[Bain's footnote 23: _The Sensation and the Idea compared_.--Great importance, in every way, attaches to the points of agreement and of difference of the Sensation and of the Idea. By the Sensation, we mean the whole state of consciousness, under an actual or present impression of sense, as in looking at the moon, in listening to music, in tasting wine. By the Idea is meant the state of mind that remains after the sensible agent is withdrawn, or that may be afterwards recovered by the force of recollection.
1. For many purposes the sensation and the idea are identical. They are compared to original and copy, which, although not in all respects of equal value, can often answer the same ends. A perfect recollection of a process that we wish to repeat, is as good as actually seeing it. For all purposes of knowledge, and of practical guidance, a faithful remembrance is equal to the real presence. So, as regards the emotional ideas, or the recollection of states of pleasure and of pain, which { 63} prompt our voluntary actions, in pursuit and in avoidance, the memory operates in the same way as the original fact, allowance being made for difference of degree. A pleasing melody induces us to listen to it, and to crave for its repetition; the after recollection of it, also moves us to hear it again. If we find ourselves in the midst of distracting noises, we are impelled to escape; the mere remembrance, at an after time, has the same influence on the will.
2. It is highly probable, if not certain, that the same nervous tracks of the brain are actuated during the sensation, and during the idea, with difference of degree corresponding to the difference of vivacity or intensity of the actual and remembered states.
Of the points wherein the Sensation and the Idea are found to differ, the most obvious is their degree of intensity. We are able to maintain in idea, the state of mind corresponding to the sight of the sun, the sound of a bell, or the smell of a rose, but we are conscious of a great inferiority in the degree or vividness of the state. The bright luminosity of the original sun turns into a feeble effect, without dazzle or excitement. The thrill of a fine musical air cannot be sustained by the mere memory of it, even in the freshness of the immediately succeeding moment. A certain pleasing remembrance attaches to a good dinner, but how far below the original! Moreover, in a complicated object of sense, a great many of the parts and lineaments drop entirely out of view. Memory is unequal to retaining, without long familiarity and practice, the exact picture of a landscape, a building, or an interior. The difference in the fulness of the idea, as compared with the sensation, is no less remarkable than the difference of vivacity or intensity. This inferiority in the idea as compared with the actuality is of very various amount; being in some cases very great, and in others very slight. The difference is in proportion to the mind's power of retentiveness, a power varying according to several circumstances or conditions, which have to be distinctly enunciated by the Psychologist. For example, it is well known, that frequency of repetition enables the idea to { 64} grow in vivacity and in fulness, and to approximate in those respects to the original. It is also known, that some minds are by nature retentive, and, by a small number of repetitions, gain the point that others reach only by a greater number.