Intuitionistic and Constructive Modal Logics for Classical Modal Logicians

Abstract

This paper gives a survey of propositional modal logic in an intuitionistic setting. Mainly intended for readers already familiar with classical modal logic, we discuss why there are two prominent families of such logics, intuitionistic modal logics (IMLs) and constructive modal logics (CMLs), what motivates their study, and how they compare with each other. Moreover, we take a deeper look at the logics lying between the CML and IML traditions, including a new logic $\textbf{KI}$ for which we prove completeness, and survey various extensions and $\Diamond$/$\Box$-free fragments. We further record several observations which seem to be implicit or absent in the literature; we point out a small defect in the original semantics for $\textbf{CK}$ and prove Lyndon interpolation theorems for $\textbf{CK}$ and $\textbf{WK}$.