On the formal character of plausible reasoning

by Alain A. Lewis

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What we mean when we say that an event is plausible when we are referring to a possible state of affairs seems somehow to be more than just the event's probability of occurrence, albeit subjectively defined. When we say that an event is plausible, we usually mean that its plausibility follows "in light of certain facts." We are thus led to a certain tentative or provisional state of mind with respect to the occurrence or nonoccurrence of events other than the one in question. It is thus permissible to regard plausibility as a relation between events (or sets of events). It often occurs that this relation, the interpretation of which is embedded in the meaning given to "in light of certain facts," is nonlogical in the sense that it is not entirely captured as an expression in ordinary propositional logic. The paper gives an approach to modeling plausible reasoning that takes this last point into account.

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