writers of metaphysic: it is
here used simply for those notions of external things, which our organs of
sense bring us acquainted with originally; and is defined a contraction, or
motion, or configuration, of the fibres, which constitute the immediate
organ of sense; which will be explained at large in another part of the
work. Synonymous with the word idea, we shall sometimes use the words
_sensual motion_ in contradistinction to _muscular motion_.

8. The word _perception_ includes both the action of the organ of sense in
consequence of the impact of external objects, and our attention to that
action; that is, it expresses both the motion of the organ of sense, or
idea, and the pain or pleasure that succeeds or accompanies it.

9. The pleasure or pain which necessarily accompanies all those perceptions
or ideas which we attend to, either gradually subsides, or is succeeded by
other fibrous motions. In the latter case it is termed _sensation_, as
explained in Sect. V. 2, and VI. 2.--The reader is intreated to keep this
in his mind, that through all this treatise the word sensation is used to
express pleasure or pain only in its active state, by whatever means it is
introduced into the system, without any reference to the stimulation of
external objects.

10. The vulgar use of the word _memory_ is too unlimited for our purpose:
those ideas which we voluntarily recall are here termed ideas of
_recollection_, as when we will to repeat the alphabet backwards. And those
ideas which are suggested to us by preceding ideas are here termed ideas of
_suggestion_, as whilst we repeat the alphabet in the usual order; when by
habits previously acquired B is suggested by A, and C by B, without any
effort of deliberation.

11. The word _association_ properly signifies a society or convention of
things in some respects similar to each other. We never say in common
language, that the effect is associated with the cause, though they
necessarily accompany or succeed each other. Thus the contractions of our
muscles and organs of sense may be said to be associated together, but
cannot with propriety be said to be associated with irritations, or with
volition, or with sensation; because they are caused by them, as mentioned
in Sect. IV. When fibrous contractions succeed other fibrous contractions,
the connection is termed _association_; when fibrous contractions succeed
sensorial motions, the connection is termed _causation_; when fibrou