McDowell has argued that external world scepticism is a pressing problem only in so far as we accept, on the basis of the argument from illusion, the claim that perceiving that p and hallucinating that p involve a highest common factor--something which functions, in the manner of the classical 'veil of ideas', as a perceptual intermediary. McDowell traces the power of this argument to disputable Cartesian assumptions about the transparency of subjectivity to itself. I argue, contra McDowell, that the reflections to be found in, paradigmatically, Descartes's First Meditation are better interpreted as offering a causal argument for scepticism that depends upon a naturalistic conception of sense experience. This is more powerful than the argument from illusion, since it requires no commitment to a highest common factor in perception, nor to the transparency of the mental. The availability of this alternative route to scepticism raises serious problems for McDowell's quietism, which aims to earn the right to avoid, rather than answer, the sceptic. Since the appeal to externalism about content cannot settle the matter, I conclude that there is, at present, an unsatisfactory stand-off between the sceptic and McDowell's position.