SELF-KNOWLEDGE, EXTERNALISM, AND SKEPTICISM by Brian P. McLaughlin and David Owens I —Brian P. McLaughlin ABSTRACT In recent years, some philosophers have claimed that we can know a priori that certain external world skeptical hypotheses are false on the basis of a priori knowledge that we are in certain kinds of mental states, and a priori knowledge that those mental states are individuated by contingent environmen- tal factors. Appealing to a distinction between weak and strong a priority, I argue that weakly a priori arguments of this sort would beg the question of whether the skeptical hypothesis under assessment is true, and that the prospect of a sound strongly a priori argument of this sort seems dim. ‘It still remains a scandal to philosophy... that the existence of things outside of us... must be accepted merely on faith, and that, if anyone thinks good to doubt their existence, we are unable to counter his doubts by any satisfactory proof.’ (Immanuel Kant, Critique of Pure Reason) A t the dawn of the Twenty First Century, many of us shrug our shoulders at talk of scandal. But there are, today, philo- sophical quarters in which Kant’s dream of answering the skeptic with a proof remains alive. There is a new line of response to external world skepticism that its proponents promise will prove that the skeptic is mistaken. My aim in this paper is to examine the prospect of its success. This new line of response presupposes a priori mental external- ism: the thesis that certain types of mental states are a priori individuated, at least in part, by contingent natural or social environmental factors. It also presupposes that at least some of the types of mental states in question are such that when we are in them, we can know in a first-person privileged way that we are; and it further presupposes that such knowledge is a kind of a priori knowledge. In the literature on privileged self-knowledge and mental exter- nalism, the idea that such knowledge is a priori is glossed by saying that certain kinds of mental states are such that we can