The Second Sex, 1949. One is not born, but rather becomes, a woman — and the century spent arguing about what that sentence means.
Simone de Beauvoir (1908–1986) published Le Deuxième Sexe in 1949, nearly a thousand pages arguing that womanhood is not a nature but a situation: produced, taught, enforced, and therefore alterable. On ne naît pas femme, on le devient. One is not born, but rather becomes, a woman.
She is one of the few women the philosophy syllabus will admit, and even then usually as Sartre's companion rather than as the author of the more durable book. That framing is itself the thing this list is about.