Rigidity, symmetry and defeasibility: On Weisberg's puzzle for perceptual justification

Joanthan Weisberg has presented a puzzle for Bayesian epistemologists (Weisberg, 2009, 2015). The puzzle highlights the inability to model a specific kind of defeasibility of evidence in a Bayesian framework. But the puzzle is of much wider interest: it arises not just for Bayesians, and not just for formal modelers, but for any theory which adopts three very plausible principles: the rigidity, symmetry, and defeasibility of evidence. In this paper I first presentWeisberg’s puzzle as originally developed, in the context of Bayesian epistemology. Then I show how the problem arises for normal epistemology aswell. I offer a solution for the normal puzzle and then carry over that solution to the formal side. I end up by suggesting that taking the puzzle seriously means facing up to the possibility that we cannot give an informative, general characterization of epistemic justification, and that we should instead rest content with a kind of particularism.