This is Volume XXIII of thirty-eight in the General Psychology series. So much has been written in recent years on the Unconscious that the time seems opportune to compare and contrast the various important theories that have held the field. Originally published in 1924, this present book aims at a general review of this kind. The subject is Modern Theories of the Unconscious, but as a study of current theories cannot well be entered upon without considering early theories, we include the latter in our general survey and devote the first chapter to their consideration. On account of its importance in modem psychology, Freud's theory is kept much in evidence throughout, and frequent references are made to it when dealing with the work of other authors.