“That which has existed with any completeness in consciousness leaves behind it, after its disappearance therefrom, in the mind or brain, a functional disposition to its reproduction or reappearance in consciousness at some future time. Of no mental act can we say that it is ‘writ in water;’ something remains from it, whereby its recurrence is facilitated. Every impression of sense upon the brain, every current of molecular activity from one to another part of the brain, every cerebral action which passes into muscular movement, leaves behind it some modification of the nerve elements concerned in its function, some after-effect, or, so to speak, memory of itself in them which renders its reproduction an easier matter, the more easy the more often it has been repeated, and makes it impossible to say that, however trivial, it shall not under some circumstances recur. Let the excitation take place in one of two nerve cells lying side by side, and between which there was not any original specific difference, there will be ever afterwards a difference between them. This physiological process, whatever be its nature, is the physical basis of memory, and it is the foundation of the development of all our mental functions. “That modification which persists, or is retained, in structure after functions, has been differently described as a residuum, or relic, or trace, or disposition, or vestige; or again as potential, latent, or dormant idea. Not only definite ideas, but all affections of the nervous system, feelings of pleasure and pain, desire, and even its outward reactions, thus leave behind them their structural effects, and lay the foundation of modes of thought, feeling, and action. Particular talents are sometimes formed quite, or almost quite, involuntarily; and complex actions, which were first consciously performed by dint of great application, become automatic by repetition; ideas which were at first consciously associated, ultimately coalesce and call one another up without any consciousness, as we see in the quick perception or intuition of the man of large worldly experience; and feelings, once active, leave behind them their large unconscious residua, thus affecting the generation of the character, so that, apart from the original or inborn nature of the individual, contentment, melancholy, cowardice, bravery, and even moral feeling are generated as the results of particular life-experiences.”