 Several arguments against internalism have prompted considerable debate in philosophy of mind. Here is a condensed version of one argument, due to Putnam (1975; though it should be said that Putnam’s original emphasis was on linguistic meaning, not on belief). Suppose that in 1750, in a far-off region of the universe, there existed a planet that was physically identical to Earth, molecule-for-molecule, in every respect but one: Where Earth had water, composed of H2O, Twin Earth had something else instead, “twater”, coming down as rain and filling streams, behaving identically to water by all the chemical tests then available, but having a different atomic formula, XYZ. Intuitively, it seems that the inhabitants of Earth in 1750 would have beliefs about water and no beliefs about twater, while the inhabitants of Twin Earth would have beliefs about twater and no beliefs about water. By hypothesis, however, each inhabitant of Earth will have a molecularly identical counterpart on Twin Earth with exactly the same brain structures (except, of course, that their brains will contain XYZ instead of H2O, but reflection on analogous examples regarding chemicals not contained in the brain suggests that this fact is irrelevant). Consequently, the argument goes, the contents of one’s beliefs do not depend entirely on internal properties of one’s brain.