"This book has been written primarily for the purpose of furnishing my students with a brief manual to supplement the lectures on Psychology. At the same time it aims to give the wider circle of scientific scholars who are interested in psychology, either for its own sake or for the sake of its applications, a systematic survey of the fundamentally important results and doctrines of modern psychology. In view of this double purpose, I have limited myself in detailing facts to that which is most important, or to the examples that serve most directly the ends of illustration, and have omitted entirely those aids to demonstration and experiment which are properly made use of in the lecture-room. The fact that I have based this treatise on the doctrines that I have come to hold as valid after long years of labor in this field, needs no special justification.