The animal body is composed of different matters, of which some are insensible, as the bones, the fat, the blood, &c. and others, as the membranes and the nerves, appear to be active substances, on which depend the action of every member. The nerves are the immediate organs of the mind, but which may be said to diversify from a difference in disposition, insomuch, that according to their position, arrangement, and quality, they transmit to the mind different kinds of sentiment, which have been distinguished by the name of sensations, and which appear, in effect, to have no resemblance to each other. Nevertheless, if we consider that all external senses are only nervous membranes, differently placed and disposed; that the nerves are the general organs of feeling, and that in the animal body, no other substance is possessed of this property, we shall be led to believe that the senses, having all one common principle, and the nerves proceeding from the same substance, though in various forms, the sensations which result from them are not so essentially different as they at first appear.
The eye ought to be regarded as an expansion of the optic nerve, whose position being more exterior than that of any other nerve, has the most quick and the most delicate sensation. It will be moved, therefore, by the smallest particles of matter, as those of light; and will consequently give us sensations of distant bodies, provided they produce or reflect those small particles. The ear is not placed so exteriorly as the eye, and in which there not being so great an expansion of nerves, will not be possessed of the like degree of sensibility, nor will it be affected by particles more gross, as those which form sounds, and will give us sensations of such distant objects as can put those particles in motion. As they are much grosser than those of light, and have less quickness, they cannot extend themselves so far; and consequently the ear will not give us sensations of objects so distant as those which the eye communicates. The membrane, which is the seat of smell, being still less furnished with nerves than the ear, it will only give us sensations of particles of matter which are more gross and less remote, such as the odour from bodies, which may be said to be the essential oils which exhale and float in the air, as light bodies swim upon the water. As the nerves are also in less quantity, and more divided over the tongue, and palate, and the odoriferous parts are not strong enough to affect them, the oily or saline parts must detach themselves from other bodies, and lodge upon the tongue to produce the sensation of taste. This sense differs materially from that of smelling, because the last brings to us sensations of things at a certain distance, but the former requires a kind of contact, which operates by the means of the fusion of certain parts of matter, such as salts, oils, &c. In short, as the nerves are minutely divided, and as the skin affords them but a very thin covering, no particles of matter so small as those which form light, sound, or odours, can affect them; and the sense of feeling gives us no sensation of distant objects, but of those only whose contact is immediate.