Page:The System of Nature.djvu/142

 brain, by a series of motion interiorly excited, is again placed in a state analogous to that in which it was when he really experienced this pain: but if he had never felt it, he would never have been in a capacity to form to himself any just idea of its excruciating torments.

The visible organs of man's body, by the intervention of which his brain is modified, take the name of senses. The various modifications which his brain receives by the aid of these senses, assumes a variety of names. Sensation, perception, and idea, are terms that designate nothing more than the changes produced in this interior organ, in consequence of impressions made on the exterior organs by bodies acting on them: these changes considered by themselves, are called sensations; they adopt the term perception when the brain is warned of their presence; ideas is that state of them in which the brain is able to ascribe them to the objects by which they have been produced.

Every sensation, then, is nothing more than the shock given to the organs, every perception is this shock propagated to the brain; every idea is the image of the object to which the sensation and the perception is to be ascribed. From whence it will be seen, that if the senses be not moved, there can neither be sensations, perceptions, nor ideas: this will be proved to those, who can yet permit themselves to doubt so demonstrable and striking a truth.

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