Book II, Chapter 3

Of the faculties of the soul, those which have been mentioned belong, as we have said, in their entirety to some living things, only some of them to others, and to certain others one alone. The faculties we spoke of are the nutritive, the appetitive, the perceptive, the faculty of movement in respect of place, and the faculty of thought. To plants there belongs the nutritive faculty only; to other things both this and the perceptive faculty. But if the perceptive, then the appetitive too. For appetite comprises desire, spirit, and wish; and all animals have at least one of the senses, namely touch; and that to which perception belongs has also pleasure and pain, and the pleasant and the painful; and whatever has these has desire as well, for desire is appetite for what is pleasant.

Further, animals have perception of food, for touch is the sense of food. All animals are nourished by things dry and moist, hot and cold, and touch is the sense of these; of the other objects of perception they have perception only incidentally, for sound contributes nothing to nourishment, nor colour, nor smell. But flavour is one of the objects of touch. Hunger and thirst are forms of desire: hunger is desire for the dry and hot, thirst for the cold and moist, while flavour is a sort of seasoning of these. These matters must be made clear later; for the present let this much be said, that those animals which have touch have appetite also. Concerning imagination (phantasia) the matter is obscure, and must be examined later. To some, besides all these, there belongs also the faculty of movement in respect of place; and to others the faculty of thought, that is, intellect, as to men and to any other being of the same kind as man, or of a higher order.

It is clear, then, that there will be a single account of soul in the same way as there is of figure. For just as in the one case there is no figure over and above the triangle and the figures which follow it in order, so here there is no soul over and above those we have mentioned. Now in the case of figures a common account could be produced, one which will fit them all but will be peculiar to no figure; and it is the same with the kinds of soul we have mentioned. Hence it is absurd, in these cases and in others, to seek the common account, which will be the account peculiar to no existing thing, and which will not answer to the proper and indivisible species, while neglecting the account which does.

The facts about soul are closely analogous to those about figures. For in the series, both of figures and of ensouled things, the prior member is always present potentially in the one that follows: for example, the triangle is present in the square, and the nutritive faculty in the perceptive. So we must inquire in each case what the soul of each thing is: what, for instance, is the soul of a plant, and what that of a man or of a beast.

We must also consider for what reason they stand in this order of succession. For without the nutritive faculty the perceptive does not exist; yet the nutritive is found separated from the perceptive in plants. Again, without the faculty of touch none of the other senses is present, whereas touch is present without the others; for many animals have neither sight nor hearing nor any sense of smell at all. And of those which have perception, some have the faculty of movement in respect of place, while others do not. Last of all, and in fewest things, come reasoning and thought. For among perishable beings, those which possess reasoning possess all the rest as well; but not all those which possess each of those others possess reasoning — indeed some have not even imagination, while others live by this alone. Concerning the contemplative intellect there is another account. It is plain, then, that the account of each of these faculties is also the most proper account of the soul.

Book II, Chapter 4

Anyone who is going to make an inquiry into these matters must first grasp what each of them is, and then go on in the same way to investigate what follows from them and the rest. But if we are to say what each of them is — what, for instance, the faculty of thought is, or the perceptive, or the nutritive — we must say still earlier what thinking is and what perceiving is; for in account the activities and the operations are prior to the faculties. And if that is so, and if prior even to these we ought to have considered their correlative objects, then for the same reason we should first determine these — that is, food, the object of perception, and the object of thought.

Hence we must speak first of food and of generation; for the nutritive soul belongs to the other kinds as well, and it is the first and most widely shared faculty of soul, that in virtue of which life belongs to all living things. Its functions are to generate and to make use of food. For the most natural of all functions in living things — in such of them as are perfect and not mutilated, and do not come to be spontaneously — is to produce another like themselves, an animal producing an animal and a plant a plant, in order that they may partake in the eternal and the divine so far as they are able. For that is what all things desire, and it is for the sake of that that they do whatever they do in accordance with nature. But "that for the sake of which" is twofold: the end at which the action aims, and the being for whose benefit it is done. Since, then, they cannot share in the eternal and the divine by unbroken continuance, because nothing perishable can persist the same and one in number, each shares in it in the way it is able, some more and some less; and what persists is not the thing itself but something like itself, one not in number but in species.

