Ralph Wedgwood (Southern California) gives a talk at the MCMP workshop "New Perspectives on External World Scepticism" (9-10 July, 2013) titled "Scepticism and Probabilism". Abstract: Philosophers who work within a broadly probabilistic epistemological framework have extensively explored the significance of inductive scepticism. But what is the significance of external-world scepticism within a probabilistic framework? Presumably, if external-world scepticism is false, then the perceptions or sensory experiences that a believer has at a given time must make some difference to the probability function that measures the degrees of belief that it is rational or justified for the believer to have at that time. Specifically, it must normally be the case that if the believer has an experience as of p’s being the case, the probability of p will be raised in some way. In general, an experience as of p’s being the case will have many other effects on the probabilities of other propositions besides p. Still, there must be some limits to the effects that this experience has on these probabilities. It is proposed here that for every experience, there is some partition of propositions such that the experience cannot change the conditional probability of any proposition conditional on any member of that partition; in other words, the rational impact of experience can be modelled by means of some kind of conditionalization. However, it seems that the effect that experience normally has on a rational believer’s degrees of belief can be defeated in some cases. Some philosophers—most notably, Jonathan Weisberg—have objected that conditionalization cannot be reconciled with a plausible account of defeasibility. A reply to this objection is offered here, including an account of how perceptual justification can be defeated. This account implies that the cases in which perceptual justification is defeated are in a sense exceptional or abnormal: it is only when certain special factors are present that perceptual justification will be defeated. The account has this implication because it relies on a principle that seems likely to play a crucial role in any adequate solution to inductive scepticism—the principle that it is rational for the believer to start out with a set of prior probabilities that are systematically biased in certain ways. In fact, it turns out that the sort of rational antecedent bias that is required to make sense of this fact about the defeasibility of perceptual justification is also enough to provide the core of an account of the rational impact of experience. In short, within a probabilistic framework, external-world scepticism and inductive scepticism turn out to have essentially the same solution.