The sense of touch is not confined to one particular organ, but is extended over the whole system, both externally and internally. It is in the hands, however, especially at the ends of the fingers, that this sense is most acute and most employed. We acquire many more ideas by the aid of this sense than by either hearing, smell, or taste. By these last we become acquainted with only one particular quality in a body, either of taste, smell, or sound; but by means of the touch we learn such qualities as heat and cold, roughness and smoothness, hardness and softness, figure, solidity, and extension. It is supposed that it is by this sense that we gain the idea of something _external_, or without ourselves. The sensation of smell would seem to be within, as an act or emotion of the soul itself. Thus also with hearing, which, being produced within the ear by the undulating air, would seem to originate within. Thus also with sensations within the eye. But when the limbs begin to move and to come in contact with outward objects, and also in contact with various parts of the body, the mind gains an idea of the existence of some outward object. This is probably the first sense by which any idea of existence is wakened in the mind. As one sense after another is called into action, the mind continually gains new ideas, and then begins its operations of comparing, abstracting, reasoning, and willing. It is by the sense of touch that we gain our ideas of _resistance_ and _extension_. In the class of ideas included under the head of ideas of resistance may be placed those of solidity, liquidity, hardness, softness, viscidity, roughness, and smoothness; these all being different names for different modes of resistance to the muscles of the hands, arms, or fingers, when applied to the bodies which have these qualities. These ideas are not gained by simple contact; their existence depends upon the contraction or expansion of the muscles, which are the organs of motion and resistance in the human body. We may suppose the infant to gain these ideas by a process somewhat similar to this: He first moves his arms by instinct, without any knowledge of the effects to follow. By this movement he gains certain ideas of the simple contractions and extension of his muscles, and learns also that by his own will he can exercise his muscles in this manner. At length he attempts to move his arm in a manner to which he has become familiar, and some object intervenes, and motion is prevented, while all his wonted muscular efforts are vain. Thus arises in his mind a new idea, of resistance, in addition to the sensations of touch and of motion, which had before been experienced.