What did Beauvoir mean by "One is not born a woman", and what is an ethics of ambiguity?

When Simone de Beauvoir wrote 'One is not born, but rather becomes, woman', she was turning the existentialist slogan that existence precedes essence into a feminist claim: womanhood is not simply handed over by nature but shaped by society. In her account, a girl is taught to be a woman and her 'feminine' destiny is imposed on her by society.

The line comes from The Second Sex, published in 1949 and written in about 14 months. Beauvoir asks 'What is woman?' and argues that man is treated as the default while woman is the 'Other': humanity is male, and man defines woman relative to himself. The book became one of her most controversial, banned by the Vatican, and is also regarded as a groundbreaking work of feminist philosophy and the starting inspiration of second-wave feminism. In a 1974 interview she remembered that Camus was furious and said she had ridiculed the French male.

Scholars still disagree about what the sentence means. Judith Butler reads it as separating sex from gender, with gender gradually acquired. Toril Moi and Nancy Bauer, among others, contest that reading. One English translation even renders the line 'becomes, woman', because Beauvoir uses the word here for woman as a construct or an idea rather than an individual.

Beauvoir also worried about existentialism's ethics. In 1945 she claimed that it was impossible to base an ethical system on Sartre's Being and Nothingness, and in The Ethics of Ambiguity she tried to supply one. Her point is that each of us is both subject and object, freedom and facticity: that is the ambiguity. Freedom, she argues, cannot be separated from responsibility to others.