Logic, medieval
Authors: Ashworth E.J. 
Published: September 2018

Medieval logic is crucial to the understanding of medieval philosophy, for every educated person was trained in logic, as well as in grammar, and these disciplines provided techniques of analysis and a technical vocabulary that permeate philosophical, scientific and theological writing. At the practical level, logic provided the training necessary for participation in the disputations that were a central feature of medieval instruction, and whose structure – with arguments for and against a thesis, followed by a resolution – is reflected in many written works. At the theoretical level, logic, like other subjects, involved the study of written texts through lectures and written commentaries. The core of the logic curriculum from the twelfth century onwards was provided by the logical works of Aristotle. These provided the material for the study of types of predication, the analysis of simple propositions and their relations of inference and equivalence, the analysis of modal propositions, categorical and modal syllogisms, fallacies, dialectical Topics, and scientific reasoning as captured in the demonstrative syllogism. Comprehensive as this list might seem, medieval logicians realized that other logical subjects needed to be investigated, and, again from the twelfth century onward, new techniques and new genres of writing appeared. The main new technique involved the use of ‘sophismata’, or puzzling cases intended to draw attention to weaknesses and difficulties in logical definitions and rules. The new genres of writing especially included works on ‘supposition theory’, which concerned the types of reference that the subjects and predicates of propositions have in different contexts, and works on ‘syncategoremata’, which concerned the effect on sense and reference produced by the presence and placing of such logical terms as ‘all’, ‘some’, ‘not’, ‘if…then’, ‘except’, and so on. Other important topics for investigation include ‘insolubles’, or semantic paradoxes, and ‘consequences’, or valid inference forms. These new developments were seen as providing a supplement to Aristotelian logic, rather than an alternative. The only context in which people occasionally suggested that Aristotelian logic was inapplicable was that of Trinitarian theology, and the only logician who deliberately set out to reform logic as a whole was Ramon Llull.
The study of medieval logic involves two kinds of difficulty. In the first place, few texts are available in translation, and indeed, many are not even available in printed form. In the second place, there is a problem of interpretation. For a very long time, the specifically medieval contributions to logic were ignored or despised, and when people began to take them more seriously, there was a strong tendency to look at them through the spectacles of modern formal logic. More recently, scholars have come to realize that medieval interests cannot be mapped precisely onto modern interests, and that any attempt, for example, to make a sharp distinction between propositional and quantificational logic is misleading. The first task of the modern reader is to try to understand what the medieval logician was really concerned with.