This book represents in substance a course of lectures and discussions given first at the University of Illinois and later at Wesleyan University. It was written to meet the needs both of the college student who has the added guidance of an instructor, and of the general reader who has no such assistance. The attempt has been made to keep the presentation simple and clear enough to need no interpreter, and by the list of readings appended to each chapter, to make a self-directed further study of any point easy and alluring. These references arc for the most part to books in English, easily accessible, and both intelligible and interesting to the ordinary untrained reader or undergraduate. Some articles from the popular reviews have been included, which, if not always authoritative, arc interesting and suggestive. The function of the instructor who should use this as a textbook would consist, first, in making sure that the text was thoroughly read and understood; secondly, in raising doubts, suggesting opposing views, conducting a discussion with the object of making the student think for himself; and, thirdly, in adding new material and illustration and directing the outside readings which should supplement this purposely brief and summary treatment. The books to which reference is made in the lists of readings, and other books approved by the instructor, should be kept upon reserved shelves for the constant use of the class in the further study of questions suggested by the text or raised in the classroom. It will be noticed that the disputes and the technical language of theorists have been throughout so far as possible avoided.