Structuralism as a Response to Skepticism

A classic argument for skepticism about the external world runs as follows. First: we cannot know that the evil-demon hypothesis is false, where the evil-demon hypothesis says that all of our sensory experiences have been produced by an evil demon. Second: if we cannot know that the evil-demon hypothesis is false, we cannot know we have hands. Conclusion: We cannot know we have hands. Generalizing: as the second premise holds equally for arbitrary empirical claims about the external world, we cannot know any such claims to be true. Here the evil-demon hypothesis is put forward as a global skeptical hypothesis: a single hypothesis whose possible truth threatens knowledge of arbitrary empirical claims all at once. Other putative global skeptical hypotheses include the brain-in-vat hypothesis, holding that we are permanently envatted brains; the simulation hypothesis, holding that we have lived our lives in a computer simulation; and certain versions of the dream hypothesis, holding that we have always been dreaming. I will call these putative global skeptical hypotheses Cartesian hypotheses, and the scenarios that they specify Cartesian scenarios. They do not all come from Descartes, and Descartes also put forward other sorts of skeptical scenario, but these scenarios have exerted an especially strong grip on the philosophical imagination. My focus here is on skepticism about the external world, so I will restrict my attention (and the label “Cartesian”) to scenarios in which a subject’s physical environment is varied globally, keeping the subject’s experiences the same. Scenarios in which only other minds are absent or in which an evil demon is interfering with one’s own mind also raise important skeptical issues, but they are beyond the scope of this paper. Cartesian hypotheses contrast with local skeptical hypotheses, whose possible truth threatens empirical claims a few at a time. For example, the painted-mule hypothesis, holding that one is currently looking at a mule painted with black and white stripes, threatens knowledge that one is looking at a zebra. Local skeptical hypotheses suffice for many skeptical purposes. Even if every empirical belief were threatened by a different local skeptical hypothesis, that would be