Otto Weininger was born on April 3, 1880 in Vienna as a son of the Jewish goldsmith Leopold Weininger and his wife Adelheid. After attending primary school and graduating from secondary school in July 1898, Weininger registered at the University of Vienna in October of the same year. He studied philosophy and psychology but took courses in natural sciences and medicine as well. Weininger learned Greek, Latin, French and English very early, later also Spanish and Italian, and acquired passive knowledge of the languages of August Strindberg and Henrik Ibsen (i.e., Swedish and Danish/Norwegian).

In the autumn of 1901 Weininger tried to find a publisher for his work Eros and the Psyche - which he submitted to his professors Jodl and Müllner as his thesis in 1902. He met Sigmund Freud who, however, did not recommend the text to a publisher. His professors accepted the thesis and Weininger received his Ph.D. degree. Shortly thereafter he became proudly and enthusiastically a Protestant.

In 1902 Weininger went to Bayreuth where he witnessed a performance of Richard Wagner’s Parsifal which left him deeply impressed. Via Dresden and Copenhagen he made his way to Christiania (Oslo) where he for the first time saw Henrik Ibsen’s liberation drama Peer Gynt on stage. Upon his return to Vienna Weininger suffered from fits of deep depression. The decision to take his own life gradually took shape in his mind; after a long discussion with his friend Artur Gerber, however, Weininger realized that “it is not yet time”.

In June 1903, after months of concentrated work, his book Sex and Character - a fundamental investigation - an attempt “to place sex relations in a new and decisive light” - was published by the Vienna publishers Braumüller & Co. The book contained his thesis to which three vital chapters were added: (XII) The Nature of Woman and her Relation to the Universe, (XIII) Judaism, (XIV) Women and Humanity.

While the book was not received negatively, it did not create the expected stir. Weininger was attacked by Paul Julius Moebius, professor in Leipzig and author of the book On the Physiological Deficiency of Women, and was accused of plagiarizing. Deeply disappointed and tortured by doubts, Weininger left for Italy.

Back in Vienna he spent his last five days with his parents. On October 3, he took a room in the house in Schwarzspanierstraße 15 where Ludwig van Beethoven died. He told the landlady that he was not to be disturbed before morning since he planned to work and then to go to bed late. This night he wrote two letters, one addressed to his father, the other one to his brother Richard, telling them that he was going to shoot himself.

On October 4, Weininger was found mortally wounded, having shot himself through the heart. He died in Wiener Allgemeines Krankenhaus at half past ten that morning. Otto Weininger was buried in the Matzleinsdorf Protestant Cemetery in Vienna. The epitaph by his father translates:

"This stone closes the resting place of a youth whose spirit never found rest on earth. And when he had made known the revelations of his spirit and of his soul, he could no longer bear to be among the living. He sought out the death precinct of one of the greatest in Vienna’s Schwarzspanier house, and there destroyed his bodily existence."

In his book Sex and Character, Weininger argues that all people are composed of a mixture of the male and the female substance, and attempts to support his view scientifically. The male aspect is active, productive, conscious and moral/logical, while the female aspect is passive, unproductive, unconscious and amoral/alogical. Weininger argues that emancipation should be reserved for the "masculine woman", e.g. some lesbians, and that the female life is consumed with the sexual function: both with the act, as a prostitute, and the product, as a mother. Woman is a "matchmaker". By contrast, the duty of the male, or the masculine aspect of personality, is to strive to become a genius, and to forego sexuality for an abstract love of the absolute, God, which he finds within himself.

A significant part of his book is about the nature of genius. Weininger argues that there is no such thing as a person who has a genius for, say, mathematics, or music, but there is only the universal genius, in whom everything exists and makes sense. He reasons that such genius is probably present in all people to some degree.

In a separate chapter, Weininger, himself a Jew who had converted to Christianity in 1902, analyzes the archetypal Jew as feminine, and thus profoundly irreligious, without true individuality (soul), and without a sense of good and evil. Christianity is described as "the highest expression of the highest faith", while Judaism is called "the extreme of cowardliness". Weininger decries the decay of modern times, and attributes much of it to feminine, and thus Jewish, influences. By Weininger's reckoning everyone shows some femininity, and what he calls "Jewishness".

Weininger's suicide in the house in Vienna where Beethoven had died, the man he considered one of the greatest geniuses of all made him a cause célèbre, inspired several imitation suicides, and turned his book into a success. The book received glowing reviews by August Strindberg, who wrote that it had "probably solved the hardest of all problems", the "woman problem".

Ludwig Wittgenstein read the book as a schoolboy and was deeply impressed by it, later listing it as one of his influences and recommending it to friends (Ray Monk: Ludwig Wittgenstein, The Duty of Genius, 1990). However, Wittgenstein's deep admiration of Weininger's thought was coupled with a fundamental disagreement with his position. Wittgenstein writes to G.E. Moore: "It isn't necessary or rather not possible to agree with him but the greatness lies in that with which we disagree. It is his enormous mistake which is great." The themes of the decay of modern civilization and the duty to perfect one's genius occur repeatedly in Wittgenstein's later writings.

Weiniger's friend Artur Gerber gave a description of Weiniger's physiognomy in "ECCE HOMO", preface to Taschenbuch und Briefe an einen Freund (E. P. Tal & Co., Leipzig/Vienna 1922):

Isolated parts of Weininger's writings were used by Nazi propaganda, despite the fact that Weininger actively argued against the ideas of race that came to be identified with the Nazis. On the other hand, Weininger's views on race cannot be reduced to a simplistic egalitarian liberalism:

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Nevertheless, Weininger's books were denounced by the Nazis..