ABSTRACT
Pierre Abelard developed a modal logic that is wide-ranging and highly original. Abelard distinguishes between the syntactic features of a proposition secundum constructionem and in sensu. Abelard's stated doctrine clearly differentiates between a modal proposition's subject and predicate in sense, and according to the construction. Abelard intermittently uses a semantics that relies on the notions of what a thing's nature allows, and what is repugnant to a thing's nature. Abelard correctly describes the logical relations and equipollences among modal propositions with singular subjects. Abelard deals with the conversion of modal propositions, arguing against those who favour a de sensu analysis of modals. Abelard points out rightly that, on their de sensu analysis, they should say that the antecedent as well as the consequent is false: the proposition 'Every non-stone is a non-man' is not possible. The point is well-taken; but it is purely ad hominem, and reveals nothing of Abelard's own views on modal conversion.
