Beauvoir began the book intending an autobiographical essay; she wrote about herself 'I am a woman' and then noticed she did not really know what that meant. That noticing led her to two years of research, and to 972 pages spanning physiology, history, psychoanalysis, and literature. The book was published in two volumes in 1949 and sold 22,000 copies in its first week.

The central thesis is in the opening sentence of the second volume: 'One is not born, but rather becomes, a woman' (*On ne naît pas femme: on le devient*). Beauvoir does not deny biological femaleness — there are bodies, hormones, reproduction — but argues that the concept 'woman' is not an inevitable essence flowing from these but a cultural category. The individual born female is transformed into 'a woman' through countless social signals received in childhood, puberty, marriage, motherhood.

Beauvoir reads this transformation through an existentialist frame. With the concepts Sartre had built in *Being and Nothingness* — the for-itself (pour-soi), the in-itself (en-soi), bad faith (mauvaise foi), the Other — she explains the experience of women. Man positions himself as the 'absolute subject' of humanity; woman is cast in the role of the 'Other' — that is, the object of another subject. This role is not biological but historical, and can therefore change.

The book provoked enormous controversy. The Vatican placed it on the Index of Forbidden Books; Albert Camus broke with Beauvoir for 'making the French man ridiculous.' The French Communist Party dismissed her as bourgeois, conservatives called the book 'an attack on the family.' Second-wave feminism — Betty Friedan, Kate Millett, Shulamith Firestone — was written directly with Beauvoir's conceptual tools. At the end of the twentieth century Judith Butler's thesis that 'gender is performative' can be read as a radicalisation of Beauvoir's 'becomes.' *The Second Sex* remains the founding text of feminist philosophy.