 All this is important for B(3), since if W. grants (2) a warrant in the absence of empirical investigation, it will be a default warrant (at least in part). And this kind of warrant exists only in ordinary cases. Thus, he grants this warrant only if he assumes that there is no Dry Earth illusion where (3) merely appears to be true. So in particular, in the context of the reductio, W. allows an apriori warrant to the *premises only if (3) is already assumed as background. QED