In Defense of Conditional Uniqueness

Abstract

The Uniqueness Thesis (U), defended by the uniquer and rejected by the permissivist, asserts that, necessarily, there is at most one rational doxastic attitude one can take towards a proposition, given a particular body of evidence. U faces a well-known, paralyzing objection from the permissivist, which I call “the simplicity objection,” which rests on the idea that evidence is not the sole determinant of rationality. In this paper, after maintaining that the ongoing dialectic between the uniquer and the permissivist has led to an exaggeration of differences, I bring into focus another, non-equivalent yet substantive (non-trivial) thesis in the vicinity, which I call “the Conditional Uniqueness Thesis” (U*), according to which if evidence is the sole determinant of rationality, then U is true. The hope is to achieve a rapprochement between the uniquer and the permissivist by showing that U* is true. To this end, I examine the argument Roger White offers in favor of U, which I call “the argument from evidential support” (AES), and argue that it is both unpersuasive for the defender of the simplicity objection and unnecessarily strong for establishing its own conclusion. I then offer a sufficiently weakened version of AES, which I call AES*, and argue that AES* is sound, if interpreted as an argument for U*.