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. Now, that the vivacity and fulness of a remembered idea should constitute the exact measure of the mind's retentiveness in that particular instance, is a thing of course. There is no other measure of retentiveness but the power of reproducing in idea, what has been before us, in actuality, or as sensation; and the greater the approach of the idea to the original sensation, the better is the retaining faculty. There is an apparent exception to this general principle. The memory of the same idea, or the same feeling, in the same person, may be at one time full and vivid, and at another time meagre and faint. In particular moments, we may recall former experiences with especial force, as if there were something that co-operated with the proper force of retentiveness. What, then, are these additional or concurring forces? Hume recognises the influence of disease in giving preternatural intensity to ideas.