other cell in the brain; thus there is also one connecting path between those two cells which were stimulated at once. Now if the two ends of an anatomical path are excited at the same time, the path itself becomes changed. The connecting way becomes a path of least resistance, and that means that if, in future, one of the two brain cells becomes excited again, the overflow of the nervous excitement will not now go on easily in all directions, but only just along that one channel which leads to that other brain cell. A theory like this explains in real explanatory terms, in ways which physics and chemistry can demonstrate as necessary, that any excitement of the odor cell runs over into the sound cell and vice versa. In short, the psychological association of ideas, which we should simply have to accept as inexplainable fact, is thus transformed into a connection which we understand as necessary; and the fact is really explained. 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.