Professor Kathleen Stock surveys the philosophical ideas that led to this point, and closely interrogates each one, from De Beauvoir’s statement that, ‘One is not born, but rather becomes a woman’ (an assertion she contends has been misinterpreted and repurposed), to Judith Butler’s claim that language creates biological reality, rather than describing it. She looks at biological sex in a range of important contexts, including women-only spaces and resources, healthcare, epidemiology, political organization and data collection.