Book III, Chapter 11

We must also inquire, in the case of the imperfect animals, which have touch alone as their sense, what it is that originates movement, and whether imagination can belong to them or not, and appetite as well. For pain and pleasure are evidently present in them; and if these, then appetite too must be present. But how could imagination be in them? Or is it that, just as they are moved indeterminately, so these too are present, but present indeterminately?

Perceptive imagination, then, as has been said, belongs to the other animals as well, but deliberative imagination only to those which are capable of reasoning. For whether one is to do this or that is already the work of reasoning; and one must measure by a single standard, for it is the greater that is pursued. Hence a man is able to make one image out of several images. And this is why the other animals are thought not to possess opinion: they do not possess that which comes from inference, whereas the latter involves the former.

Hence desire does not possess the deliberative faculty. Yet sometimes it overcomes wish and sets it moving, and at other times wish overcomes desire — one desire moving another, as one ball moves another — whenever incontinence arises. But by nature the higher is always the more authoritative and originates movement, so that three movements are by now set going. The faculty of knowledge, however, is not moved, but remains at rest.

And since the one supposition and account is universal, while the other is of the particular — for the one says that such and such a man ought to do such and such a thing, while the other says that this thing here and now is of such a kind, and that I am a man of such a kind — it is this latter opinion that originates movement, not the universal one; or rather both do, but the one is more at rest, the other not.

Book III, Chapter 12

The nutritive soul, then, must belong to everything that lives and has soul, from birth until destruction. For what has come to be must have growth, prime, and decay, and these are impossible without nourishment. It is necessary, therefore, that the nutritive faculty be present in all things that grow and decay.

But perception is not necessary in all living things. For those whose body is simple cannot have touch, and without touch nothing can be an animal; nor can those things have perception which are not receptive of the forms without the matter. But an animal must have perception, if nature makes nothing in vain. For all things that exist by nature are for the sake of something, or else will be concomitants of things that are for the sake of something. If, then, every body capable of travelling from place to place were without perception, it would be destroyed and would never reach its end, which is nature's work. For how will it be nourished? Stationary things do indeed have their nourishment from the place in which they have grown; but it is not possible for a body to have soul and a discriminating intellect and yet not have perception, if it is not stationary but has come to be. Nor, indeed, is it possible even if it were ungenerated. For why should it have soul without perception? Either because this is better for the soul or because it is better for the body. But as things are, neither is the case: for the soul will not think any the better for it, nor will the body be at all the more on account of it. No body, therefore, that is not stationary has soul without perception.

But if in fact it has perception, the body must be either simple or mixed. Simple it cannot be, for then it would not have touch, and it must have touch. This is clear from the following. Since an animal is an ensouled body, and every body is tangible, and the tangible is what is perceptible by touch, the body of the animal too must be capable of touch, if the animal is to be preserved. For the other senses perceive through other things, as do smell, sight, and hearing; but a thing that is in contact, if it has no perception, will not be able to avoid some things and take others. And if this is so, it will be impossible for the animal to be preserved. Hence taste also is a kind of touch, for it is of nourishment, and nourishment is tangible body; whereas sound and colour and smell do not nourish, nor do they produce either growth or decay. So taste too must be a kind of touch, because it is the perception of what is tangible and nutritive. These senses, then, are necessary to the animal, and it is evident that an animal cannot exist without touch.

But the other senses are for the sake of well-being, and belong already not to any chance kind of animal but to certain kinds only: for instance, they must belong to the kind that travels from place to place. For if such an animal is to be preserved, it must perceive not only when in contact but also at a distance. And this would come about if it were able to perceive through a medium, the medium being affected and moved by the object of perception, and the animal itself by the medium. For just as that which produces movement in respect of place causes change up to a certain point, and that which has pushed makes another thing push in turn, and the movement takes place through an intermediate — the first mover pushing without being pushed, the last being pushed only without pushing, and the intermediate doing both, there being many intermediates — so it is with alteration, except that here the thing alters while remaining in the same place. For instance, if one were to dip something into wax, the wax would be moved just so far as one dipped it; a stone would not be moved at all, but water would be moved far. Air is moved farthest of all, and both acts and is acted upon, provided it remains continuous and one. Hence, in the case of reflection, it is better to say that the air is affected by the shape and the colour, so far as it remains one, than that the visual ray goes out and is bent back. And upon a smooth surface the air is one; hence it in turn moves the sight, just as if the impression in the wax were transmitted right through to the further side.

Book III, Chapter 13

That the body of an animal cannot be simple — I mean, for instance, made of fire or of air — is evident. For without touch it can have no other sense whatever, since every ensouled body is capable of touch, as has been said. The other elements, apart from earth, might indeed become sense organs, but all of them produce perception by perceiving through something else, that is, through media. Touch, however, consists in touching the objects themselves, which is why it has this name. Yet the other sense organs also perceive by contact, but through something else; touch alone seems to perceive through itself. So the body of the animal could be composed of none of these elements.

Nor, indeed, could it be made of earth. For touch is, as it were, a mean of all tangible qualities, and its sense organ is receptive not only of all the differences of earth, but also of hot and cold and all the other tangible qualities. And this is why we do not perceive with our bones and hair and parts of that sort, because they are made of earth. And plants for this reason have no perception, because they are made of earth. But without touch no other sense can be present, and this sense organ is made neither of earth nor of any other one of the elements.

It is evident, then, that animals must die if deprived of this sense alone. For nothing can have this sense unless it is an animal, nor, being an animal, must it have any other sense than this. And this is why the other objects of perception do not destroy the animal by their excesses — colour, for instance, and sound and smell — but only destroy the sense organs, except incidentally, as when along with the sound there occurs a thrust and a blow, and when by sights and by smell other things are set in motion which destroy by contact. Flavour too destroys in so far as it happens at the same time to be tangible. But the excess of tangible qualities, such as of hot and cold and hard, does away with the animal itself. For an excess of any object of perception does away with the sense organ, and so the tangible does away with touch; and it is by touch that life is defined, for it has been shown that without touch it is impossible for a thing to be an animal. Hence the excess of tangible qualities destroys not only the sense organ but also the animal, because this is the one sense the animal must have.

The other senses the animal has, as has been said, not for the sake of its being but for the sake of its well-being: sight, for instance, since it lives in air and water, and generally in what is transparent, in order that it may see; taste, on account of the pleasant and the painful, in order that it may perceive what is pleasant in its food, and may desire it and be moved; hearing, in order that something may be signified to it; and a tongue, in order that it may signify something to another.