AGM Revision Research Articles

Three Scenarios for the Revision of Epistemic States

This position paper discusses the difficulty of interpreting the iterated belief revision problem. Axioms of iterated belief revision are often presented as extensions of the AGM axioms, upon receiving a sequence of inputs, likely to alter not only the belief set, but also the epistemic entrenchment relation underlying the revision operator. Iterated belief revision presupposes that more recent inputs have priority over less recent ones. We argue that this view of iterated revision is at odds with the suggestion of Gardenfors and Makinson, that belief revision and non-monotonic reasoning are two sides of the same coin. It is not clear that non-monotonic reasoning modifies the ranking of possible worlds implicit in default rules. We lay bare three different paradigms of revision based on specific interpretations of the epistemic entrenchment implicitly at work and of the input information. If the epistemic entrenchment stems from default rules and the input is a specific piece of evidence, then AGM revision is a matter of changing plausible conclusions, and iterated revision makes no sense. However, if the epistemic entrenchment encodes uncertain factual evidence and the input information as well, then iterated revision reduces to prioritized merging. A third problem where iteration makes sense corresponds to the revision, by the addition of new default rules, of a conditional knowledge base describing background information. The three scenarios are compared with similar problems in the framework of probabilistic reasoning.

Credibility-limited Functions for Belief Bases

This thesis consists in six articles and a comprehensive summary. • The pourpose of the summary is to introduce the AGM theory of belief change and to exemplify the diversity and significance of the research that has been inspired by the AGM article in the last 25 years. The research areas associated with AGM was divided in three parts: criticisms, where we discussed some of the more common criticisms of AGM. Extensions where the most common extensions and variations of AGM are presented and applications where we provided an overview of applications and connections with other areas of research. • Article I elaborates on the connection between partial meet contractions [AGM85] and kernel contractions [Han94a] in belief change theory. Also both functions are equivalent in belief sets, there are notequivalent in belief bases. A way to define incision functions (used in kernel contractions) from selection functions (used in partial meet contractions) and vice versa is presented. It is explained under which conditions there are exact correspondences between selection and incision functions so that the same contraction operations can be obtained by using either of them. • Article II proposes an axiomatic characterization for ensconcement-based contraction functions, belief base functions proposed byWilliams and relates this function with other kinds of base contraction functions. • Article III adapts the Ferme and Hansson model of Shielded Contraction [FH01] as well as Hansson et all Credibility-Limited Revision [HFCF01] for belief bases, to join two of the many variations of the AGM model [AGM85], i.e. those in which knowledge is represented through belief bases instead of logic theories, and those in which the object of the epistemic change does not get the priority over the existing information as it is the case in the AGM model. • Article IV introduces revision by comparison a refined method for changing beliefs by specifying constraints on the relative plausibility of propositions. Like the earlier belief revision models, the method proposed is a qualitative one, in the sense that no numbers are needed in order to specify the posterior plausibility of the new information. The method uses reference beliefs in order to determine the degree of entrenchment of the newly accepted piece of information. Two kinds of semantics for this idea are proposed and a logical characterization of the new model is given. • Article V focuses on the extension of AGM that allows change for a belief base by a set of sentences instead of a single sentence. In [FH94], Fuhrmann and Hansson presented an axiomatic for Multiple Contraction and a construction based on the AGM Partial Meet Contraction. This essay proposes for their model another way to construct functions: Multiple Kernel Contraction, that is a modification of Kernel Contraction,proposed by Hansson [Han94a] to construct classical AGM contractions and belief base contractions. • Article VI relates AGM model with the DFT model proposed by Carlos Alchourron [Alc93]. Alchourron devoted his last years to the analysis of the notion of defeasible conditionalization. His definition of the defeasible conditional is given in terms of strict implication operator and a modal operator f which is interpreted as a revision function at the language level. This essay points out that this underlying revision function is more general than AGM revision. In addition, a complete characterization of that more general kind of revision that permits to unify models of revision given by other authors is given.

Belief Fusion: Aggregating Pedigreed Belief States

We introduce a new operator – belief fusion – which aggregates the beliefs of two agents, each informed by a subset of sources ranked by reliability. In the process we define pedigreed belief states, which enrich standard belief states with the source of each piece of information. We note that the fusion operator satisfies the invariants of idempotence, associativity, and commutativity. As a result, it can be iterated without difficulty. We also define belief diffusion; whereas fusion generally produces a belief state with more information than is possessed by either of its two arguments, diffusion produces a state with less information. Fusion and diffusion are symmetric operators, and together define a distributive lattice. Finally, we show that AGM revision can be viewed as fusion between a novice and an expert.

Irrevocable belief revision and epistemic entrenchment

In recent papers [10, 11] Krister Segerberg introduced Irrevocable Belief Revision, as closely related to AGM revision [2]. In this paper we present irrevocable belief revision in terms of an epistemic entrenchment relation. 1

Distance based revision of preferential logics

We first analyze AGM revision as conditions on choice functions for sets of models. This abstraction seems to us to capture the essentials of classical revision, it also immediately reveals the connection between revision and ranked preferential models, and gives further insight into the distance semantics for revision as developed by Lehmann, Magidor, and Schlechta. Our analysis shows how to apply the essential ideas of revision to other situations than classical theories and formulas, we exemplify this by examining preferential databases. We revise one preferential logic or database, |∼ with another one, |∼'. The basic idea is to describe such a logic as a partial order, either as the order of a preferential model which defines the logic, or as the order between formulas defined by the logic. A partial order can be seen as the set of total orders which extend it, and, given a distance on the set of total orders, we can define a revision as follows: |∼* |∼' will be the logic corresponding to the partial order generated by those total orders extending (the order of) |∼', which are closest to the set of total orders extending (the order of) |∼. We thus give a semantical approach to the problem. A representation result is proven.

Symmetry and Belief Revision

This paper continues the recent tradition of investigating iterated AGM revision by reasoning directly about the dynamics for total pre-order (“implausibility ordering”) representations of AGM revision functions. We reorient discussion, however, by proving that symmetry considerations, almost by themselves, suffice to determine a particular, AGM-friendly implausibility ordering dynamics due to Spohn 1988, which we call “J-revision”. After exploring the connections between implausibility ordering dynamics and the social choice theory of Arrow 1963, we provide symmetry arguments in the social choice-theoretic framework for an interesting generalization of J-revision due to Nayak 1994. We conclude by arguing that the symmetry principles that uniquely favor J-revision and its generalizations are importantly expressive of the purely qualitative framework for representing beliefs that distinguishes the AGM program. Our results therefore comprehensively vindicate Spohn's 1988 conjecture that essentially J-revision is the best that can be done by way of a purely qualitative model of belief revision.