The diaries and other papers of the Oxford classics teacher Arthur Sidgwick (1840–1920) show how men like Sidgwick used ancient Greek to demarcate the boundaries of an elite male social, emotional, and educational sphere, and how that sphere became more porous at the turn of the twentieth century through processes such as university coeducation. Progressive dons like Sidgwick stood by women's equality in principle but were troubled by the potential loss of an exceptional environment of intense friendships forged within intellectually rigorous single-sex institutions. Several aspects of Sidgwick's life and his use of Greek exemplify these tensions: his marriage, his feelings about close male friends, his life as a college fellow, his work on behalf of the Oxford Association for the Education of Women, and his children's lives and careers. The article recovers a lost world in which Greek was an active conversational language, shows how the teaching of classics and the inclusion of women were intimately connected in late-nineteenth-century Oxford, and suggests some reasons why that world endured for a certain period of time but ultimately came to an end. It offers a new way of explaining late-nineteenth-century cultural changes surrounding gender by placing education and affect firmly at their center.