We embarked on the writing of these chapters with a very definite aim in mind: namely, to free experimental psychology proper from the encroachments of its popular namesake and to provide some concrete examples of what the psychologist actually does. We attempted throughout to bear in mind the interested layman who asks in genuine perplexity whether it is really possible to make experiments with human beings. We have tried to show what constitutes a psychologist’s problem, some of the methods by which he tackles it, and the significance of the more general conclusions which he reaches.