It is the extreme mobility of which man is capable, owing to his peculiar organization, that distinguishes him from other beings that are called insensible or inanimate; the different degrees of this mobility, of which the individuals of his species are susceptible, discriminate them from each other; make that incredible variety, that infinity of difference which is to be found, as well in their corporeal faculties, as in those which are mental or intellectual. From this mobility, more or less remarkable in each human being, results wit, sensibility, imagination, taste, &c.: for the present, however, let us follow the operation of the senses; let us examine in what manner they are acted upon, and are modified by exterior objects:--we will afterwards scrutinize the re-action of the interior organ or brain. The eyes are very delicate, very movable organs, by means of which the sensation of light or colour is experienced: these give to the brain a distinct perception, in consequence of which, man forms an idea, generated by the action of luminous or coloured bodies: as soon as the eyelids are opened, the retina is affected in a peculiar manner; the fluid, the fibres, the nerves, of which they are composed, are excited by shocks which they communicate to the brain; to which they delineate the images of the bodies from which they have received the impulse; by this means, an idea is acquired of the colour, the size, the form, the distance of these bodies: it is thus that may be explained the mechanism of _sight_. The mobility and the elasticity of which the skin is rendered susceptible, by the fibres and nerves which form its texture, accounts for the rapidity with which this envelope to the human body is affected when applied to any other body; by their agency, the brain has notice of its presence, of its extent, of its roughness, of its smoothness, of its surface, of its pressure of its ponderosity, &c. Qualities from which the brain derives distinct perceptions, which breed in it a diversity of ideas; it is this that constitutes the _touch_ or _feeling_. The delicacy of the membrane by which the interior of the nostrils is covered, renders them easily susceptible of irritation, even by the invisible and impalpable corpuscles that emanate from odorous bodies: by these means sensations are excited, the brain has perceptions, and generates ideas: it is this that forms the sense of _smelling_. The mouth, filled with nervous, sensible, movable, irritable glands, saturated with juices suitable to the dissolution of saline substances, is affected in a very lively manner by the aliments which pass through it for the nourishment of the body; these glands transmit to the brain the impressions received: perceptions are of consequence; ideas follow: it is from this mechanism that results _taste_.