suppose a person to be born without feeling, and to arrive at man’s estate, he could not from his present visible impressions judge what others would follow upon varying the circumstances. Thus the picture of a knife, drawn so well as to deceive his eye, would not, when applied to another body, produce the same change of visible impressions, as a real knife does, when it separates the parts of the body through which it passes. But the touch is not liable to these deceptions. As it is therefore the fundamental source of information in respect of the essential properties of matter, it may be considered as our first and principal key to the knowledge of the external world.
When we apply the parts of our bodies to each other, particularly our hands to the several parts of the surface of our bodies, we excite vibrations in both parts, viz. both in the hands, and in that part of the surface which we touch. Suppose the hand to pass over the surface gradually, and the first impression will remain the same, while the last alters perpetually, because the vibrations belonging to the last are excited in different nerves, and by consequence enter the brain, or spinal marrow, at different parts. And this difference in the last impression or its vibrations, corresponding always to the part on which the impression is made, will at last enable us to determine immediately what part of our bodies we touch; i.e. what is the distance of the part touched from the mouth, nose, shoulder, elbow, or other remarkable part, considered as a fixed point. For by passing frequently from the mouth, nose, &c. to the part under consideration, children learn this very early, even without attending to it at all explicitly.
Sight also helps us to judge of this distance in the parts, which are frequently exposed to view, and this in proportion to that frequency.
Let us suppose then, that we are able to determine at once what external part of our bodies we touch, i.e. to determine how it is situated in respect of the other parts, and to shew the corresponding part in the body of another person; it will follow, that if a like impression be made not by our own hand, but by that of another, or by any foreign body, we shall know at once the part on which it is made. We shall also, supposing us arrived at a sufficient degree of voluntary power over the muscles, be able at once to put our hand upon the part on which the impression is made.
By degrees we shall learn to distinguish the part, not only when an impression like the gentle ones of our hands is made upon it, but also when a vivid, rude, or painful one is. For,
