A regress or vicious circularity supposed to infect attempts to answer scepticism. Suppose a sceptic to raise a doubt about a claim that something is known. Any answer citing, for instance, an observation, needs to suppose that the observation meets some criterion for knowledge. But then how do we know that this is the right criterion? The question is raised in Sextus Empiricus, and was the subject of a 1973 book The Problem of the Criterion by Roderick Chisholm.