Jonathan Vogel

This dissertation begins with a general discussion of the role of epistemic principles in arguments for and against Cartesian skepticism . The skeptic may be viewed as trying to establish that, according to non-arbitrary epistemic principles we ordinarily accept, we have no knowledge of the external world. So construed, the skeptic's challenge cannot be dismissed. It can, however, be refuted, particularly if the epistemic principles invoked by the skeptic prove to be invalid. ;One epistemic principle is that of the closure of knowledge under known logical implication: that if a person knows A, and knows that A entails B, then that person knows, or can know, B. ;The validity of the closure principle has been challenged recently by Fred Dretske and Robert Nozick. Dretske's case against the principle rests on some apparent counterexamples. I maintain that these examples don't have the force Dretske thinks, and that other, even stronger examples fail to undermine the closure principle. Nozick bases his criticism of the closure principle on a plausible assumption about the conditions for knowledge. I argue that this assumption is deeply flawed, especially in light of certain facts about inductive knowledge. ;I next consider and defend the claim that justification is closed under known logical implication. On the assumption that knowledge does require justification, this result supports two further conclusions: that knowledge itself is likely to obey closure, and that, even if it doesn't, the skeptic could still proceed with his argument unimpeded. In connection with these issues, I discuss some relevant views of Peter Klein, and argue that his attempted refutation of skepticism isn't fully successful. ;A principal result is that rejection of the closure principle doesn't provide a legitimate way to avoid Cartesian skepticism. The discussion also indicates that a convincing refutation of the skeptic may be found in an appeal to principles of inference to the best explanation. I distinguish various skeptical hypotheses and show that they may be rejected on grounds of explanatory inadequacy. When faced with this sort of response, Cartesian skepticism either becomes untenable or collapses into skepticism about induction.