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_.