This work is designed to be an introduction to abnormal psychology. The problems considered, however, belong equally to normal psychology in that they are problems of psycho-physiological functions and mechanisms. I have made no attempt to develop any particular school of psychological theory but rather, so far as may be, to gather together the knowledge already gained and lay a foundation which can be built upon by any school for the solution of particular problems, especially those of special pathology. I have therefore endeavored to avoid controversial questions although this, of course, has not been wholly possible, and indeed so far as special pathological conditions (the psychoses) have been considered, it has been for the purpose of providing data and testing the principles adduced. The inductive method, alone, I believe, as in the physical sciences, can enable us to arrive at sound conclusions—justify the formulation of theories to explain psychological phenomena. Because of the very difficulties of this field of research—one of which is that of submitting to experimental conditions complex psychological phenomena having so many factors—it is all the more incumbent that the inductive method should be employed. To my way of thinking we should begin at the bottom and build up bit by bit, drawing, as we go, no wider conclusions than the facts developed warrant; or if we do, these should be recognized clearly as working hypotheses or speculative theories. Skyscrapers should not be erected until the foundations have been examined to see if they will bear the superstructure. That I have wholly succeeded in so rigorously restricting my own endeavors I can scarcely hope. I trust, however, that I have succeeded in consistently maintaining the distinction between facts and their interpretations. The present volume consists of selected lectures (with the exception of four) from courses on abnormal psychology delivered at the Tufts College Medical School (1908-10) and later at the University of California (1910).[1] These again were based on a series of papers on the Unconscious published in the Journal of Abnormal Psychology (1908-9) of which they are elaborations. Since the lectures were delivered a large amount of new material has been incorporated and the subject matter considered in more detail and more exhaustively than was practical before student bodies. The four additional lectures (X, XI, XII and XIII) appeared in abbreviated form in the same Journal (Oct., Nov., 1912) under the title “The Meaning of Ideas as Determined by Unconscious Settings.” The lecture form has been retained, offering as it does many advantages where, in the exposition of a difficult subject, much that is elemental needs to be stated.