In Abelard and Heloise, a dual intellectual biography of Peter Abelard (1079–1142) and Heloise (d. 1164), I argue that there is a fundamental continuity to the evolution of Abelard’s thought from his early concern with dialectic, to his growing interest in theology in the 1120s and in ethical questions in the 1130s. Heloise was much more than the disciple and lover of Abelard. She emerges as a distinct thinker in her own right, deeply versed in classical ideals of friendship and ethics, which she wished to apply to her relationship to Abelard. While they have both functioned as mythic figures in the western imagination, I argue that both participated in a broader 12th-century renewal of interest in both classical literature and in religious reform. I examine Abelard’s dialectic as a theory not just about universals, but about language as a whole. Tracing the maturing of his dialectic from his earliest glosses to the Logica ‘Ingredientibus’, written after his early affair with Heloise (1115–17), I argue that Abelard was initially unable to come to terms with the ethical questions presented by Heloise in her side of messages (Epistolae duorum amantium) they exchanged during those years. After Abelard became a monk at St Denis, he started to write about theology. I trace the evolution of his theological interests, from his early concern with linguistic concerns to increasing preoccupation with the Holy Spirit and divine goodness, as manifest in Jesus. After Heloise, abbess of the Paraclete from 1129, responded to his Historia calamitatum and demanded he pay greater attention to the community he had founded, Abelard started to devote much more attention to commenting on Scripture and to reflecting on the ethical questions with which she had always been concerned. Accusations spread by St Bernard that Abelard promoted heresy distort the true character of his contribution to theology, on which Heloise exercised a profound influence.