Dream Psychology is an 1899 book by Sigmund Freud, the founder of psychoanalysis, in which the author introduces his theory of the unconscious with respect to dream interpretation, and discusses what would later become the theory of the Oedipus complex. Freud revised the book at least eight times and, in the third edition, added an extensive section which treated dream symbolism very literally, following the influence of Wilhelm Stekel. Freud said of this work, "Insight such as this falls to one's lot but once in a lifetime." Dated 1900, the book was first published in an edition of 600 copies, which did not sell out for eight years. The Interpretation of Dreams later gained in popularity, and seven more editions were published in Freud's lifetime. Because of the book's length and complexity, Freud also wrote an abridged version called On Dreams. The original text is widely regarded as one of Freud's most significant works.