Probabilistic semantics for epistemic modals: normality assumptions, conditional epistemic spaces, and the strength of `must' and `might'

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Abstract
The epistemic modal auxiliaries 'must' and 'might' are vehicles for expressing the force with which a proposition follows from some body of evidence or information. Standard approaches model these operators using quantificational modal logic, but probabilistic approaches are becoming increasingly influential. According to a traditional view, 'must' is a maximally strong epistemic operator and 'might' is a bare possibility one. A competing account---popular amongst proponents of a probabilisitic turn---says that, given a body of evidence, 'must p' entails that Pr(p) is high but non-maximal and 'might p' that Pr(p) is significantly greater than 0. Drawing on several observations concerning the behavior of 'must', 'might' and similar epistemic operators in evidential contexts, deductive inferences, downplaying and retractions scenarios, and expressions of epistemic tension, I argue that those two influential accounts have systematic descriptive shortcomings. To better make sense of their complex behavior, I propose instead a broadly Kratzerian account according to which 'must p' entails that Pr(p) = 1 and 'might p' that Pr(p) > 0, given a body of evidence and a set of normality assumptions about the world. From this perspective, 'must' and 'might' are vehicles for expressing a common mode of reasoning whereby we draw inferences from specific bits of evidence against a rich set of background assumptions---some of which we represent as defeasible---which capture our general expectations about the world. I will show that the predictions of this Kratzerian account can be substantially refined once it is combined with a specific yet independently motivated `grammatical' approach to the computation of scalar implicatures. Finally, I discuss some implications of these results for more general discussions concerning the empirical and theoretical motivation to adopt a probabilisitic semantic framework.
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Archival date: 2021-07-13
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