Product logic and the deduction theorem

Abstract

In this paper we prove the following negative result: Product Logic [9] does not have the Deduction Theorem, that is, there is no binary defined connective in the language of Product Logic such that the Deduction Theorem is satisfied with respect to it. We prove this theorem mainly by using algebraic methods: we prove that Product Logic is algebraizable, that the variety of Product Algebras is its equivalent quasivariety semantics and that this variety has no equationally definable principal congruences.