This paper begins by applying a version of Timothy Williamson’s anti-luminosity argument to normative properties. This argument suggests that there must be at least some unknowable normative facts in normative Sorites sequences, or otherwise we get a contradiction given certain plausible assumptions concerning safety requirements on knowledge and our doxastic dispositions. This paper then focuses on the question of how the defenders of different forms of metaethical anti-realism (namely, error theorists, subjectivists, relativists, contextualists, expressivists, response dependence theorists, and constructivists) could (...) respond to the explanatory challenge created by the previous argument. It argues that, with two exceptions, the metaethical anti-realists need not challenge the argument itself, but rather they can find ways to explain how the unknowable normative facts can obtain. These explanations are based on the idea that our own attitudes on which the normative facts are grounded need not be transparent to us either. Reaching this conclusion also illuminates how metaethical anti-realists can make sense of instances of normative vagueness more generally. (shrink)