Reasoning about the trinity: A modern formalization of a medieval system of trinitarian logic

In Christian discourse, paralogisms arise when one blindly applies classical logic to the traditional definition of the trinity. Some people conclude, on the basis of these paralogisms, that logic cannot be coherently applied to the trinity. Others conclude that it can, but that the right logic is not classical logic, but an extension of it. An anonymous treatise from the late Middle Ages addresses the problems of paralogisms by introducing a semi-formal theory of predication and syllogistic reasoning which can be applied to both the trinity and to other subject matter. In this paper we formalize the theory presented in the medieval text, providing the contemporary philosopher and logician with a sound logic for reasoning about the trinity.