FORCE OF TOUCH TO MAKE SENSATIONS OBJECTIVE.
54. It has been said that touch is the surest, and perhaps the only witness of the existence of bodies; for without it, all sensations would be nothing more than simple modifications of our being, to which we could attribute no external object. But this I do not believe to be true. We receive by touch an impression, just as we do by the other senses; this impression is in all cases an affection of our being, and not something external. When, from the continuance of these impressions, their order, and their independence of our will, we judge them to proceed from objects without us, our judgment is true not only of impressions of the sense of touch, but also of those of the other senses.
55. One of the reasons whereon it has been attempted to base the superiority of touch to attest the existence of bodies, is that it gives us the idea or sensation of extension; for if we suppose a man to be deprived of all his senses but that of touch, and to pass his hand over the surface of a body, he will experience that continuity of the sensation which involves extension. This observation of those who maintain the supremacy of touch does not prove what they propose. If we pass our sight over various objects, or the different parts of one object, we shall experience the sensation of continuity just as clearly as by touch. We cannot conceive why the sensation of extension must be any clearer when the hand is passed along a balustrade than when it is seen by the eyes.
56. The advocates of this opinion assert that we acquire by the touch of our body a double sensation, which we do not by the other senses. If we pass our hand over our forehead, we feel with both our hand and our forehead, and so verify a continuity of sensations, all originating and terminating in ourselves. Thus we are conscious that both the sensations of our hand and our forehead belong to us.
But this reason, by some deemed conclusive, is nevertheless exceedingly futile; it labors under the sophism called by dialecticiansbegging of the question, for it supposes what was to be proved. The man destitute of all senses but that of touch will, indeed, experience the two sensations and their continuity; but what can he infer from them? Does he even know that he has either hand or forehead? Suppose him not to know, how is he to acquire this knowledge? Both sensations belong to him, and of this he is internally conscious; but whence they came, he knows not. Does the coincidence of the two sensations, perchance, prove something in favor of the existence of his forehead and hand, objects of which we suppose him to have no idea?If this coincidence proved what is pretended, with still greater reason would it prove the combination of some senses with others to elevate us to the knowledge of the existence of bodies, and consequently that this knowledge is not produced exclusively by touch. Whenever I have the sensation of the motion of placing my hand before my eyes, I find that I lose sight of the objects before me, and in their room is presented another always the same, my hand. If, from this coincidence, I infer the existence of external objects, the supremacy of touch is destroyed, for sight, also, acts a part in the formation of such a judgment. I also observe that when I have the sensation of clapping my hands together, I experience the sensation of hearing the noise of their contact; if, therefore, coincidence is of any account, hearing as well as touch comes in. What I say concerning the clapping of my hands, is applicable to what I experience when I pass a hand over any part of my body, for instance my arm, so as to produce some noise. In this case there are two sensations coincident and continuous.
It will, perhaps, be replied, that these examples refer to different senses, and produce sensations of different kinds. This, however, is of no consequence; for, if the being that perceives, infers the existence of objects from the coincidence of various sensations, the supremacy of touch is destroyed, which is what we undertook to demonstrate.
57. The sensation of the hand is not that of the forehead, for the one is warmer or colder, harder or softer than the other, and so the sensation caused by the hand upon the forehead will not be the same as that produced upon the hand by the forehead. It is to be observed, that the less difference we suppose between the two sensations, the less lively will be the perception of their duality, and consequently the less marked the coincidence on which the judgment is founded. Thus by rigorously analyzing this matter we discover that the diversity of sensations contributes in an especial manner to form judgments of the existence of objects, and therefore the combination of two senses will more conduce to this end than two sensations of one sense. Far, then, from its being necessary to consider touch as alone or superior upon this point, it is only to be held as auxiliary to the other senses.
In truth, it is almost beyond doubt that the sense of touch also requires the aid of the other senses, and that the judgments resulting from it are similar to those coming from the other senses. It is probable that only after repeated trials do we refer the sensation of touch to the object that causes it, or even to the part affected. The man who has had his arm amputated, feels pain as if he still preserved it; and this is because a repetition of acts has formed the habit of referring the cerebral impression to the point where the nerves transmitting it terminate. There is, therefore, no necessary relation between the sense of touch and the object; and this sense is, like the others, liable to illusions. Therefore, it is not exact to say that the idea of body springs up under our hand, if this be understood as excluding touch; for the same is true of the other senses, particularly of sight.