Four-valued logics of truth, non-falsity, exact truth, and material equivalence

TL;DR

This paper explores and axiomatizes various combinations of four-valued logics based on truth, non-falsity, exact truth, and material equivalence, extending Belnap--Dunn logic to more expressive systems.

Contribution

It introduces axiomatizations for all combinations of four-valued logics, enabling complex implications involving multiple propositional features.

Findings

Axiomatization of combined four-valued logics

Formalization of implications involving multiple features

Extension of Belnap--Dunn logic to new logical systems

Abstract

The four-valued semantics of Belnap--Dunn logic, consisting of the truth values True, False, Neither, and Both, gives rise to several non-classical logics depending on which feature of propositions we wish to preserve: truth, non-falsity, or exact truth (truth and non-falsity). Interpreting equality of truth values in this semantics as material equivalence of propositions, we can moreover see the equational consequence relation of this four-element algebra as a logic of material equivalence. In this paper we axiomatize all combinations of these four-valued logics, for example the logic of truth and exact truth or the logic of truth and material equivalence. These combined systems are consequence relations which allow us to express implications involving more than one of these features of propositions.