Semantic results for ontic and epistemic change

TL;DR

This paper explores semantic properties of an epistemic logic with dynamic operators, demonstrating how information and world changes can be modeled and related through semantic results involving bisimilarity and event sequences.

Contribution

It provides three new semantic results connecting epistemic and ontic changes within a unified framework using pointed Kripke models.

Findings

Any two information states can be connected by an event.

Events with simple postconditions are bisimilar to more complex ones.

Complex events can be decomposed into sequences of simpler events.

Abstract

We give some semantic results for an epistemic logic incorporating dynamic operators to describe information changing events. Such events include epistemic changes, where agents become more informed about the non-changing state of the world, and ontic changes, wherein the world changes. The events are executed in information states that are modeled as pointed Kripke models. Our contribution consists of three semantic results. (i) Given two information states, there is an event transforming one into the other. The linguistic correspondent to this is that every consistent formula can be made true in every information state by the execution of an event. (ii) A more technical result is that: every event corresponds to an event in which the postconditions formalizing ontic change are assignments to `true' and `false' only (instead of assignments to arbitrary formulas in the logical language). `Corresponds' means that execution of either event in a given information state results in bisimilar information states. (iii) The third, also technical, result is that every event corresponds to a sequence of events wherein all postconditions are assignments of a single atom only (instead of simultaneous assignments of more than one atom).