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On The Soul   

animals. 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. Many a

contrary is transformed into its other and vice versa, where neither

is even a quantum and so cannot increase in bulk, e.g. an invalid into

a healthy subject. It is clear that not even those contraries which

satisfy both the conditions mentioned above are food to one another in

precisely the same sense; water may be said to feed fire, but not fire

water. Where the members of the pair are elementary bodies only one of

the contraries, it would appear, can be said to feed the other. But

there is a difficulty here. One set of thinkers assert that like

fed, as well as increased in amount, by like. Another set, as we

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