The twelfth century was one of the most important and exciting periods in the history of logic. At the start of the century, the production of elementary glosses on ancient texts gave way to a sophisticated commentary literature in which writers developed and debated their own theories concerning what we would now classify as ontology and philosophical logic. Most famous today are the disputes over the status of universals; the present chapter, however, focuses on the less well-known – but, I believe, more important – work done on theories of meaning, modality, and the relation of logical consequence. Many of the works that have survived from the twelfth century are anonymous, but fortunately at least some of those by Peter Abaelard do bear his name: in particular his survey of logic, the Dialectica (probably written around 1112) and a set of commentaries on the books of the logica vetus known as the Logica “Ingredientibus” (probably written between 1115 and 1120). Abaelard is the outstanding logician of this period and is, indeed, one of the greatest of all logicians. His work in these areas fundamentally shaped later development in logic; what follows is essentially an account of his views and of the problems to which they gave rise.

To grasp the importance and originality of Abaelard’s work, it is first necessary to understand in some detail the character of the semantical and logical theories that Boethius bequeathed to the Middle Ages. These were transmitted in his translations of both Porphyry’s Isagoge and Aristotle’s Categories and De interpretatione, together with his own Introductio ad syllogismos categoricos and De syllogismo categorico (which together paraphrase Prior Analytics I.1–7), his treatises De hypotheticis syllogismis, De differentiis topicis, and De divisione, and his commentaries on the Categories, as well as on the Isagoge and De interpretatione, both of which he commented on twice.