
She asked what it means to be a woman — and showed that the answer was not biology but history, not nature but construction.
De Beauvoir grew up in bourgeois Paris, won the agrégation in philosophy second in the country — after Sartre — and spent her life arguing that the concepts of freedom and situation, abstract in Sartre's hands, must be worked out in the concrete conditions of actual lives. The Second Sex opens with a question so simple it had never been properly asked: what is a woman? Not a biological given, she argued, but a social construction — one is not born a woman, one becomes one. The book mapped the mechanisms of that becoming with philosophical precision and empirical depth, drawing on biology, psychoanalysis, history, and literature. It is the founding text of second-wave feminism. Her novels and memoirs are inseparable from her philosophy: she demonstrated that the existentialist commitment to freedom and responsibility applied to actual people in actual bodies, not just to abstract consciousness. She is not Sartre's partner. He was hers.

“One is not born, but rather becomes, a woman.”
“To will oneself free is also to will others free.”
In 1929, both Sartre and de Beauvoir sat the agrégation examination in philosophy. She finished second; he placed first. They began a lifelong partnership that was philosophical as well as personal, choosing not to marry and to live separately while remaining each other's primary intellectual interlocutors until his death.
Published in 1949, The Second Sex caused a scandal in France. The Vatican put it on the Index of Forbidden Books. Camus complained that de Beauvoir had made French men look ridiculous. In two volumes it analyzed the historical and existential situation of women, asking not what women are but what they have been made to become.
De Beauvoir worked within the phenomenological tradition Husserl founded, applying its method of describing lived experience to the situation of women — a subject Husserl himself had not considered.
Butler took de Beauvoir's claim that 'one is not born but becomes a woman' further than de Beauvoir intended: not just gender roles but the categories of sex themselves are culturally constituted and performatively maintained.