In recent years psychology has considerably expanded and en­ riched its relations with medical practice, first and foremost with psychiatry. This orientation toward experimental abnormal psy­ chology has been closely tied to the practical tasks of psychiatry: differential diagnosis, establishment of the structure and extent of impairment, and the dynamics of mental disorders as affected by treatment, etc. Experimental abnormal psychology has been no less important for the theoretical problems of psychology and psychiatry. The study of pathological changes in mental processes helps in dealing with questions about the structure and formation of mental activity. The research findings of abnormal psychology also have important implications for overcoming biologizing tendencies in the interpre­ tation of human psychology. The present book does not try to provide an exhaustive expo­ sition of all divisions of abnormal psychology. It introduces the reader only to those problems which at the present time seem to be best worked out experimentally: the breakdown of intellectual capacity, thought disorders, the methodology of setting up an ex­ periment in the psychiatric clinic, and certain questions relating to motivational disturbances and psychological growth and decay. Some rewritten sections from the author's earlier book, "The Pathology of Thinking," have been included. v vi FOREWORD The present volume is intended for psychology students, for psycholOgists, and for physicians working in psychiatry.