 Democritus’ student Metrodorus of Chios says at the beginning of his book On Nature “None of us knows anything, not even this, whether we know or we do not know; nor do we know what ‘to not know’ or ‘to know’ are, nor on the whole, whether anything is or is not” (Cicero, Acad. 2.73; trans. Lee (2010) = DK 70B1; SE M 7.48, 87–8; Eusebius, Praep. evang. 14.19.9). This formulation reflects awareness of the fact that a simpler statement than the one reported in Cicero, such as “there is no knowledge,” can be turned against itself. In particular, Metrodorus recognizes the role that understanding concepts plays in any such statement. Does its proponent know something, merely by virtue of understanding what the terms she uses in her philosophizing refer to? Sextus presents Metrodorus’ pronouncement as related to an enigmatic idea that he ascribes to two other philosophers, the Democritean Anaxarchus of Abdera and the Cynic Monismus (SE M 7.87–88). Both are said to have likened existing things to a stage-painting. This comparison, which Burnyeat captures in the catch-phrase “all the world’s a stage-painting,” is open to a range of interpretations (Burnyeat 2017). For Monismus, Burnyeat argues, it is likely to have had a moralistic upshot, along the lines of “all is vanity.” In Democritean philosophy, and insofar as later skepticism recalls the saying, it is bound to be a proposal in epistemology (or possibly epistemology and metaphysics).