The Role of Perspectives in the Interpretation of Defeasible Reasoning

Non-monotonicity in logic is a symptom that may have many causes. In the formalisation of defeasible reasoning, an epistemic diagnosis has largely prevailed according to which some inferences are non-monotonic because they are provisionally drawn in the absence of relevant or complete information. The Gabbay-Makinson rules for cumulative consequence relations are a paradigmatic example of this epistemic approach. In this paper a different approach to defeasible reasoning is introduced, based on the idea of inferential perspectives. According to this approach, some inferences are non-monotonic because they are drawn as from another reasoner’s perspective. Rules are introduced and discussed for a Perspectival Calculus, which show both similarities and interesting differences with respect to cumulative systems.