AbstractSimone de Beauvoir is renown for The Second Sex (1949), a work now considered to be a feminist classic. Nevertheless, when Beauvoir wrote this book she did not explicitly endorse the women's movement, nor did she associate her analysis with the women's liberation. It took twenty-one years after the publication before she publicly declared herself a feminist, but from that point on she was a dedicated feminist. How can her development from a gender blind young philosopher to a radical feminist activist be explained? In this article I argue that her less known moral philosophy might provide an answer, as it might be understood as the foundation for her later philosophical analysis and political commitments. In her existentialist ethics she assets that freedom to be the normative core value, and develops an ethical justification for why we should defend our own as well as the freedom of others. However, when this idealistic and abstract moral philosophy was applied to the concrete situation of women, she discovered a reality permeated with gendered structures that impeded women's possibilities of transcendence and to attain freedom. An examination of the philosophical link between Beauvoir's ethics, The Second Sex and her feminist analysis also reveals, Pettersen argues, what might happen when a gender blind moral philosophy is faced with a gendered reality. NORWEGIAN ABSTRACT: Hvordan kunne Simone de Beauvoir allerede i 1949 skrive Det annet kjønn uten tilknytning til en kvinnebevegelse, og uten å oppfatte seg som feminist? Svaret er trolig at hennes mindre kjente moralfilosofi danner grunnlaget for senere analyser, og også forklarer utviklingen fra kjønnsblind ung filosof til radikal feministisk aktivist. Forbindelsen mellom Beauvoirs etikk og senere femi- nistiske analyser viser dessuten hva som kan skje når idealistisk moralfilosofi møter en kjønnet virkelighet.