Inquisitive Neighborhood Logic

TL;DR

This paper introduces an inquisitive modal logic for neighborhood models, featuring a new strict conditional operator and unary modalities, with proven expressive equivalence to bisimilarity and a complete axiomatization.

Contribution

It develops a novel inquisitive modal logic with a strict conditional operator and unary modalities, expanding the expressive and axiomatic framework for neighborhood models.

Findings

Expressive power matches bisimilarity in neighborhood models

Certain language fragments are invariant under neighborhood modifications

Conditional modality is not definable from unary modalities

Abstract

We explore an inquisitive modal logic designed to reason about neighborhood models. This logic is based on an inquisitive strict conditional operator, which quantifies over neighborhoods, and which can be applied to both statements and questions. In terms of this operator we also define two unary modalities that function respectively as a universal and existential quantifier over neighborhoods. We prove that the expressive power of this logic matches the natural notion of bisimilarity in neighborhood models. We show that certain fragments of the language are invariant under certain modifications of the set of neighborhoods, and use this to show that our conditional modality is not definable from the induced unary modalities, and that questions embedded on the right of this conditional are indispensable. We provide a sound and complete axiomatization of our logic, both in general and in restriction to some salient frame classes, and discuss the relations between our logic and other modal logics interpreted over neighborhood models.