Normal Natural Deduction Proofs (in Non-classical Logics)

Abstract

We provide a theoretical framework that allows the direct search for natural deduction proofs in some non-classical logics, namely, intuitionistic sentential and predicate logic, but also in the modal logic S4. The framework uses so-called intercalation calculi to build up broad search spaces from which normal proofs can be extracted, if a proof exists at all. This claim is supported by completeness proofs establishing in a purely semantic way normal form theorems for the above logics. Logical restrictions on the search spaces are briefly discussed in the last section together with some heuristics for structuring a more efficient search. Our paper is a companion piece to [15], where classical logic was treated.