The soul is the cause and the principle of the living body. But "cause" and "principle" are used in many senses. The soul is a cause in the three senses we have distinguished: it is a cause as the source from which movement comes, as that for the sake of which, and as the substance of ensouled bodies. That it is a cause as substance is clear; for substance is the cause of being to all things, and for living things to be is to live, and the soul is the cause and principle of this. Further, actuality (entelecheia) is the account of that which exists potentially. It is evident too that the soul is a cause as that for the sake of which. For just as intellect acts for the sake of something, so also does nature, and this is nature's end; and in living things this is what the soul is, and it is so in accordance with nature. For all natural bodies are instruments of the soul — those of plants no less than those of animals — since they exist for the sake of the soul. But "that for the sake of which" has two senses: the end aimed at, and the being for whose benefit it is. Moreover the soul is that from which movement in respect of place first proceeds, though not all living things possess this faculty. Alteration and growth also occur in virtue of soul; for perception seems to be a kind of alteration, and nothing perceives which does not possess soul. It is the same with growth and decay; for nothing decays or grows naturally unless it is nourished, and nothing is nourished which does not share in life.

Empedocles has not spoken well on this point, when he adds that growth occurs in plants downwards when they take root, because earth is naturally carried in that direction, and upwards because fire is carried in the other. For he does not take "up" and "down" rightly. Up and down are not the same for all things as they are for the universe; rather, as the head is in animals, so are the roots in plants, if we are to call organs different or the same by their functions. Besides this, what is it that holds together the fire and the earth when they are carried in contrary directions? For they will be torn apart unless there is something to prevent it; and if there is such a thing, that is the soul, and it is the cause of being nourished and of growing.

Some, again, think that the nature of fire is without qualification the cause of nourishment and growth; for fire alone among bodies, or among the elements, is evidently nourished and made to grow. Hence one might suppose that in plants and in animals it is fire that does the work. But fire is in a way a joint cause, though not the cause without qualification; the soul is that rather. For the growth of fire goes on without limit so long as there is fuel to burn, whereas in all things constituted by nature there is a limit and a ratio determining both size and growth; and these belong to soul, not to fire, and to form rather than to matter.

Since one and the same faculty of the soul is both nutritive and generative, we must first determine the question of food; for it is by this function that the faculty is marked off from the others. Now food seems to be the contrary fed upon by its contrary — yet not every contrary by every contrary, but only such contraries as not only come to be out of one another but also derive growth from one another. For many things come to be out of one another, but not all of them are quantities: the healthy, for instance, comes to be out of the sick. And even those contraries which do serve as food do not appear to be food to one another in the same way: water is food to fire, but fire does not nourish water. In the simple bodies, then, it is above all in these that the one is food and the other the thing fed.

But a difficulty arises. Some say that like is nourished by like, just as like grows by like; others, as we have said, hold the opposite view, that contrary is nourished by contrary, on the ground that like is unaffected by like, whereas food is changed and digested, and change is in every case into the opposite or into the intermediate. Again, the food is acted upon in some way by the thing fed, but the thing fed is not acted upon by the food, just as the carpenter is not acted upon by his material, but the material by him; the carpenter merely changes from inactivity into activity. But it makes a difference whether food is what is added last or what is added first. If both are food, the one undigested and the other digested, it would be possible to speak of food in both ways: in so far as it is undigested, contrary is nourished by contrary, and in so far as it is digested, like by like. So it is evident that in a way both parties speak rightly and in a way not rightly.

Since nothing is nourished which does not share in life, what is nourished must be the ensouled body, and it is nourished qua ensouled; so that food too is relative to what is ensouled, and not incidentally so. But to be food and to be productive of growth are different things. For in so far as the ensouled thing is a quantity, food is productive of growth; but in so far as it is a "this" and a substance, it is food. For food preserves the substance, and the thing continues to exist just so long as it is fed. And food is productive of generation — not of the thing fed, but of something like the thing fed; for the substance of that thing already exists, and nothing generates itself, but only preserves itself. Hence this principle of the soul is a faculty of such a kind as to preserve that which possesses it, precisely as such a thing; and food makes it able to be active. Therefore, when deprived of food, it cannot exist.

Since there are three things involved — that which is nourished, that by which it is nourished, and that which nourishes — that which nourishes is the primary soul, that which is nourished is the body which possesses this soul, and that by which it is nourished is the food. And since it is right to name everything from its end, and the end here is to generate another like itself, the primary soul will be that which generates another like itself. Now "that by which it is nourished" is twofold, just as "that by which one steers" is twofold — the hand and the rudder: the one both moves and is moved, the other is moved only. Further, all food must be capable of being digested, and it is heat that produces digestion; hence every ensouled thing possesses heat. We have now said in outline what food is; it must be made clear later in the discussions proper to it.