Simone de Beauvoir took the central insight of existentialism and turned it into a revolution in how we think about gender. Existentialism holds that human beings have no fixed, given essence — we become who we are through our situation and choices. Beauvoir asked what follows if this is applied to being a woman. Her answer, opening the second volume of The Second Sex, is one of the most quoted lines in modern philosophy: one is not born, but rather becomes, a woman.
The claim is precise. Beauvoir is not denying biological differences; she is denying that 'woman,' as a social role laden with expectations, passivity, and limits, follows from biology. That role is constructed — by upbringing, culture, and a society that has long defined man as the norm and woman as deviation from it. What is presented as women's timeless 'nature' is really a situation that has been imposed, and, crucially, a situation that could be otherwise. In showing this, she gave later feminist thought its founding framework.
The OtherFreedom, ethics, and ambiguity.
Central to her analysis is the idea of the Other. Societies, she argued, have positioned man as the essential Subject and woman as the inessential Other — defined always in relation to men rather than in her own right. This structure, she showed, runs through myth, history, and everyday life, shaping how women are seen and how they come to see themselves. Naming it made it possible to contest it, and the concept of Othering has since travelled far beyond feminism.
Beauvoir was also a serious ethical philosopher. In The Ethics of Ambiguity, she developed an existentialist morality grounded in freedom: because we are radically free and our freedom is bound up with the freedom of others, an authentic life means willing not only our own liberty but everyone's. She rejected the idea that existentialism leads to selfishness or despair, arguing instead that it grounds a real responsibility toward others. A philosopher, novelist, and public intellectual, and a lifelong partner of Sartre, she is now recognised as a major thinker whose influence on ethics and feminism keeps growing.