Most people have noticed that vision can play tricks. A straight stick submerged in water looks bent, though it is not; railroad tracks seem to converge in the distance, but they do not; and a page of English-language print reflected in a mirror… thing as knowledge of an external world. According to that view, it is at least logically possible that one is merely a brain in a vat and that one’s sense experiences of apparently real objects (e.g., the sight of a tree) are produced by carefully engineered electrical stimulations. Again, given