The regress argument against Cartesian skepticism

Does the external world exist, as it so concretely seems? Perhaps not, says the Cartesian skeptic. After all, if you were dreaming or hallucinating, if an evil demon were bent on deceiving you, or if, god forbid, you were a brain in a vat, things might appear to you in just the same way. Why think, then, that you are experiencing good old reality rather than some dream or other fiction? Absent any way of distinguishing these cases, the reasonable thing to do is to cast a skeptical eye on your usual beliefs: you should doubt, you should have reservations about, whether the external world exists. Descartes, of course, thought that there was a way of distinguishing the cases, that proceeded circuitously through the fact that (though he could doubt whether the external world existed) he could not doubt whether he existed; such knowledge of the Cartesian ego, in combination with a supposedly undeceiving deus ex machina – revealed by the light of pure reason – served, Descartes thought, to get back the world. Descartes’s answer is unsatisfyingly ad hoc, whatever one’s theistic inclinations. Nor have recent answers to the Cartesian skeptic been much better. Moore (1939) maintains that we may rest with what we naturally believe, or presuppositions thereof; but in context, this seems to beg the question, or at least not properly engage the skeptical concern, and similarly for views on which we need not rule out every conceivable defeater of our ordinary beliefs. Russell (1912, 1997) maintains that we may infer to the existence of the external world, as the best explanation of the pattern of our sense experience; but what qualifies the usual explanation as ‘best’? Comprehensive skeptical scenarios also explain this pattern, and some on arguably simpler grounds. Relatedly, attempts (along lines of Dretske 1970 and Goldman 1976) to dismiss these scenarios as ‘irrelevant’ presuppose that we have some independent handle on what is actually the case; but this presupposition is exactly what the skeptic’s cases aim to undermine. There is a better route to the rejection of Cartesian skepticism. Suppose, as a result of contemplation of the Cartesian scenarios, I become skeptical about