De Anima

That there is no other sense distinct from the five, by which I mean sight, hearing, smell, taste, touch, anyone may convince himself on the following grounds. Let us assume that, as a matter of fact, we have sensation of every sensible object for which touch is the appropriate sense, all qualities of the tangible, as such, being perceptible to us through touch. Let us further assume that, when any sense is lacking to us, an organ of sense must also be lacking; and further, that whatever we perceive by actual contact is perceptible by touch, a sense which we do possess, while whatever we perceive mediately and not by actual contact is perceptible by means of the elements, namely, air and water. And here are implied two cases. Suppose, first, we have perception by one and the same medium of two several things, different in kind from one another, then whoever possesses the appropriate sense-organ must be percipient of both: as, for example, if the sense-organ consists of air and air is also the medium of both sound and colour. Next suppose several media to transmit the same object, as both air and water transmit colour, both being transparent, then he who possesses one of these alone will perceive whatever is perceptible through both media. Now, of the elements, air and water are the only two of which sense-organs are composed. For the pupil of the eye is of water, and the ear is of air, and the organ of smell is of one or the other, while fire, if present anywhere, enters into all, since nothing can be sentient without warmth. Earth, again, belongs to none of the sense-organs, or, at most, is a constituent peculiar to touch. It follows, then, that outside water and air there is no sense-organ. Now sense-organs composed of air and water certain animals do, in fact, possess. We may infer, then, that all the senses are possessed by those animals which are fully developed and are not crippled: even the mole is found to have eyes beneath its skin. And thus, unless there exists some unknown body or some property different from any possessed by any of the bodies within our experience, there can be no sixth sense which we lack.

Nor, again, can there be any special sense-organ for the common sensibles, which we perceive incidentally by every sense; for example, motion, rest, figure, magnitude, number, unity. For all of these we perceive by motion. Thus it is by motion that we perceive magnitude, and consequently figure, figure being one variety of magnitude; while that which is at rest we perceive by the fact that it is not moved. Number we perceive by the negation of continuity and by the special sense-organs also: for each sensation has a single object. Clearly, then, it is impossible that there should be a special sense for any one of these; for example, motion: for in that case we should perceive them in the same way as we now perceive sweetness by sight (and this we do because we have a sense which perceives both, and by this we actually apprehend the two simultaneously when they occur in conjunction). Otherwise we should never have more than an incidental perception of them; as of Cleon's son we perceive not that he is Cleon's son, but that he is a white object, and the fact of his being Cleon's son is accessory to the whiteness. But of the common sensibles we have already a common perception, which is direct and not indirect, so that there cannot be a special sense for them. For, if there were, we should never perceive them otherwise than in the way in which we said we saw Cleon's son.

But the various senses incidentally perceive each other's proper objects, not as so many separate senses, but as forming a single sense, when there is concurrent perception relating to the same object; as, for instance, when we perceive that gall is bitter and yellow. For it is certainly not the part of any other sense to declare that both objects are one and the same. Hence you are sometimes deceived and, on observing something yellow, fancy it to be gall.

But, it might be asked, why have we several senses, instead of only one? I answer, it is in order that we may not be so likely to overlook the common attributes, such as motion, magnitude, number, which accompany the special sensibles. For, if sight had been our only sense and whiteness its object, we should have been more apt to overlook the common sensibles and to confuse all sensibles, because colour and magnitude, for instance, must always go together. As it is, the fact that the common attributes are found in the object of another sense also shows that they are severally distinct.