Abstract

The concept of epistemic defeat or defeasibility has come to occupy an important place in contemporary epistemology, especially in relation to the closely allied concepts of justified belief, warrant, and knowledge. These allied concepts signify positive epistemic appraisal or positive epistemic status. As a first approximation, defeasibility refers to a belief’s liability to lose some positive epistemic status, or to having this status downgraded in some particular way. For example, a person may be epistemically justified in believing some proposition p at one time, but then the belief might become less justified or even unjustified at some later time. Moreover, beliefs may also be prevented from having or acquiring some positive epistemic status in the first place. So more generally, defeasibility refers to a kind of epistemic liability or vulnerability, the potential of loss, reduction, or prevention of some positive epistemic status. A defeater is, broadly speaking, a condition that actualizes this potential. This article begins by outlining two general types of defeaters: propositional defeaters and mental state defeaters. Propositional defeaters are conditions external to the perspective of the cognizer that prevent an overall justified true belief from counting as knowledge. Mental state defeaters are conditions internal to the perspective of the cognizer (such as experiences, beliefs, withholdings) that cancel, reduce, or even prevent justification.