Richard Freiherr von Krafft-Ebing (August 4 1840- December 22 1902), German psychiatrist, wrote Psychopathia Sexualis (1886), a famous study of sexual perversity, and remains well-known for his coinage of the term sadism.
After Krafft-Ebing graduated in medicine and finished his specialisation in psychiatry, he worked in several asylums, but he soon felt that the way those institutions worked deceived him and decided to become an educator. He became a professor at Strasbourg, Graz and Vienna, and also a forensic expert at the Austrian capital.
Krafft-Ebing wrote and published several articles on psychiatry, but his book Psychopathia Sexualis (Psychopathy of Sex), became his best-known work.
After interviewing many homosexuals, both as his private patients and as a forensic expert, and reading some works in favour of gay rights (male homosexuality had become a criminal offence in Germany and the Austro-Hungarian Empire by that time; unlike lesbianism, but discrimination against lesbians functioned equally), Krafft-Ebing reached the conclusion that both male and female homosexuals did not suffer from mental illness or perversion (as persistent popular belief held), and became interested in the study of the subject.
Krafft-Ebing elaborated an evolutionist theory considering homosexuality as an anomalous process developed during the gestation of the embryo and foetus, evolving into a sexual inversion of the brain. Some years later, in 1901, he corrected himself in an article published in the Jahrbuch für sexuelle Zwischenstufen, changing the term anomaly to differentiation. He thus revealed himself, if not as the first, at least as one of the first professionals seeing homosexuals as normal people with a different sexuality.
But his final conclusions remained forgotten for years, partly because Sigmund Freud's theories captivated the attention of those that considered homosexuality a psychological problem (the majority at the time), and partly because Krafft-Ebing had incurred some enmity from the Austrian Catholic church by associating the desire for sanctity and martyrdom with hysteria and masochism (besides denying the perversity of homosexuals).
Some years later Krafft-Ebing's theory led other specialists on mental studies to reach the same conclusion and to the study of transgenderism (or transsexuality) as another differentiation correctable by means of surgery (rather than by psychiatry or psychology).
Note that many contemporary psychiatrists no longer consider homosexual practices as pathological (as Krafft-Ebing did in his first studies): partly due to new conceptions, and partly due to Krafft-Ebing's own self-correction.