This article offers a systematic treatment of Beauvoir's reading of biology in The Second Sex. Following Gatens 's suggestion that this chapter has not received the scholarly consideration it demands and deserves, it explains key aspects of Beauvoir's relationship to biological reason by re-telling the story of Beauvoir's early life from the perspective of her scientific education, rationally reconstructing her argument in the chapter on "Biological Data," and exploring the philosophical orientation of her argument using the Frankfurt School model of (...) 'immanent critique.' By illuminating Beauvoir's reading of biology, this article contributes to our understanding of her philosophy while also deflating the widespread assumption that existentialist philosophy is inherently 'anti-science.'. (shrink)