Vagueness and Uncertainty

Abstract

Recent forms of expressivism attempt to explain the sense in which certain propositions are ‘non-factual’ in terms of principles about attitudes towards those propositions. Following recent expressivist accounts of conditionals and modals, a version of expressivism about vagueness is explored, which maintains that to have a credence in a vague proposition is just to have your credences in the precise propositions distributed in a certain way. Whilst this form of expressivism is ultimately rejected, a consequence of the view can be exploited to partially capture the intuition that certain subject matters are non-factual. This principle, Rational Supervenience’, effectively states that all disagreements about the vague ultimately boil down to disagreements about the precise: any two rational priors that agree about all precise propositions agree about everything. While the Principle of Plenitude states that there is a proposition occupying every evidential role, Rational Supervenience entails conversely that every proposition occupies some evidential role.