it certainly is, but not the principal cause, that is
rather the soul; for while the growth of fire goes on without limit so
long as there is a supply of fuel, in the case of all complex wholes
formed in the course of nature there is a limit or ratio which
determines their size and increase, and limit and ratio are marks of
soul but not of fire, and belong to the side of formulable essence
rather than that of matter.
Nutrition and reproduction are due to one and the same psychic
power. It is necessary first to give precision to our account of food,
for it is by this function of absorbing food that this psychic power
is distinguished from all the others. The current view is that what
serves as food to a living thing is what is contrary to it-not that in
every pair of contraries each is food to the other: to be food a
contrary must not only be transformable into the other and vice versa,
it must also in so doing increase the bulk of the other.
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