"One Is Not Born, but Rather Becomes, a Woman"
In The Second Sex Beauvoir proposed this later widely circulated core thesis, holding that femininity is not an inborn biological essence but a social product "made" through a whole later process of construction — family upbringing, social custom, cultural narrative; this analytical framework distinguishing "gender" from "biological sex" thereafter became one of the most basic and important theoretical tools in the entire field of gender studies.
A "Contractual" Lifelong Partnership
Beauvoir maintained with Sartre an open partnership of more than fifty years on the principle that "a necessary love can allow contingent loves"; the two never married yet always kept a deep intellectual exchange and mutual support, and this in-its-time controversial relationship was itself, in a sense, a personal practice of her existentialist-feminist stance that "a woman should freely choose her own path in life rather than be bound by traditional marital norms."