From the PREFACE. The aim of this work is given in its title. First, it is an "introduction" only, and does not go into the details or the literature of the subject. The aim is to point out the highways of psychology, rather than its myriad byways. Secondly, it is an "introduction to psychological theory," and aims less at a knowledge of facts than at an understanding of principles. Until principles are settled there is no bar to the most fantastic theories and interpretations. These principles being illustrated in the most common facts of experience, it is not necessary to psychological insight to make an anthology of madhouse and hospital stories. Such a procedure has about the same relation to psychology that the various books of "wonders" or the "brilliant experiments" of the popular lecturer have to sober physical science. An odor of quackery is perceptible in both cases. The plan of the work precludes much attention to physiological psychology. Whatever the merits of this science may be, it presupposes pure psychology. If our aim is to give a physiological explanation of psychological facts, we must' first know the facts. Or if our aim is the more modest one of finding the physical conditions or attendants of mental facts, again we must know the facts. But this knowledge is not possible by the way of physiology, and in any case the mental facts remain what they always were. Their likenesses and differences and essential nature would not be changed if physiology were supreme. Even the "new psychology " would not give us new mental facts, but only a new interpretation of the old facts. The Zeitgeist itself begins at last to see this; and the naive onslaughts on the "old psychology" are happily growing fewer. Psychological literature shows very marked progress in this respect within the last twenty years. Physiology remains a most estimable science, but the physiological reconstruction of psychology has been postponed. The study of the physical conditions of our mental life has a pathological and practical importance; but it does not promise any valuable psychological results, at least for those who can distinguish between the physical conditions and the mental facts which they condition. The limitation of plan involves many omissions; and in these there will seem to be a measure of arbitrariness. Hence many will not find here what they want, and probably still more will find what they do not want. There seems to be no way of adjusting so grave a difficulty except by maintaining, on the one hand, freedom to publish, and, on the other, freedom not to read.