Thomas Aquinas, Non-Normal Modal Logics and the Problem of Future Contingent Events

Thomas Aquinas maintained that God foreknows future contingent events and that his foreknowledge does not entail that they are necessarily the case. More specifically, he stated that if God knows a future contingent event, this future contingent event will be necessarily the case de sensu composito, but not de sensu diviso. After emphasizing the unified nature of Aquinas' notion of necessity, we propose an interpretation of his theses by restating them within the framework of non-normal modal logics. In this framework, the K-axiom does not hold, i.e. the necessity operator does not distribute over the material implication. Moreover, assuming that Aquinas rejected the K-axiom is not only consistent, but also leads to a logical framework that allows us to understand other theses maintained by the Doctor Angelicus. In particular, we argue that Aquinas' remarks on the principle of non contradiction rest on an impossible worlds semantics for non-normal modal logics.