Book III, Chapter 8

Now, summing up what has been said about the soul, let us say again that the soul is in a way all existing things. For existing things are either objects of perception or objects of thought, and knowledge is in a way the objects of knowledge, and perception the objects of perception. But in what way this is so we must inquire.

Knowledge, then, and perception are divided so as to answer to their objects: potential knowledge and perception to potential objects, actual knowledge and perception to actual objects. And the perceptive and the knowing faculties of the soul are potentially these objects, the one the knowable and the other the perceptible. And they must be either the things themselves or their forms. But not the things themselves, surely; for it is not the stone that is in the soul, but its form. So that the soul is like the hand; for the hand is a tool of tools, and the intellect is a form of forms, and sense a form of perceptible things.

And since, as it seems, no thing exists separately apart from perceptible magnitudes, the objects of thought are present in the perceptible forms — both what are spoken of by abstraction and all the states and affections of perceptible things. And for this reason one who perceived nothing would neither learn anything nor understand anything; and when one contemplates, one must at the same time contemplate some image; for the images are like sense-impressions, except that they are without matter.

But imagination is something other than assertion and denial; for the true and the false are an interweaving of thoughts. Yet in what will the primary thoughts differ, so as not to be images? Surely not even the rest are images, but they do not exist without images.

Book III, Chapter 9

Since the soul of animals has been defined in terms of two capacities — by the power of discrimination, which is the work of thought and of perception, and further by the power of originating movement in respect of place — let so much stand as our determination about perception and about intellect. But concerning that which originates movement, what part of the soul it is, we must inquire: whether it is some one part of it, separable either in magnitude or in account, or the whole soul; and, if it is some part, whether it is a special part over and above those customarily spoken of and those we have already discussed, or one of these.

A difficulty presents itself at once as to how we ought to speak of parts of the soul, and how many there are. For in a way they appear infinite in number, and not only those which some mention when they make their divisions — the calculative, the spirited and the appetitive — or, as others have it, that which has reason and that which is without reason. For, judged by the very differences on the strength of which they separate these, other parts also will appear which have a wider interval between them than these: namely the parts of which we have just now spoken, the nutritive, which belongs both to plants and to all animals, and the perceptive, which one could not easily set down either as without reason or as having reason.

Further, there is the imaginative faculty, which in its being is other than all of them, while it involves much difficulty to say with which of them it is the same and from which it is different, if one is going to posit separate parts of the soul. And besides these there is the faculty of desire, which would seem to be different from all of them both in account and in capacity. And it is surely absurd to tear this apart; for wish arises in the calculative part, while appetite and spirit arise in the part without reason; and if the soul is three things, there will be desire in each of them.

And now indeed we come to that with which our present discussion is concerned: what is it that moves the animal in respect of place? For the movement in respect of growth and decay, which belongs to all living things, would seem to be caused by that which belongs to all, the generative and nutritive faculty. Concerning inhalation and exhalation, and sleep and waking, we must inquire later, for these too involve much difficulty. But about movement in respect of place we must consider what it is that moves the animal with the movement of progression.

That it is not the nutritive capacity is clear. For this movement is always for the sake of something, and is attended either by imagination or by desire; for nothing that is not desiring or avoiding something is moved, except by force. Further, if it were the nutritive capacity, plants too would be capable of movement, and would have some organic part instrumental to this movement. Similarly it is not the perceptive capacity either; for there are many animals which have perception but are stationary and unmoved throughout their lives. If, then, nature does nothing in vain and omits nothing that is necessary, except in maimed and imperfect creatures — and animals of this kind are perfect and not maimed, a sign of which is that they are generative and have a prime and a decline — it follows that they would also have had the organic parts required for progression.

