This explains, further, the following difference between the other
senses and touch. In the case of all the others excess of intensity in
the qualities which they apprehend, i.e. excess of intensity in
colour, sound, and smell, destroys not the but only the organs of
the sense (except incidentally, as when the sound is accompanied by an
impact or shock, or where through the objects of sight or of smell
certain other things are set in motion, which destroy by contact);
flavour also destroys only in so far as it is at the same time
tangible. But excess of intensity in tangible qualities, e.g. heat,
cold, or hardness, destroys the animal itself. As in the case of every
sensible quality excess destroys the organ, so here what is tangible
destroys touch, which is the essential mark of life; for it has been
shown that without touch it is impossible for an animal to be. That is
why excess in intensity of tangible qualities destroys not merely
the organ, but the animal itself, because this is the only sense which
it must have.
All the other senses are necessary to animals, as we have said,
not for their being, but for their well-being. Such, e.g. is sight,
which, since it lives in air or water, or generally in what is
pellucid, it must have in order to see, and taste because of what is
pleasant or painful to it, in order that it may perceive these
qualities in its nutriment and so may desire to be set in motion,
and hearing that it may have communication made to it, and a tongue
that it may communicate with its fellows.
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