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deontic logic by incorporating a nonmonotonic reasoning mechanism that enables previously derived obligations to be withdrawn when new, potentially conflicting information comes in.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.

翻译：针对Horty提出的关切，本文发展了一种基于两层偏好的语义框架来建模可废止的条件义务。该框架扩展了用于二元道义逻辑的汉森-刘易斯式偏好语义，通过纳入非单调推理机制，使得在新出现的可能冲突信息传入时能够撤销先前推导出的义务。该方案具有双偏好特征：通过使用世界上的两种排序——理想性与正常性——分别采用独立的排序方法来弥补早期方法的不足。在非单调层面，本文考量了包括前件强化、包含原则和无灾难原则在内的一系列公设。此外，本文还建立了与所谓约束型输入/输出逻辑（一种基于不同方法论的标准规范性推理框架）之间的关联。