In this paper we address the problem of combining a logic with nonmonotonic modal logic. In particular we study the intuitionistic case. We start from a formal analysis of the notion of intuitionistic consistency via the sequent calculus. The epistemic operator M is interpreted as the consistency operator of intuitionistic logic by introducing intuitionistic stable sets. On the basis of a bimodal structure we also provide a semantics for intuitionistic stable sets.

The main results of the paper are the following: For each monadic prepositional formula which is classically true but not intuitionistically so, there is a continuum of intuitionistic monotone modal logics L such that L+ is inconsistent.There exists a consistent intuitionistic monotone modal logic L such that for any formula of the kind mentioned above the logic L+ is inconsistent.

Kripke-style models with two accessibility relations, one intuitionistic and the other modal, are given for analogues of the modal systemK based on Heyting's prepositional logic. It is shown that these two relations can combine with each other in various ways. Soundness and completeness are proved for systems with only the necessity operator, or only the possibility operator, or both. Embeddings in modal systems with several modal operators, based on classical propositional logic, are also considered. This paper lays the ground for (...) an investigation of intuitionistic analogues of systems stronger thanK. A brief survey is given of the existing literature on intuitionistic modal logic. (shrink)

We proved in [1] that there exist a continuum consistent monotone intuitionistic modal logics which do not admit the law of the excluded middle p ∨ ¬p. Rieger [2] and Nishimura [3] introduced a sequence of formulas ϕ0, ϕ1, . . . , ϕω of one variable p such that for any intuitionistic formula ϕi containing only the variable p there exists a formula ϕi from this sequence equivalent to ϕ in the intuitionistic propositional logic . In [5] V. Tselkov (...) has proved that for each i ≥ 4 there exist at least countably many consistent monotone intuitionistic modal logics which do not admit the formula ϕi. In this paper we strengthen this result by showing that there exist continuum of such logics. We construct an example of monotone intuitionistic modal logic not admitting all formulas ϕi, where i ≥ 4 and i =6 ω. Using similar constructions we prove that there exist at least countably many maximal monotone intuitionistic modal logics. Let us mention that in the classical case Makinson [6] has shown that there are exactly three such logics. (shrink)