TOUCH, THE FUNDAMENTAL SENSE
IT IS CLEARLY IMPOSSIBLE FOR THE BODY OF AN animal to be a simple element: to be, I mean, fire or air. For no other sense can exist without touch; since every animate body, as we have seen, is tactual. The sense organs, indeed, can be constituted of other elements (except earth) for they effect sensation by perceiving through some other thing, i.e. through a medium. But touch occurs by immediate contact with things; that is why it has this name. Other senses no doubt also perceive by contact, but through something else; touch alone, it would seem, through itself. Hence the body of an animal will not be of such elements as these; nor yet be earthy. For touch is a kind of mean between all tangible objects, and is receptive, not only of all the differences that characterise earth but also heat and cold and all other tangible qualities. This is why we do not feel with the bones or hair or other such parts,--because they are earthy. Plants, too, have no sensation, for the same reason,--they are of earth; and without touch there can be no other sense; and this sense does not consist either of earth or of any other [single] element.§§ 865-8
It is then evident that animals must perish if they are deprived of this sense alone. For it is neither possible for what is not an animal to possess this, nor, to be an animal, is any other necessary. And this again is why no other sense-object (e.g. colour, smell and sound) will destroy the animal by excessive intensity, but only the [corresponding] faculty of sense;--except incidentally, as when, for example, together with a sound, a thrust and a blow take place; or when other [things] are set in motion, by objects seen or smelled, which destroy by contact; and flavour too, in so far as it happens also to be tangible, may destroy thus. But an excess of tangible qualities, of heat or cold or hardness, destroys the animal itself. For as every excess in a sense-object is destructive of the sense, so may the tangible destroy touch; and by touch life itself is defined. For it has been shown that without touch no animal can exist. Hence an excess in the tangible destroys not the sense only, but the whole animal; for only this one sense is necessary to animals.§§ 869-71
As we have said, the animal has other senses, not for being, but for well-being. Thus [it has] sight so that [living] in air, water, or generally in what is transparent, it may see. Taste it has on account of the pleasant or unpleasant, that it may perceive what it desires in food, and be moved towards it. Hearing it has that it may receive signs, and a tongue to signify something to another.§§ 872-4