The words "suggestion" and "suggestibility" are the playthings of the tyro. He flourishes them as the key to most of the situations presented by human behavior as exhibited in crowds, mobs, and audiences. The reactions of one to another, as those of salesman and purchaser also, are often "explained" by the application of one or the other of these words. In this article we (i) discuss two definitions of suggestion and suggestibility, and (2) describe the conditions that affect both. This should enable us (3) to understand the limitations of suggestion and suggestibility as means of arriving at large social unities. Titchener defines suggestion as "any stimulus, external or internal, accompanied or unaccompanied by consciousness, which touches off a determining tendency."' For example, in the simple reaction experiment the instruction to react on a given signal sets off a determining tendency which releases the reaction movement. What made the reactor ready to accept instruction? What brought him into the laboratory? What brought him to the university? What brought him to seek an education in any university? In each case a previous suggestion. The reactioh to this train of previous suggestion, each in its turn, has developed a complex disposition because of which the reaction is made as a matter of course, once the stimulus is presented. This definition makes suggestion no different from a command or a sensory stimulus. To understand the response to a command or a sensory stimulus we must assume that a tendency or a disposition has already been prepared which is of such a nature that it may be touched off by the appropriate word, gesture, or other stimulus. We would not command an ox to attend to the demonstration of a geometrical proposition because we assume that the