Williamson (2000a) has argued that positive introspection is incompatible with inexact knowledge. His argument relies on a margin-for-error requirement for inexact knowledge based on a intuitive safety principle for knowledge, but leads to the counterintuitive conclusion that no possible creature could have both inexact knowledge and positive introspection. Following Halpern (2004) I put forward an alternative margin-for-error requirement that preserves the safety requirement while blocking Williamson’s argument. I argue that the infallibilist conception of knowledge that underlies the new requirement provides a better account of inexact knowledge and higher-order knowledge than both Williamson’s and Halpern’s. The positive introspection axiom (or principle KK) states that if one knows, one knows that one knows (Hintikka, 1962). It is routinely used in formal epistemology, at least as a reasonable idealisation. Inexact knowledge is knowledge that fails to distinguish close possi...