De Anima

The enquirer who approaches this subject must ascertain what each of these faculties is before he proceeds to investigate procedure. the questions next in order and so forth. But if we are asked to state what each of these is; that is to say, what the cognitive, sensitive and nutritive faculties respectively are, we must begin by stating what the act of thinking is and what the act of sensation is. For activities and functions are logically prior to faculties. But, if so, and if a study of the correlative objects should have preceded, these objects will for the same reason have to be defined first: I mean, nutriment and the sensible and intelligible. Consequently we have first to treat of nutriment and of generation.

The nutritive soul belongs to other living things as well as man, being the first and most widely distributed faculty, in virtue of which all things possess life. Its functions are reproduction and assimilation of nutriment. For it is the most natural function in all living things, if perfect and not defective or spontaneously generated, to reproduce their species; animal producing animal and plant plant, in order that they may, so far as they can, share in the eternal and the divine. For it is that which all things yearn after, and that is the final cause of all their natural activity. Here final cause is an ambiguous term, which denotes either the purpose for which, or the person for whom, a thing is done. Since, then, individual things are incapable of sharing continuously in the eternal and the divine, because nothing in the world of perishables can abide numerically one and the same, they partake in the eternal and

divine, each in the only way it can, some more, some less. That is to say, each persists, though not in itself, yet in a representative which is specifically, not numerically, one with it.

Now the soul is cause and origin of the living body. But cause and origin are terms used in various senses: accordingly soul is cause in the three senses of the word cause already determined. For the soul is the cause of animate bodies as being in itself the origin of motion, as final cause and as substance. Clearly it is so as substance, substance being the cause of all existence. And for living things existence means life, and it is the soul which is the cause and origin of life. Furthermore, actuality is the notion or form of that which has potential existence. Manifestly, too, the soul is final cause. For nature, like intelligence, acts for a purpose, and this purpose is for it an end. Such an end the soul is in animals, and this in the order of nature, for all the natural bodies are instruments of soul: and this is as true of the bodies of plants as of those of animals, shewing that all are means to the soul as end; where end has two senses, the purpose for which and the person for whom. Moreover, the soul is also the origin of motion from place to place, but not all living things have this power of locomotion. Qualitative change, also, and growth are due to soul. For sensation is supposed to bea sort of qualitative change, and nothing devoid of soul has sensation. The same holds of growth and decay. For nothing undergoes natural decay or growth except it be nourished, and nothing is nourished unless it shares in life.

Empedocles is mistaken in adding that in plants, in so far as they strike their roots downwards, growth takes place because the earth in them has a natural tendency in this direction and that, when they shoot upwards, it is because the fire in them has a similar tendency upwards. He is wrong in his view of up and down. For up and down are not the same for all individuals as for the universe. On the contrary, the roots of plants correspond to the heads of animals, if we are to

make identity and diversity of organs depend upon their functions. Besides, what is it that holds together the fire and the earth, tending, as they do, in opposite directions? For they will be rent asunder, unless there is something to prevent it: while, if there is, it is this which is the soul and the cause of growth and nourishment.

Some hold the nature of fire to be singly and solely the cause of nourishment and growth. For it would seem that fire is the only body or element which of itself is nourished and grows. Hence fire might be supposed to be the operative cause, both in plants and animals. Whereas, though it is in a sense a joint cause, it is not a cause absolutely: it is rather the soul which is so. For fire goes on growing to infinity, as long as there is fuel to be consumed, but in natural wholes there is always a limit or proportion which determines growth and size. But this belongs to the soul and not to fire, to form rather than' to matter.

The nutritive faculty of the soul being the same as the reproductive, it is necessary first to give a definition of nutriment. For it is by the nutritive function that this faculty is separated off from the others. The common view is that contrary is nutriment to contrary; though not in every case, but wherever each of two contraries is not only generated by, but derives growth from, the other. For many things are derived from one another, but not all of them are quantities: thus the sick man becomes well. But it is found that even the contraries supposed to derive growth from each other are not fed by one another in the same way: while water serves to feed fire, fire is not nutriment to water. It would seem, then, that it is in the simple bodies above all that of two contraries one is nutriment and the other is nourished. Yet here is a difficulty. It is said by the one side that like is nourished by, as well as derives its growth from, like; while the others, again, as we explained, hold that contrary is nourished by contrary, on the ground that like cannot be affected by like, while food undergoes change and is digested. Now change is always in the direction of the opposite, or of the intermediate state. Further, nutriment is acted upon by that which it nourishes, and not the latter by the former: just as

the carpenter is not affected by his material, but on the contrary the material by the carpenter. The carpenter merely passes to activity from inaction. But it makes a difference whether by nutriment we mean the final, or the primary, form of what is added. If both are nutriment, the one as undigested, the other as digested, it will be possible to use the term nutriment in conformity with both theories. For, in so far as it is undigested, contrary is nourished by contrary: and, in so far as it is digested, like by like. So that clearly both sides are in a manner partly right and partly wrong. But, since nothing is nourished unless it possesses life, that which is nourished must be the animate body as such: so that nutriment also is relative to the animate being which it nourishes: and this not incidentally merely.

There is, however, a difference between nutritivity and conducivity to growth. In so far as the animate thing is quantitative, what is taken promotes growth; in so far as it is a definite individual, what is taken nourishes. For the animate thing preserves its substance or essential nature and exists as long as it is nourished: and it causes the production, not of that which is nourished, but of another individual like it. Its essential nature already exists, and nothing generates itself, it only maintains its existence. Hence the above described principle of the soul is the power to preserve in existence that which possesses it in so far as it is a definite individual, while nutrition prepares it for activity. Therefore it cannot live when deprived of nutriment. There are, then, these three things, that which is nourished, that with which it is nourished, and that which nourishes it. The last of the three is the primary soul, that which is nourished is the body which contains the soul, that wherewith it is nourished is nutriment. As, however, it is right to name all things from the end they subserve, and the end here is reproduction of the species, the primary soul is that which is capable of reproducing the species. That with which the living thing is nourished may be understood in two senses, just as that with which one steers may mean the hand or the rudder; the former, the hand, both causing motion and being moved, the latter, the rudder, being simply moved. Now it is necessary that all food should be capable of digestion, and digestion is promoted by heat; this explains why every animate thing has warmth. This, then, is an outline of what nutriment is. It must be more clearly defined hereafter in the discussion devoted specially to it.