 The most radical challenge to Lewis came from Quine (1951 [1953]) who argued that the distinction between so called a priori truths and a posteriori truths is just one of degree. The argument has two steps. First, empirical hypotheses have implications for experience only in conjunction with various empirical generalizatons and other background assumptions, e.g. about the circumstances of perception. Recalcitrant experience thus tells us only that some belief or assumption in the total set that implies a contrary experience is false, not which one, and thus any statement can be held true, no matter what experience brings, so long as we make enough adjustments to the rest of our beliefs and assumptions. Second, empirical hypotheses can’t logically imply anything about experience except against a background of assumed laws of logic. Recalcitrant experience can, in principle, then, lead us to revise an assumed logical principle in our web of belief rather one of our other beliefs. 