Semantic Incompleteness of Liberman et al. (2020)'s Hilbert-style System for Term-modal Logic K with Equality and Non-rigid Terms

TL;DR

This paper demonstrates that the Hilbert-style system for a specific term-modal logic with equality and non-rigid terms is semantically incomplete, using a novel Kripke semantics to show certain valid formulas are not derivable.

Contribution

It proves the semantic incompleteness of Liberman et al.'s Hilbert-style system for minimal normal term-modal logic with equality and non-rigid terms, introducing a new Kripke semantics.

Findings

Certain valid formulas are not derivable in the system

A non-standard Kripke semantics is introduced

Semantic incompleteness is established

Abstract

In this paper, we prove the semantic incompleteness of the Hilbert-style system for the minimal normal term-modal logic with equality and non-rigid terms that was proposed in Liberman et al. (2020) "Dynamic Term-modal Logics for First-order Epistemic Planning." Term-modal logic is a family of first-order modal logics having term-modal operators indexed with terms in the first-order language. While some first-order formula is valid over the class of all frames in the Kripke semantics for the term-modal logic proposed there, it is not derivable in Liberman et al. (2020)'s Hilbert-style system. We show this fact by introducing a non-standard Kripke semantics which makes the meanings of constants and function symbols relative to the meanings of relation symbols combined with them.