The same reasoning holds good for the tangible quality as for the Touch; for if the Touch be not a single but a manifold sense, it follows that tangible qualities must be manifold also. Now, it is difficult to determine whether the Touch is a manifold or a single sense, and difficult also to say what the organ may be which is percipient of tangible qualities; that is, whether or not it is the flesh, and that which, in other creatures, is analogous to flesh; but yet the flesh is only the medium, and the essential organ, therefore, must be something different from flesh, and situated internally.
Each sense seems to be perceptive of only one contrary, as Sight of white and black, Hearing of acute and grave, and Taste of bitter and sweet; but several contraries belong to the sense of Touch, as hot and cold, wet and dry, hard and soft, with others. There is, it is true, a kind of solution for this difficulty, in that the other senses also admit of several contraries; as in the voice there are not only the acute and grave but also the strong and weak, the rough and smooth, with yet other contrasts; and there are many and varied shades of colour. Still it is not clear what that subjacent something is, which is to the tangible impression what the Hearing is to Sound.
Is then the sentient organ placed or not within the flesh, or is it the flesh itself which is immediately perceptive? It does not appear that any indication can be obtained upon this point from sensation being simultaneous with the tangible impression; for, situated as we are, were any one to extend a membranelike substance over his flesh, the party would be equally sensible when touched, and sensible at the moment of contact; and yet, clearly, the sentient organ cannot be in that membrane. It may be, however, that if the membrane were a congenital part of the body, sensation would pass through it more rapidly. Thus, this part of the body appears to be disposed towards us as air would be, had air been diffused around us; for it would seem to us as though by some one sense we perceive sound, colour, and odour, and as though sight, hearing, and smell, are one and the same sense. But now, as the motions emanating from external objects are distinguishable by the medium through which they are conveyed, the sentient organs alluded to must manifestly be different also. With respect to the Touch, however, this is still obscure, for it is impossible that a living body should be constituted out of air or water, as it must have some solidity; and there remains only this conclusion, that it must be a mixture of earth, and such other particles as have affinity with flesh, and the analogue of flesh. Thus, the body has, of necessity, been adapted for being the medium for the tangible sense, through which the several tangible impressions are to be conveyed; and that the impressions are manifold is shewn in the tongue being perceptive of tangible as well as sapid qualities. We are sensible, in fact, by this organ of all tangible as well as sapid qualities; and were the rest of the flesh, like the tongue, sensible of savour, then "Taste" and "Touch" would seem to be one and the same sense; but now we perceive, since they are not convertible, that they must be distinct senses.
It may be a question whether, as all bodies have depth, that is the third magnitude, any two bodies, which have between them another body, can be in contact; for neither the humid nor the liquid is incorporeal, as each must, of necessity, be water or hold water; and thus, it follows that, as the extreme parts of bodies in the water are not dry, the water, with which their extremities are covered, must be interposed between them. If this be true, then it is impossible that one body, when in the water, should be in immediate contact with another; and this holds good for bodies in the air; for the air is in the same relation to bodies in air which water is to bodies in water; but owing to our being in the air, the fact as readily escapes us, as it does aquatic animals, from their being in water, that water is in immediate contact with water. It may then be asked whether there is but one mode of impression for all the senses, or whether it is different for different senses, seeing that taste and touch are acted upon by contact, and the other senses from a distance? But yet this is a seeming difference only, for we perceive the hard and the soft, as we do the odorous, the sonorous, and the visible, through media; with this difference, that the former impressions are made by objects close to, and the latter by objects at a distance from us. On which account, as we perceive all things through a medium, the medium, in the case of bodies close to us, escapes our attention; but if, as we have already said, we could be sensible of all tangible impressions through a membraneous substance, without our being conscious of their having been so transmitted, we should then be situated as we now are, when in water or air; for so situated, we seem to touch bodies directly, and to have no impression from them through a medium.
But tangible differ from visible and sonorous impressions, in that the latter are perceived by the medium acting in some way upon us, while the former are perceived, not by, but together with, the medium, like a man who is struck through his shield; —for it is not the shield which, having been struck, strikes him, but the shield and he are simultaneously struck together. To use a general expression, the flesh and the tongue seem to be in the same relation to the touch which air and water are to sight, hearing, and smell; —are disposed towards that organ, that is, as each of those elements is to each of those senses. When the sentient organ itself is touched, no sensation can there or then be produced, any more than a white object can be seen when placed immediately over the surface of the eye; and thus it is evident that the part perceptive of tangible impressions must be within. Thus, it should be with the touch, as with the other senses; and if objects, when placed upon an organ, are not perceived, but, when placed upon the flesh, they are perceived, we must conclude that the flesh is only the medium for tangible impressions.
The distinctions of the body, as body, are tangible distinctions, and by these I mean distinctions such as distinguish the elements, as hot, cold, dry and moist, upon which we have heretofore spoken in our treatise upon the Elements. The organ which perceives those distinctions is that of Touch; and the part in which resides, primarily, the so-called sense of Touch is, in potentiality, what tangible impressions are in reality; for all sensation is a kind of impression. So that whatever, by its agency, makes something else to be as itself, can do so only from that something being already, as itself, in potentiality. Hence, we are not sensible of hot and cold, hard and soft, when manifested in the same degree as in ourselves, but perceive them only when in excess, as if the sensibility were some kind of mean between the contraries of sentient impressions, and able, as such, to judge of sentient perceptions. The mean, in fact, is critical—for it is either of the extremes in its relation to the other; and as that which is to perceive white and black may be neither one nor the other actually, and yet both potentially, so it is with the other senses, and with touch, which may be neither hot nor cold.
As vision was said to be, in some sense, perceptive of the visible and the invisible, and the other senses equally of their opposites, so Touch may be said to be perceptive of the tangible and the intangible; and by intangible is meant as well what differs but slightly from what is tangible, as air for instance, as what is in such excess as to be destructive of all sensation.
We have thus then spoken, although but superficially, upon each of the senses.