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.

Indeed, not only the interior organ perceives the modifications it receives from without, but again it has the faculty of modifying itself; of considering the changes which take place in it, the motion by which it is agitated in its peculiar operations, from which it imbibes new perceptions and new ideas. It is the exercise of this power to fall back upon itself, that is called _reflection_.

From this it will appear, that for man to think and to reflect, is to feel, or perceive within himself the impressions, the sensations, the ideas, which have been furnished to his brain by those objects which