From Richard Fumerton's Metaepistemology and Skepticism:
"This reminds us, of course, of Quine's injunction to naturalize epistemology. Quine suggested that we give ourselves full access to the deliverances of science when it comes to understanding how we have knowledge of the world around us. Contemporary externalists have simply given us more detailed metaepistemological views which allow us to rationalize following the injunction to naturalize epistemology. If the mere reliability of a process, for example, is sufficient to give us justified belief, then if that process is reliable we can use it to get justified belief wherever and whenever we like.(Hat tip to Agent Intellect.)
All of this will, of course, drive the skeptic crazy. You cannot use perception to justify the reliability of perception! You cannot use memory to justify the reliability of memory! You cannot use induction to justify the reliability of induction! Such attempts to respond to the skeptic's concerns involve blatant, indeed pathetic, circularity. Frankly, this does seem right to me and I hope it seems right to you, but if it does, then I suggest that you have a powerful reason to conclude that externalism is false. I suggest that, ironically, the very ease with which externalists can deal with the skeptical challenge at the next level betrays the ultimate implausibility of externalism as an attempt to explicate concepts that are of philosophical interest. If a philosopher starts wondering about the reliability of astrological inference, the philosopher will not allow the astrologer to read in the stars the reliability of astrology. Even if astrological inferences happen to be reliable, the astrologer is missing the point of a philosophical inquiry into the justifiability of astrological inference if the inquiry is answered using the techniques of astrology. The problem is perhaps most acute if one thinks about first-person philosophical reflection about justification. If I really am interested in knowing whether astrological inference is legitimate, if I have the kind of philosophical curiosity that leads me to raise this question in the first place, I will not for a moment suppose that further use of astrology might help me find the answer to my question. Similarly, if as a philosopher I start wondering whether perceptual beliefs are accurate reflections of the way the world really is, I would not dream of using perception to resolve my doubt. Even if there is some sense in which the reliable process of perception might yield justified beliefs about the reliability of perception, the use of perception could never satisfy a philosophical curiosity about the legitimacy of perceptual beliefs. When the philosopher wants an answer to the question of whether memory gives us justified beliefs about the past, that answer cannot possibly be provided by memory."
