French philosopher, novelist, and memoirist — one of the founding figures of existentialism and the author of what is still the most influential single book of 20th-century feminist theory. The Second Sex (1949), written in a year and a half of relentless research, turns the existentialist vocabulary of Sartre (her partner from 1929 to his death) into an analytical weapon: "One is not born, but rather becomes, a woman." Femininity is not a natural essence but a socially produced situation — historical, contingent, maintained by the countless practices through which girls are trained into a secondary role and women kept there.
Her central structural claim is that in patriarchal society woman is constituted as the Other against which Man defines himself as the default human subject. This is a Hegelian move — the master–slave dialectic read through existentialism — and it allows Beauvoir to mount a phenomenology of lived female experience across biology, psychoanalysis, history, myth, and everyday life that still reads as startlingly fresh. The book was an international scandal and an international sensation; the Vatican put it on the Index.
Beauvoir was also a significant philosopher in her own right beyond feminist theory. The Ethics of Ambiguity (1947) is the clearest statement of an existentialist ethics — a domain Sartre himself never satisfactorily developed. Her memoirs, starting with Memoirs of a Dutiful Daughter (1958), are among the great 20th-century autobiographies. She was politically active throughout her life — against the Algerian War, for abortion rights (signing the Manifesto of the 343 in 1971) — and her later old-age study La Vieillesse (1970) anticipated much later thinking about aging and social invisibility.