Since the time of Democritus, one of the chief irritants producing the pearl of philosophy has been the challenge of skepticism, the worry that false sense impressions are sometimes indistinguishable from true ones. The experience of illusions of perception is familiar—the oar that appears bent in water, the tower that appears round from a distance but square nearby. As Aristotle explains, “Which, then, of these impressions are true and which are false are not obvious, for the one set is no more true than the other, but both are alike. And this is why Democritus, at any rate, says that either
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