On the intuitionistic force of classical search

Abstract

The combinatorics of classical propositional logic lies at the heart of both local and global methods of proof-search enabling the achievement of least-commitment search. Extension of such methods to the predicate calculus, or to non-classical systems, presents us with the problem of recovering this least-commitment principle in the context of non-invertible rules. One successful approach is to view the non-classical logic as a perturbation on search in classical logic and characterize when a least-commitment (classical) search yields sufficient evidence for provability in the (non-classical) logic. This technique has been successfully applied to both local and global methods at the cost of subsidiary searches and is the analogue of the standard treatment of quantifiers via skolemization and unification. In this paper, we take a type-theoretic view of this approach for the case in which the non-classical logic is intuitionistic. We develop a system of realizers (proof-objects) for sequents in classical propositional logic (the types) by extending Parigot's λμ-calculus, a system of realizers for classical free deduction ( cf. natural deduction). Our treatment of disjunction exploits directly the multiple-conclusioned form of LK as opposed to the single-conclusioned form of LJ. Consequently, it requires the addition of another binding operator, called ν, to λμ. This choice is motivated by our concern to reflect the properties of classical proof-search in the system of realizers. Using this framework, we illustrate the sense in which intuitionistic search can be viewed as a perturbation on classical search. As an application, we develop a proof procedure based on the natural extension of the notion of uniform proof to the multiple-conclusioned classical sequent calculus Harrop fragment of intuitionistic logic. This paper develops the proof-theoretic aspects of the approach.