But neither, again, is it the calculative part, that is, what is called intellect, that originates movement. For the theoretical intellect thinks nothing that is practicable, and says nothing about what is to be avoided or pursued, whereas movement always belongs to something that is avoiding something or pursuing something. But even when it does contemplate something of this sort, it does not thereupon bid us avoid it or pursue it: for instance, it often thinks of something fearful or pleasant, and yet does not bid us fear, though the heart is moved, or, if the object be pleasant, some other part. Again, even when the intellect commands and thought bids us avoid something or pursue it, a man is not moved, but acts in accordance with his appetite — the incontinent man, for example. And in general we see that the man who possesses the art of medicine does not thereby heal, which shows that something else is in control of acting in accordance with knowledge, and not the knowledge itself.

But neither, again, is desire in control of this movement; for the self-controlled, though they desire and have appetite, do not do the things for which they have the desire, but follow the intellect.

Book III, Chapter 10

These two, at any rate, appear to be originators of movement — either desire or intellect — if one sets down imagination as a kind of thinking. For men often follow their imaginations contrary to their knowledge, and in the other animals there is neither thinking nor reasoning, but imagination. Both of these, then, are capable of originating movement in respect of place, intellect and desire; but by intellect is meant that which reasons for the sake of something, that is, the practical intellect, which differs from the theoretical by its end. And every desire too is for the sake of something; for that of which there is desire, this is the starting point of the practical intellect, and the last step is the starting point of action.

Hence it is with good reason that these two appear as that which originates movement, desire and practical thought. For the object of desire originates movement, and it is on this account that thought originates movement, because its starting point is the object of desire. And imagination too, when it originates movement, does not originate it without desire.

There is, then, some one thing which originates movement, the faculty of desire. For if two things originated movement, intellect and desire, they would do so in virtue of some common form. But as it is, intellect is not seen to originate movement without desire, for wish is a desire, and whenever a man is moved in accordance with reasoning he is moved also in accordance with wish; whereas desire originates movement even contrary to reasoning, for appetite is a kind of desire. Now intellect is always right, but desire and imagination may be right or not right. Hence it is always the object of desire that originates movement, but this is either the good or the apparent good; not every good, however, but the practicable good. And the practicable is that which admits of being otherwise.

That, then, it is such a capacity of the soul that originates movement, namely the one called desire, is evident. But for those who divide the parts of the soul, if they divide and separate them according to their capacities, a great many parts result: the nutritive, the perceptive, the intellectual, the deliberative, and further the desiderative; for these differ from one another more than do the appetitive and the spirited.

But since desires arise which are contrary to one another — and this happens whenever reason and appetite are opposed, and it occurs in those creatures which have a perception of time (for the intellect bids us hold back on account of what is to come, whereas appetite has regard to what is immediate; for what is pleasant at the moment appears both pleasant without qualification and good without qualification, because we do not see the future) — that which originates movement will be one in form, namely the faculty of desire, in so far as it is desiderative, and first of all the object of desire (for this originates movement without itself being moved, by being thought of or imagined), while in number the things which originate movement are more than one.

Now since there are three things — first, that which originates movement; second, that by means of which it originates movement; third, that which is moved — and since that which originates movement is twofold, the one unmoved, the other both originating movement and itself moved: the unmoved is the practicable good, while that which both originates movement and is moved is the faculty of desire (for that which is moved is moved in so far as it desires, and desire, as actuality, is a kind of movement), and that which is moved is the animal. But the instrument with which desire originates movement is at this point something bodily; hence it must be examined among the functions common to body and soul.

For the present let it be said summarily that that which originates movement instrumentally is to be found where beginning and end coincide, as in a hinge-joint; for there the convex and the concave are, the one an end and the other a beginning, and on this account the one is at rest while the other is moved, the two being different in account though inseparable in magnitude. For everything is moved by pushing and pulling; and hence, as in a circle, something must remain fixed, and it is from this that the movement must begin.

In general, then, as has been said, in so far as an animal is capable of desire, so far is it capable of moving itself; and it is not capable of desire without imagination, while all imagination is either calculative or perceptive. In this last, then, the other animals also have a share.