In the numerous references which we have hitherto made to the faculty of Sensation, as a display of the operation of the nervous system, we have considered it as indicating merely the presence of bodies, and as giving no information respecting their character. If we attend more minutely to this faculty, we shall find, that all the sensitive parts of the body, are not equally capable of warning us of the presence of the same kind of objects. The rays of light make no im pression upon the tongue or the fingers, indicating their presence, while they act with energy on the eye. The vibrations of the air make no impression on the eye, the mouth, or the nose, while they instantly act upon the ear. Sensation, therefore, is a generic term, intimating the capability of the nervous system to receive impressions of external objects; and it includes as many species as there are impressions calculated to act on one organ, and not upon another, distinguished by this common property, that they intimate the presence of objects.

The number of impressions which may be regarded as distinct species, is more extensive than is generally imagined, and would justify us in considering the term Sensation as the index of an order or class, rather than of a subordinate division. Philosophers, however, have agreed to reduce our sensations to five kinds, namely, those of Touch, Sight, Hearing, Taste, and Smell, to which I have ventured to add Heat.