Simone de Beauvoir

Beauvoir was the youngest person ever to pass France's agrégation in philosophy (she came in second; the jury gave Sartre first place by a hair, and its members later admitted that of the two they had thought her the real philosopher). She refused for most of her life to be called a philosopher – Sartre got that title in their partnership – and yet her The Second Sex (1949) is one of the most influential philosophical works of the 20th century.

Her argument, applying existentialist categories to women's lives, is that "woman" is not a biological essence; it is a situation – a social position constructed historically, in which women have been made the Other to the male Self. To be born female is not yet to be a woman; one becomes one through a thousand cultural acts. The book launched the second wave of feminism.

Her Ethics of Ambiguity (1947) is one of the clearest statements of existentialist ethics, in many ways more rigorous than Sartre's own attempts. Her late memoirs and The Coming of Age (on aging) are masterpieces in their own right.