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.