The System of Nature, or, the Laws of the Moral and Physical World. Volume 1

by baron d' Paul Henri Thiry Holbach

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.

The ear, whose conformation fits it to receive the various impulses of air, diversely modified, communicates to the brain the shocks or sensations; these breed the perception of sound, and generate the idea of sonorous bodies: it is this that constitutes hearing.

Such are the only means by which man receives sensations, perceptions, and ideas. These successive modifications of his brain are effects produced by objects that give impulse to his senses; they become themselves causes, producing in his soul new modifications, which are denominated thought, reflection, memory, imagination, judgment, will, action; the basis, however, of all these is sensation.

To form a precise notion of thought, it will be requisite to examine, step by step, what passes in man during the presence of any object whatever. Suppose for a moment this object to be a peach: this fruit makes, at the first view, two different impressions on his eyes; that is to say, it produces two modifications, which are transmitted to the brain, which on this occasion experiences two new perceptions, or has two new ideas or modes of existence, designated by the terms colour and rotundity; in consequence, he has an idea of a body possessing roundness and colour: if he places his hand on this fruit, the organ of feeling having been set in action, his hand experiences three new impressions, which are called softness, coolness, weight, from whence result three new perceptions in the brain, he has consequently three new ideas: if he approximates this peach to his nose, the organ of smelling receives an impulse, which, communicated to the brain, a new perception arises, by which he acquires a new idea, called odour: if he carries this fruit to his mouth, the organ of taste becomes affected in a very lively manner: this impulse communicated to the brain, is followed by a perception that generates in him the idea of flavour. In re-uniting all these impressions, or these various modifications of his organs, which it have been consequently transmitted to his brain; that is to say, in combining the different sensations, perceptions, and ideas, that result from the impulse he has received, he has an idea of a whole, which he designates by the name of a peach, with which he can then occupy his thoughts.

From this it is sufficiently proved that thought has a commencement, a duration, an end; or rather a generation, a succession, a dissolution, like all the other modifications of matter; like them, thought is excited, is determined, is increased, is divided, is compounded, is simplified, &c. If, therefore, the soul, or the principle that thinks, be indivisible; how does it happen, that this soul has the faculty of memory, or of forgetfulness; is capacitated to think successively, to divide, to abstract, to combine, to extend its ideas, to retain them, or to lose them? How can it cease to think? If forms appear divisible in matter, it is only in considering them by abstraction, after the method, of geometricians; but this divisibility of form exists not in Nature, in which there is neither a point, an atom, nor form perfectly regular; it must therefore be concluded, that the forms of matter are not less indivisible than thought.

What has been said is sufficient to show the generation of sensations, of perceptions, of ideas, with their association, or connection in the brain: it will be seen that these various modifications are nothing more than the consequence of successive impulses, which the exterior organs transmit to the interior organ, which enjoys the faculty of thought, that is to say, to feel in itself the different modifications it has received, or to perceive the various ideas which it has generated; to combine them, to separate them, to extend them, to abridge them, to compare them, to renew them, &c. From whence it will be seen, that thought is nothing more than the perception of certain modifications, which the brain either gives to itself, or has received from exterior objects.