The traditional epistemological approach towards judgments like BACHELORS ARE UNMARRIED or ALL KNOWLEDGE IS TRUE is that they are justified or known on the basis of understanding alone. In this paper, I develop an understanding-based account which takes understanding to be a sufficient source of epistemic justification for the relevant judgments. Understanding-based accounts face the problem of the rational revisability of almost all human judgments. Williamson has recently developed a reinforced version of this problem: the challenge from expert revisability. This (...) is the problem that even the best candidate judgments for understanding-based justification can be rationally rejected by the relevant experts, who may not even have any disposition or inclination to accept these judgments. However, I argue that expert revisability is fully compatible with the proposed understanding-based epistemology, because expert revisability is true of sufficient sources of epistemic justification in general. A remaining metaphysical worry is that understanding might end up being ‘too thin’ to play the envisaged epistemological role. This worry can be countered with a novel metaphysics of understanding involving second-order dispositions. (shrink)