Now under special conditions, the ideas making up an experience at any given moment tend to become organized into a system or complex, so that when we later think of the experience or recall any of the ideas belonging to it, the complex as a whole is revived. This is one of the principles underlying the mechanism of memory. Thus it happens that memory may, to a large extent, be made up of complexes. These complexes may be very loosely organized in that the elementary ideas are weakly bound together, in which case, when we try to recall the original experience, only a part of it is recalled. Or a complex may be very strongly organized, owing to the conditions under which it is formed, and then a large part of the experience can be recalled. In this case, any idea associated with some element in the complex may, by the law of association, revive the whole original complex. If, for instance, we have gone through a railroad accident involving exciting incidents, loss of life, etc., the words "railroad," "accident," "death," or a sudden crashing sound, or the sight of blood, or even riding in a railroad train may recall the experience from beginning to end, or at least the prominent features in it, i.e., so much as was organized. The memory of the greater part of this experience is well organized, while the earlier events and those succeeding the accident may have passed out of all possibility of voluntary recall. To take an instance commonplace enough but which happens to have just come within my observation: A fireman was injured severely by being thrown from a hose wagon rushing to a fire against a telegraph pole with which the wagon collided. He narrowly escaped death. Although three years have passed he still cannot ride on a wagon to a fire without the memory of the whole accident rising in his mind. When he does so he again lives through the accident, including the thoughts just previous to the actual collision when, realizing his situation, he was overcome with terror, and he again manifests all the organic physical expressions of fear, viz.: perspiration, tremor, and muscular weakness. Here is a well-organized and fairly limited complex.