Book II, Chapter 1

Let what has been handed down by our predecessors concerning the soul (psuchē) stand as said. Let us now make a fresh start, as it were from the beginning, and try to determine what soul is and what would be the most general account of it.

We say, then, that substance is one kind among the things that are; and of substance, one sort is matter, which in itself is not a particular thing (a "this"), another is shape and form, in virtue of which a thing is already called a particular thing, and a third is what is composed of these two. Now matter is potentiality, while form is actuality (entelecheia); and this in two ways, in the way that knowledge is and in the way that contemplating is.

It is bodies above all that seem to be substances, and among bodies the natural ones; for these are the principles of the rest. Of natural bodies, some have life and some do not; and by life we mean self-nourishment and growth and decay. Hence every natural body that partakes of life will be a substance, but a substance in the sense of a composite.

But since it is a body of such a kind — namely, one having life — the body cannot be soul; for the body is not among the things said of a subject, but is rather itself subject and matter. It is necessary, then, that the soul be substance in the sense of the form of a natural body having life potentially. But substance is actuality; and so the soul is the actuality of a body of this kind.

Actuality, however, is spoken of in two ways: as knowledge is, and as contemplating is. It is plain, then, that the soul is actuality in the way that knowledge is; for both sleeping and waking depend upon the soul's being present, and waking is analogous to contemplating, sleep to possessing knowledge without exercising it. And in one and the same subject knowledge is prior in coming to be. Hence the soul is the first actuality of a natural body having life potentially.

And such is any body that is equipped with organs. The parts of plants too are organs, though altogether simple ones: the leaf, for instance, is a covering for the seed-vessel, and the seed-vessel a covering for the fruit; and the roots are analogous to the mouth, for both draw in nourishment. If, then, one must state something common to every kind of soul, it will be that the soul is the first actuality of a natural body equipped with organs.

Hence one need not ask whether the soul and the body are one, any more than whether the wax and its shape are one, or in general whether the matter of each thing and that of which it is the matter are one. For while "one" and "being" are said in many ways, that which is so called in the strict sense is actuality.

In general terms, then, it has been said what the soul is: it is substance in the sense of the account of a thing's being. That is to say, it is the essence of a body of the kind described. Suppose that some instrument — an axe, say — were a natural body: then being an axe would be its substance, and this would be its soul; and if this were separated from it, it would no longer be an axe, except homonymously. But as it is, it is only an axe; for the soul is not the essence and account of a body of that sort, but of a natural body of a certain kind, one having in itself a principle of motion and of rest.

We must consider what has been said in application to the parts as well. For if the eye were an animal, sight would be its soul; for sight is the substance of the eye corresponding to its account. And the eye is the matter of sight, so that when sight fails there is no longer an eye, except homonymously, as an eye of stone or an eye in a picture is one. What holds of the part must then be applied to the whole living body; for as the one part stands to the other, so does perception as a whole stand to the whole perceptive body as such.

But that which is potentially such as to live is not the body that has cast off its soul, but the body that possesses it; while the seed and the fruit are potentially a body of this kind. As, then, cutting and seeing are actualities, so too is waking; but as sight is, and the capacity of the organ, so is the soul, while the body is that which exists potentially. Yet just as the eye is pupil together with sight, so in the other case the animal is soul together with body.

That the soul, then, is not separable from the body — or that certain parts of it are not, if it is by nature divisible — is not obscure; for the actuality of some parts of the soul belongs to the bodily parts themselves. Nevertheless, nothing prevents some parts from being separable, since they are the actualities of no body. Again, it is unclear whether the soul is the actuality of the body in the way that a sailor is of a ship. Let the soul, then, be determined and sketched in outline in this fashion.

Book II, Chapter 2

But since it is out of what is unclear yet more evident to us that what is clear and more intelligible in respect of the account comes to be, we must try to go over the soul again in this way. For a defining account ought not only to make plain the fact, as most definitions do, but the cause too ought to be present in it and to show itself. As it is, the formulae of definitions are like conclusions. For instance: what is squaring? The construction of an equilateral rectangle equal in area to an oblong one. But such a definition is the formula of a conclusion; whereas he who says that squaring is the finding of a mean proportional states the cause of the thing.

