Defeasibility in Epistemology

This dissertation explores some ways in which logics for defeasible reasoning can be applied to questions in epistemology. It's naturally thought of as developing four applications: The first is concerned with simple epistemic rules, such as ``If you perceives that X, then you ought to believe that X'' and ``If you have outstanding testimony that X, then you ought to believe that X.'' Anyone who thinks that such rules have a place in our accounts of epistemic normativity must explain what happens in cases where they come into conflict —such as one where you perceive a red object and are told that it is blue. The literature has gone in two directions: The first suggests that rules have built-in unless-clauses specifying the circumstances under which they fail to apply; the second that rules do not specify what attitudes you ought to have, but only what counts in favor or against having those attitudes. I express these two different ideas in a defeasible logic framework and demonstrate that there's a clear sense in which they are equivalent. The second application uses a defeasible logic to solve an important puzzle about epistemic rationality, involving higher-order evidence, or, roughly, evidence about our capacities for evaluating evidence. My solution has some affinities with a certain popular view on epistemic dilemmas. The third application, then, is a characterization of this conflicting-ideals view in logical terms: I suggest that it should be thought of as an unconventional metaepistemological view, according to which epistemic requirements are not exceptionless, but defeasible and governed by a comparatively weak logic. Finally, the fourth application is in the burgeoning debate about the epistemic significance of disagreement. The intuitive conciliatory views say, roughly, that you ought to become less confident in your take on some question X, if you learn that an epistemic equal disagrees with you about X. I propose to think of conciliationism as a defeasible reasoning policy, develop a mathematically precise model of it, and use it to solve one of the most pressing problems for conciliatory views: Given that there are disagreements about these views themselves, they can self-defeat and issue inconsistent recommendations