Unjustified Defeaters and the Role of Incoherence in Epistemic Defeat

Abstract This paper examines the possibility of unjustified defeaters and the explanatory role of incoherence in epistemic defeat. I reconstruct Peter Graham and Jack Lyons’s objections to Jennifer Lackey’s psychological defeaters, paying special attention to their rejection of unjustified defeaters and to their claim that incoherence plays no indispensable explanatory role in defeat. First, I argue that denying the possibility of unjustified defeaters leads to implausible verdicts about rationality in cases involving incoherent belief sets. Second, I argue that incoherence is essential for explaining the epistemic damage generated by such defeaters, even when a subject’s total evidence remains unchanged. Finally, I examine different ways of accommodating this phenomenon and argue that, regardless of whether it is understood in terms of full loss of justification, structural irrationality, or some combination of the two, incoherence remains an indispensable component of its explanation. I conclude that, although evidentialist accounts plausibly explain many ordinary cases of defeat, they cannot fully account for defeat involving unjustified defeaters, where incoherence itself becomes the source of the epistemic downgrade.