Questions as to Touch.—Passing now from the sense of sight to that of touch, we find similar questions discussed among philosophers respecting the precise information afforded by this sense. Does touch give us immediate perception of externality, extension, form, hardness, softness, etc., including the various mechanical properties of bodies? To this sense it has been common to ascribe these faculties of perception. They are so attributed by Reid, Upham, Wayland, and, I believe, by modern writers generally, with the exception of Brown and Hamilton.
Probability of another Source of Information.—It may be questioned, I think, whether, as regards some of these qualities at least, it is not rather the consciousness of resistance to muscular effort, than the sense of touch, properly speaking, that is the informing source. So, for example, as to hardness; the application of an external body lightly to the hand awakens the sense of touch, but conveys no idea of hardness. Let the same object be allowed to rest with gradually increasing weight upon the hand until it becomes painful, and we get the idea of weight, gravitation, but not of the hardness or impenetrability of the object. It is only when our muscular effort to move or penetrate the external body is met and resisted by the same, that we learn the impenetrability of the opposing body.
Other Perceptions attributable to the same Source.—So with regard to externality, extension, and form. When an external object, a cube, for example, or an ivory ball, is placed on the palm of the hand, sensation is awakened, but is that sensation necessarily accompanied with the perception of the external object as such? Does the mere tactual sensation, in the first instance, and of itself, inform us that there is something external to ourselves, that what we feel is not a part of our own organism? We are conscious of a change in the sensation of the part affected, but are we immediately conscious that this change is produced by something external? Let there be given, however, the consciousness of resistance to our muscular movements, as when the cube or ball, for instance, prevents the effort to close the hand, or when our locomotion is impeded by the presence of some obstacle, and will not the same resistance inform us of the extension of the resisting body, and so of its form and figure? We learn whereabout in space this resistance occurs, and where it ceases. The tactual sensation would indeed very soon come to our aid in this cognition, and serve as a guiding sense, even in the absence of the former. The question is, whether this alone would, in the first instance, give us such cognitions?
Our first Ideas of Extension, how derived.—We have had reference in this discussion only to the qualities of external bodies. There can be little question that our first ideas of extension are derived from our own sentient organism, the consciousness of sensations in different parts of the body, distinct from, and out of each other, thus affording the knowledge of an extended sentient organization. The idea of externality, or outness, and extension, thus acquired, the transition is easy from the perception of our own bodies as possessing these qualities, to the cognizance of the same qualities in external objects.
§ VI.—Credibility of our Sensations and Perceptions.
Denied by some.—There have always been those who were disposed to call in question the testimony of the senses. Such were the Eleatics and the Skeptics among the Greek philosophers, and there have not been wanting among the moderns minds of acuteness and ingenuity that have followed in the same path. While admitting the phenomena of sense, the appearance of things as being so and so, they have called in question the corresponding objective reality. Things appear to me to be thus and thus—such and such impressions are made on my senses—that I cannot deny; but how do I know that the reality corresponds to my impressions, or, in fact, that there is any reality? How know we our senses to be reliable? What evidence have we that they do not habitually deceive us?
Evidence demanded.—It were perhaps a sufficient answer to this question to reply, What evidence have we, or can we have, that they do deceive us? In the absence of all evidence to the contrary, is it not more reasonable to suppose that our perceptions correspond to realities, than that they are without foundation, uncaused, or caused by something not at all answering to the apparent object of perception; more reasonable to suppose that there is a real table or book answering to my perception of one, than that I have the perception while there is no such reality? It remains with those, then, who question and deny the validity of sense-perception, to show reasons for such denial. And this becomes the more imperative on them, inasmuch as they contradict the common belief and universal opinion of mankind—nay, what, in spite of all their arguments, is still, by their own confession, their own practical conviction and belief.
Evidence impossible.—But whence is this evidence to come? Where is it to be sought? How are we to prove that sense deceives us, except by arguments drawn from sense? And if sense is not reliable in the first instance, why rely upon it in the second, to prove that it is not reliable? If the senses do habitually deceive us, manifestly it can never be shown that they do. And, even if this could be shown, it would be impossible to find any thing better to rely upon in their stead. We have these guides or none. We have these instruments of observation provided for the voyage of life. We may pronounce them worthless and throw them into the sea, but we cannot replace them.
Inconsistent and contradictory Testimony of Sense.—But it may be replied that the testimony of sense is often inconsistent with itself, and contradictory of itself. What is sweet to one is sour and bitter to another. What seems a round tower in the distance becomes a square one as you approach; and the straight stick that you hold in your hand appears crooked when thrust into the water. There is in reality, however, no contradiction or inconsistency in the cases supposed. The change of circumstances accounts in every instance for the change of appearance. In the case of the stick, for example, the different density of the water accounts for the refraction of the rays of light that pass through it, and this accounts for the crooked appearance of the stick that is only partially submerged. So in the other cases; it is no contradiction that an object which appears round at a distance of ten miles, should appear square at the distance of so many rods—or that the taste of two persons should not agree as to the savor of a given object.