This simple scheme of the physiology of association for a hundred years has given a most decided impulse to the progress of psychology. As the association process can so easily be expressed in physiological terms, the aim was prevalent to understand the interplay of mental life more and more as the result of association. The underlying thought of this whole association psychology was thus a conviction that whenever two mental experiences occur together, either of them keeps the tendency to reawaken the other at a later time. Through the endless combination which life's impressions awaken in the mind from the first hour after birth, the whole stream of memory images and thoughts and aims and imaginations is thus to be explained.
The whole theory of physiological associationism works evidently with two factors. First, there are millions of brain cells of which each one may have its particular quality of sensation, and second, each brain cell may work with any degree of energy, to which the intensity of the sensation would correspond. If I distinguish ten thousand different pitches of tone, they would be located in ten thousand different cell groups, each one connected through a special fiber with a special string in the ear. And each of these tones may be loud or faint, corresponding to the amount of excitement in the particular cell group. Every other variation must then result from the millionfold connections between these brain cells. Indeed, the brain furnishes all possibilities for such a theory. We know how every brain cell resolves itself into tree-like branch systems which can take up excitements from all sides, and how it can carry its own excitement through long connecting fibers to distant places, and how the endings of these fibers clasp into the branches of the next cell, allowing the propagation of excitement from cell to cell. We know further how large spheres of the brain are confined to cells of particular function, that for instance cells which serve visual sensations are in the rear part of the brain hemispheres, and so on. Finally we know how millions of connecting fibers represent paths in all directions, allowing very well a coöperation by association between the most distant parts of the brain. The theories found their richest development, when it was recognized that large spheres of our brain centers evidently do not serve at all merely sensory states, but that their cells have as their function only the intermediating between different sensory centers. Such so-called association centers are thus like complex switchboards between the various mental centers. Their own activity is not accompanied by any mental content, but has only the function of regulating transmission of the excitement from the one to the other. Above all their operation would make it possible that through associative processes, the wonderful complexity of our trains of thought may be reached.