Modalities in ecumenical systems

Abstract

The discussion about how to put together Gentzen’s systems for classical and intuitionistic logic in a single unified system is back in fashion. Indeed, recently Prawitz and others have been discussing the so-called Ecumenical Systems, where connectives from these logics can co-exist in peace. In Prawitz’ system, the classical logician and the intuitionistic logician would share the universal quantifier, conjunction, negation, and the constant for the absurd, but they would each have their own existential quantifier, disjunction, and implication, with different meanings. Prawitz’ main idea is that these different meanings are given by a semantical framework that can be accepted by both parties. In a recent work, an Ecumenical sequent calculus and a nested system were presented, and some very interesting proof theoretical properties of the systems were established. In this work the notion of truth in Prawitz Ecumenical Logic will be extended, so to define Ecumenical alethic modalities.