Abstract
Since the early 1980s, logical theories of belief revision have offered formal methods for the transformation of knowledge bases or “corpora” of data and beliefs. Early models have dealt with unconditional acceptance and integration of potentially belief-contravening pieces of information into the existing corpus. More recently, models of “non-prioritized” revision were proposed that allow the agent rationally to refuse to accept the new information. This paper introduces 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. We use reference beliefs in order to determine the degree of entrenchment of the newly accepted piece of information. We provide two kinds of semantics for this idea, give a logical characterization of the new model, study its relation with other operations of belief revision and contraction, and discuss its intuitive strengths and weaknesses.
Highlights
Since the early 1980s, logical theories of belief revision have offered formal methods for the transformation of knowledge bases or “corpora” of data and beliefs
The basic idea of this paper is to model degrees of acceptance by what is known in the literature as epistemic entrenchment [13,14,42]
A detailed study and self-contained axiomatizations of the three special cases of severe withdrawal, irrevocable belief revision and irrefutable belief revision is deferred to a companion paper to the present one [43]
Summary
Belief change theories have been facing a dilemma. In qualitative theories of the AGM variety [1,13,16] agents accept new incoming information without further. In contrast to previous studies in belief change of the AGM variety, we will study a simple model of implementing revision-by-comparison in terms of a function K ◦α β that takes two sentences α and β as arguments. We have mentioned that the reference sentence is usually supposed to be sufficiently highly entrenched as compared to the (negation of the) input sentence This is the paradigm case of application of revision-by-comparison, and in this case our operation will behave like an ordinary kind of AGM revision by the input sentence. The reference sentence is too weakly entrenched relative to the (negation of the) input sentence, the attempted revision will fail and end up with a contraction of the belief set, more precisely a severe withdrawal of the reference sentence [34] This is the first limiting case for which our operation reduces to an operation that is well known from previous literature. A detailed study and self-contained axiomatizations of the three special cases of severe withdrawal, irrevocable belief revision and irrefutable belief revision is deferred to a companion paper to the present one [43]
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