A General Framework of Epistemic Forgetting and its Instantiation by Ranking Functions

TL;DR

This paper develops a unified framework for epistemic forgetting, extending existing syntactic and semantic methods to richer epistemic states, and evaluates multiple concrete forgetting operations within this framework.

Contribution

It introduces five general types of epistemic forgetting and instantiates them with seven concrete operations for ranking functions, providing a comprehensive axiomatic evaluation.

Findings

Seven concrete forgetting operations evaluated against multiple postulates.

Identifies key differences and similarities among various forgetting operators.

Provides a unified, axiomatic framework for epistemic forgetting in richer semantic structures.

Abstract

Forgetting as a knowledge management operation deliberately ignores parts of the knowledge and beliefs of an agent, for various reasons. Forgetting has many facets, one may want to forget parts of the syntax, a proposition, or a conditional. In the literature, two main operators suitable for performing forgetting have been proposed and investigated in depth: First, variable elimination is a syntactical method that blends out certain atomic variables to focus on the rest of the language. It has been mainly used in the area of logic programming and answer set programming. Second, contraction in AGM belief revision theory effectively removes propositions from belief sets under logical deduction. Both operations rely mainly on classical logics. In this article, we take an epistemic perspective and study forgetting operations in epistemic states with richer semantic structures, but with clear links to propositional logic. This allows us to investigate what forgetting in the epistemic background means, thereby lifting well-known and novel forgetting operations to the epistemic level. We present five general types of epistemic forgetting and instantiate them with seven concrete forgetting operations for Spohn's ranking functions. We take inspiration from postulates of forgetting both from logic programming and AGM theory to propose a rich landscape of axioms for evaluating forgetting operations. Finally, we evaluate all concrete forgetting operations according to all postulates, leading to a novel comprehensive overview highlighting differences and commonalities among the forgetting operators.