Let us say, then, taking this as the starting point of our inquiry, that the ensouled is distinguished from the unensouled by living. But since living is spoken of in many ways, we say that a thing lives if but one of the following is present in it: intellect, perception, motion and rest in respect of place, and further the motion involved in nutrition, and decay and growth. Hence all growing things too are held to live; for they evidently have in themselves a capacity and principle of this sort, through which they take on growth and decay in opposite directions. For they do not grow upwards without growing downwards, but alike in both directions and in every direction, and they are nourished and continue to live to the end, so long as they are able to take nourishment.

This capacity can be separated from the others, but in mortal beings the others cannot be separated from it. And this is plain in the case of growing things, for no other capacity of soul belongs to them.

Living, then, belongs to living things through this principle; but a thing is an animal primarily through perception. For even those things which do not move or change their place, but do have perception, we call animals, and not merely say that they live. And of perception, the first that belongs to all is touch. And as the nutritive can be separated from touch and from perception generally, so touch can be separated from the other senses. By "nutritive" we mean that part of the soul in which plants too have a share; while all animals evidently have the sense of touch. For what cause each of these holds we shall say later.

For the present let just this much be said: that the soul is the principle of the things mentioned, and is defined by them — by the nutritive, the perceptive, the thinking, and by motion.

But whether each of these is a soul or a part of a soul, and, if a part, whether such as to be separable only in account or also in place — about some of these it is not hard to see, but some present a difficulty. For just as, in the case of plants, some when divided are evidently able to go on living even when separated from one another, the soul in them being in actuality one in each plant but potentially several, so we see the same thing happening, with respect to other differentiae of soul, in the case of insects that are cut in two. For each of the parts has perception and motion in respect of place; and if perception, then imagination (phantasia) and desire as well. For where there is perception there is also pain and pleasure, and where these are, there is of necessity appetite too.

But concerning the intellect and the contemplative faculty nothing is yet clear; it seems, however, to be a different kind of soul, and this alone admits of being separated, as the everlasting is separated from the perishable. As for the remaining parts of the soul, it is plain from what has been said that they are not separable, as some assert. That they are different in account, however, is plain; for to be perceptive and to be capable of opinion are different, if indeed perceiving is different from opining; and likewise with each of the others mentioned.

Again, in some animals all of these belong, in others some of them, and in others only one. This is what makes the difference among animals; but for what cause, we must inquire later. Much the same has come about in the case of the senses as well: some animals have them all, some only certain of them, and some only the one that is most necessary, touch.

But that by which we live and perceive is spoken of in two ways, just as is that by which we know; for by the latter we mean in one sense knowledge and in another the soul, since we say that we know by each of these. And similarly with that by which we are healthy: in one sense it is health, in another a certain part of the body, or even the whole of it. Now of these, knowledge and health are the shape and a kind of form and account, and as it were the actuality of what is receptive — the one of what is capable of knowledge, the other of what is capable of health. For the actuality of the agents seems to reside in the thing acted upon and disposed. The soul, then, is that by which primarily we live and perceive and think; so that it will be an account and a form, and not matter and subject.

For, as we said, substance is spoken of in three ways: as form, as matter, and as what is composed of both; and of these, matter is potentiality and form actuality. And since it is the composite that is ensouled, the body is not the actuality of the soul, but the soul is the actuality of a certain body. And for this reason those are right in their supposition who hold that the soul neither exists without a body nor is itself a kind of body; for it is not a body, but something belonging to a body, and for this reason it is present in a body, and in a body of a certain kind — not as our predecessors fitted it into a body without further determining in what body and of what sort, although it is evident that not any chance thing receives any chance thing.

And it turns out so on reflection too; for the actuality of each thing naturally comes to be in that which is it potentially, that is, in its own proper matter. That the soul, then, is a certain actuality and account of that which has the capacity to be of such a kind, is plain from these considerations.