Simone de Beauvoir published this in 1949, and feminism gained its philosophical foundation. Her central argument, that women are not born but made, that femininity is a cultural construction, changed how millions of people understood gender. The book is long, drawing on biology, history, psychoanalysis, and literature. It was immediately controversial and has remained controversial, criticized from every direction. But the core insight has become so absorbed into culture that it is difficult to remember how revolutionary it once was. This is the book that made it thinkable.