according to which a name refers to an object by virtue of a causal connection with the object as mediated through communities of speakers. He points out that proper names, in contrast to most descriptions, are rigid designators: that is, a proper name refers to the named object in every possible world in which the object exists, while most descriptions designate different objects in different possible worlds. For example, "Richard Nixon" refers to the same person in every possible world in which Nixon exists, while "the person who won the United States presidential election of 1968" could refer to Nixon, Humphrey, or others in different possible worlds. Kripke also raised the prospect of a posteriori necessities—facts that are necessarily true, though they can be known only through empirical investigation. Examples include "Hesperus is Phosphorus", "Cicero is Tully", "Water is H2O", and other identity claims where two names refer to the same object. According to Kripke, the Kantian distinctions between analytic and synthetic, a priori and a posteriori, and contingent and necessary do not map onto one another. Rather, analytic/synthetic is a semantic distinction, a priori/a posteriori is an epistemic distinction, and contingent/necessary is a metaphysical distinction. Finally, Kripke gave an argument against identity materialism in the philosophy of mind, the view that every mental particular is identical with some physical particular. Kripke argued that the only way to defend this identity is as an a posteriori necessary identity, but that such an identity—e.g., that pain is C-fibers firing—could not be necessary, given the (clearly conceivable) possibility that pain could be separate from the firing of C-fibers, or the firing of C-fibers be separate from pain. (Similar arguments have since been made by David Chalmers.) In any event, the psychophysical identity theorist, according to Kripke, incurs a dialectical obligation to explain the apparent logical possibility of these circumstances, since according to such theorists they should be impossible. Kripke delivered the John Locke Lectures in philosophy at Oxford in 1973. Titled Reference and Existence, they were in many respects a continuation of Naming and Necessity, and deal with the subjects of fictional names and perceptual error. In 2013 Oxford University Press published the lectures as a book, also titled Reference and Existence. In a 1995 paper, philosopher Quentin Smith argued that key concepts in Kripke's new theory of reference originated in the work of Ruth Barcan Marcus more than a decade earlier.