Defeasible Conditional Obligation in a Two-tiered Preference-based Semantics (Extended Version)

2026-04-28

AI Summary

This study addresses the challenge of modeling defeasible conditional obligations in dynamic informational environments, particularly the inability of existing semantic frameworks to retract previously derived obligations in the presence of conflicting information. To overcome this limitation, the paper proposes a two-layered preferential semantic framework that distinguishes between ideality-based and normality-based world orderings. By integrating a Hansson-Lewis style dyadic deontic logic with non-monotonic reasoning mechanisms, the framework satisfies key metatheoretic principles such as immunity to strengthening of the antecedent, inclusion, and avoidance of drowning. This approach not only enables dynamic adjustment and principled retraction of defeasible obligations but also establishes a formal connection with constraint-based input/output logics, thereby overcoming significant shortcomings of traditional models.

Abstract

In response to a concern raised by Horty, this paper develops a two-tiered, preference-based semantic framework for modeling defeasible conditional obligations. The paper extends a Hansson-Lewis style preference semantics for dyadic deontic logic by incorporating a nonmonotonic reasoning mechanism that enables previously derived obligations to be withdrawn when new, potentially conflicting information comes in. The account is bi-preferential: two orderings--ideality and normality--on worlds are employed to address shortcomings in earlier approaches, with a separate ranking method for each. At the nonmonotonic layer, a number of postulates are considered, including antecedent strengthening, inclusion and no-drowning. A connection is established with so-called constrained input/output (I/O) logic--an existing standard for normative reasoning based on a different methodology.

Problem

• defeasible conditional obligation
• preference-based semantics
• nonmonotonic reasoning
• dyadic deontic logic
• normative reasoning

Innovation

• defeasible conditional obligation
• preference-based semantics
• nonmonotonic reasoning
• bi-preferential ordering
• input/output logic