many (perhaps constituting all of one's essential knowledge of the individual named), or a weighted average of these descriptions. Kripke found this account to be deeply flawed, for a number of reasons. Notably: We can successfully refer to individuals for whom we have no uniquely identifying description. (For example, a speaker can talk about Phillie Sophik even if one only knows him as 'some poet'.) We can successfully refer to individuals for whom the only identifying descriptions we have fail to refer as we believe them to. (Many speakers have no identifying beliefs about Christopher Columbus other than 'the first European in North America' or 'the first person to believe that the earth was round'. Both of these beliefs are incorrect. Nevertheless, when such a person says 'Christopher Columbus', we acknowledge that they are referring to Christopher Columbus, not to whatever individual satisfies one of those descriptions.) We use names to speak hypothetically about what could have happened to a person. A name functions as a rigid designator, while a definite description does not. (One could say 'If Aristotle had died young, he would never have taught Alexander the Great.' But if 'the teacher of Alexander the Great' were a component of the meaning of 'Aristotle' then this would be nonsense.) A causal theory avoids these difficulties. A name refers rigidly to the bearer to which it is causally connected, regardless of any particular facts about the bearer, and in all possible worlds where the bearer exists. The same motivations apply to causal theories in regard to other sorts of terms. Putnam, for instance, attempted to establish that 'water' refers rigidly to the stuff that we do in fact call 'water', to the exclusion of any possible identical water-like substance for which we have no causal connection. These considerations motivate semantic externalism. Because speakers interact with a natural kind such as water regularly, and because there is generally no naming ceremony through which their names are formalized, the multiple groundings described above are even more essential to a causal account of such terms. A speaker whose environment changes may thus observe that the referents of his terms shift, as described in the Twin Earth and Swampman thought experiments. Variations Variations of the causal theory include: The causal-historical theory of reference is the original version of the causal theory. It was put forward by Keith Donnellan in 1972 and Saul Kripke