In On Certainty, Wittgenstein conceives a novel way of dispelling skeptical doubts about our knowledge of the external world. He acknowledges that in his attempt to refute the skeptic, Moore uncovered epistemologically relevant propositions such as ‘I know that this is a hand’. But he denies that appealing to such truisms is likely to succeed in refuting skepticism–not because they cannot be doubted, but because they are not objects of knowledge in the first place. Rather than refuting skepticism about the external world, Moore’s truisms form the background against which claims to knowledge and doubt are reasonable, meaningful, or justified. By incorporating such conditions for doubt, Wittgenstein seems to provide an effective rationale against skepticism: not only do we lack reasons for certain doubts about our knowledge of the external world, but those doubts also presuppose that much of what we take to know is exempt from them. This paper critically examines the claim that doubt requires reasons and uses Descartes’ famous dream argument as an example to show that not every doubt can be easily dispelled by invoking it.