parents; of the aliments with which his frame has been nourished;
besides a thousand trivial, inappreciable causes, which congregating
themselves by degrees produce in him the gouty humour; the effect of
which is to make him feel in an acute and very lively manner. The pain
of the gout engenders in his brain an idea, so modifies it that it
acquires the faculty of representing to itself, of reiterating as it
were, this pain when even he shall be no longer tormented with the gout:
his 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.