  Two kinds of replies to this objection are worth considering. The first, suggested in the brief statement of Quine’s new methodology, is to reject the old problem as based on the false presupposition that one can vindicate scientific and other knowledge on the basis of propositions that are knowable a priori. The difficulty with this response is that it raises the difficult issues of whether a priori knowledge is possible and whether we can determine an answer to that question a posteriori. Another reply, perhaps more in keeping with the spirit of naturalized epistemology, is not to dismiss global skepticism out of hand, but to allow science to attempt to explain science within its own terms, recognizing that the effort will be circular but defending its legitimacy. The effort is legitimate, it might be argued, because the circularity does not guarantee the success of the effort. The appeal to Darwin, for example, may not succeed, either because Darwin’s theory is overthrown or because a necessary background assumption (say that our native inductive tendencies are gene linked) is contradicted by the evidence. In fact, Quine allowed that the project to explain science from within science could defeat itself, in the end vindicating the skeptic (Quine 1981, p. 22). On this approach knowledge claims are always open to questioning and perhaps rejection down the road. Any justification that is offered for induction or claims of knowledge based on induction is ultimately fallible and revisable.