Abstract: This chapter articulates the requirements of fallible a priori justification. It distinguishes two senses of fallibility: c-fallibility, justification that does not guarantee truth; and p-fallibility, justification that is defeasible. It argues that although these senses are logically independent of one another, there are some significant relations between them mediated by the concepts of self-revision, overriding defeater, and undermining defeater. It is shown that several alternative fallibilist accounts of a priori justification face difficulties that are avoided by the account defended in Chapter 2.

Keywords: a priori, c-fallibility, defeater, fallible, fallibilist, justification, p-fallibility, overriding, self-revision, undermining,