Heinrich Kaan?s fascinating work?part medical treatise, part sexual taxonomy, part activist statement, and part anti-onanist tract?takes us back to the origins of sexology. He links the sexual instinct to the imagination for the first time, creating what Foucault called ?a unified field of sexual abnormality.? Kaan?s taxonomy consists of six sexual aberrations: masturbation, pederasty, lesbian love, necrophilia, bestiality, and the violation of statues. Kaan not only inaugurated the field of sexology, but played a significant role in the regimes of knowledge production and discipline about psychiatric and sexual subjects. 00As Benjamin Kahan argues in his Introduction, Kaan?s text crucially enables us to see how homosexuality replaced masturbation as the central concern of Euro-American sexual regulation. Kaan?s work (translated into English for the first time here) opens a new window onto the history of sexuality and the history of sexology and reconfigures our understanding of Richard von Krafft-Ebing?s book of the same name, published some forty years later.