The British philosopher G. E. Moore (1873-1958) famously offered an externalist reply to radical skepticism. Recall that radical skeptics are not sure of the existence of any material objects, not even the existence of their own hands since they might merely be having the experience of having hands while, in reality, not having any hands. To refute such radical skeptics, Moore would sometimes dramatically hold up one hand and assert, “Here is one hand.” Then he would raise his other hand and say, “Here is another.” He would conclude from this vivid demonstration that there are at least two material objects existing in the so-called external world. And from this conclusion, he drew a further conclusion: that the external world exists.
Now it is very tempting to make fun of Moore in providing such a simple argument. But he was a fiercely intelligent person, and he knew exactly what he was doing. What he was doing was calling to everyone’s attention that the starting place of the skeptic—that I know my experience but not what lies beyond it—is not in fact more obvious than the starting place of the non-skeptic—that, in fact, we typically do know that we have hands (as well as many other things). In some cases, we may not know that we have hands. If we have been in a terrible accident, for example, and the ends of our arms are wrapped in thick wads of bandages, then we may not be sure that we still have hands. But ordinarily we are quite confident that we have hands, and the burden is on the skeptic to offer some positive reason for thinking we are mistaken about this. If we are sure we have hands, then the proof of the existence of the external world is relatively straightforward: “Here is one hand … and here is another.”
An externalist, following Moore’s line of thought, might well argue that the GDD is an abuse of language and the meanings of words (this particular line of thought is associated with Ludwig Wittgenstein). For consider, how do we learn how to use words like “know” and “illusion” and “deceive” and “doubt”? We learn them in very ordinary situations, in classrooms and at home and in theaters and on the street. We learn, for example, that stage magicians deceive us with hidden pockets and trap doors and sleight of hand. We learn about mirages and optical illusions. We learn about liars and cover-ups and conspiracies. In these ordinary situations, there aren’t any deep puzzles about what we know or what it means to be deceived. And we learn when it is appropriate to doubt or how severe our doubt should be in particular real circumstances. It is not easy—it requires a lot of thought and experiment and so on—but the challenges we face are familiar and common.
Then we walk into a philosophy class, and we are asked to apply the concepts that we learned in ordinary circumstances to circumstances that are unlike anything we have seen before: deceiving gods and demons and mad scientists who exert malicious control over everything we experience. But our ordinary concepts are not meant to hold up in such extraordinary circumstances! It is a bit like learning that all numbers greater than zero are either even or odd, and then being asked whether infinity is even or odd. It is not just a hard question, it is an impossible one. To this extent, the skeptic raising the GDD scenario is pushing our concepts well past their breaking points. By changing frameworks, the externalist is trying to pull us back into the circumstances where those concepts are meant to apply.