If, for instance, we have experienced 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, or at least the prominent features in it. 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 come within my recent observation: a fireman, hurrying to a fire, was injured severely by being thrown from a hose-wagon against a telegraph-pole with which the wagon collided. He narrowly escaped death. Although three years have elapsed he still cannot ride on a wagon to a fire without the memory of substantially 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. It is also plainly an imperative memory, that is to say, any stimulus-idea associated with some element in the complex reproduces the experience as memory whether it is wished or not. Try as hard as he will he cannot prevent its recurrence. The stimulus that excites such involuntary memories may be a spoken word (as in the psycho-galvanic and other associative experiments which we shall consider in a later lecture), or it may be a visual perception of the environment—of a person or place—or it may be a repetition of the circumstances attending the original experience, however induced. The phenomenon may also be regarded as an automatism or automatic process. As the biological instinct of fear is incorporated in the complex it is also a _phobia_. _Why_ our fireman suffered the intense terror that he did at the time of the accident, why he experienced the thoughts which surged into his mind, why he suffered this emotional experience, while another man going through the same accident suffers no more than the physical injury (if any) at the time, and why the experience continues to recur as an imperative memory are problems which we are not considering now. The fact is that he did suffer the terror and its agonizing thoughts, and, this being the case, their constant recurrence, i.e., the reproduction of the experience, is a memory. And this memory consists of a well organized complex of ideas, feelings, and physiological accompaniments. I emphasize this point because an imperatively recurring mental experience of this sort is a psychosis, and, so far as the principle of memory enters into it, so far memory becomes a part of the mechanism of obsessions.