Sexual perversion—however defined—has always been a staple of Western history and even a celebrated part of its culture; de Sade's works are still considered "classics" of literature and philosophy. Psychoanalytic investigations into the subject did not begin until 1905 when Freud wrote "Three Essays on the Theory of Sexuality," wherein he considered perversions to be a clear matter of arrested development, the intrusion of elements of infantile sexuality into the world of adult genitality.