De Anima

It is evident that the body of an animal cannot be uncompounded; I mean, it cannot consist entirely of fire, for instance, or of air. An animal, unless it has touch, can have no other sense, the animate body being always, as we have remarked, capable of tactile sensation. Now the other elements, with the exception of earth, would make sense-organs: but it is always indirectly and through media that such organs effect sensation. Touch, however, acts by direct contact with objects: hence its name. The other sense-organs, it is true, also perceive by contact, but it is by indirect contact: touch alone, it would seem, perceives directly in and through itself. Thus, then, no one of the three elements referred to can constitute the body of the animal. Nor indeed can it be of earth. For touch is a sort of mean between all tangible qualities, and its organ is receptive not only of all the distinctive qualities of earth, but also of heat and cold and all other tangible qualities. And this is why we do not perceive anything with our bones and our hair and such parts of us, namely, because they are of earth. And for the same reason plants, too, have no sensation, because they are composed of earth. Without touch, however, there can be no other sense; and the organ of this sense does not consist of earth nor of any other single element.

Thus it is evident that this is the only sense the loss of which necessarily involves the death of the animal. For it is not possible for anything that is not an animal to have this sense, nor is it necessary for anything that is an animal to have any other sense besides this. And this explains another fact. The other sensibles—I mean, colour, sound, odour—do not by their excess destroy the animal, but only the corresponding sense-organs: except incidentally, as when concurrently with the sound some thrust or blow is given, or when objects of sight or smell move something else which destroys by contact. Flavour, again, destroys only in so far as it is at the same time tactile. Tangible qualities, on the other hand, as heat, cold and hardness, if in excess, are fatal to the living animal. For excess of any sensible object is fatal to the organ, and so consequently excess of the tangible object is fatal to touch. And it is by this sense that the life of the animal is defined, touch having been proved to be indispensable to the existence of an animal. Hence excess in tangible qualities destroys not only the sense-organ, but also the animal itself. For touch is the one sense that the animal cannot do without. The other senses which it possesses are, as we have said, the means, not to its being, but to its well-being. Thus the animal has sight to see with, because it lives in air or water or, speaking generally, in a transparent medium. It has taste on account of what is pleasant and painful, to the end that it may perceive what is pleasant in food and feel desire and be impelled to movement. It has hearing in order that information may be conveyed to it, and a tongue, that in its turn it may convey information to its fellow.