7.  The word idea has various meanings in the 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 fibrous and sensorial motions reciprocally introduce each other in progressive trains or tribes, it is termed catenation of animal motions.  All these connections are said to be produced by habit; that is, by frequent repetition.

12.  It may be proper to observe, that by the unavoidable idiom of our language the ideas of perception, of recollection, or of imagination, in the plural number signify the ideas belonging to perception, to recollection, or to imagination; whilst the idea of perception, of recollection, or of imagination, in the singular number is used for what is termed “a reflex idea of any of those operations of the sensorium.”