According to John McDowell’s version of disjunctivism, a perceptual experience has both a property that it shares with a subjectively indistinguishable illusory experience as well as a property that it does not share with a subjectively indistinguishable illusory experience. McDowell is also an infallibilist about justification; accordingly, he holds that a perceptual experience justifies a belief in virtue of the latter property. In this paper, I defend McDowell against an argument that purports to show that perceptual experiences justify beliefs only (...) in virtue of the former property, the property that they share with illusory experiences. The argument is a version of Michael Huemer’s self-defeat argument for phenomenal conservatism; in Sect. 2 I show how the argument seemingly applies to show that McDowell’s infallibilism is false. I respond on behalf of McDowell to Huemer’s argument by developing McDowell’s idea of knowledge as cognitive purchase on a fact: I explain both why this idea requires infallibilism about justification and how this idea allows a response to Huemer’s argument. (shrink)