 The basic philosophical idea behind G2D—that subjects have apriori access to reference-fixing criteria for their words but not to the modal profile of the subject matter picked out—has been suggested by a number of theorists. David Lewis, in particular, was a powerful champion of the idea that we can give apriori definitions for terms whose precise reference we do not understand. Lewis articulated the ‘analytic functionalist’ approach to specifying the meaning of mental predicates and of theoretical terms in science (1966; 1970; 1972; 1979; 1980; 1994); and he was also an early advocate of a generalized 2D approach to semantics (1981; 1994). Other influential proponents of the idea that we can have implicit knowledge of reference-fixing criteria without knowing the modal profile of the reference include Michael Dummett (1973; 1981), Gareth Evans (1982), and John Searle (1983). Early proponents of an explicitly two-dimensional semantics for names and natural kind terms include Harry Deutsch (1990, 1993), Ulrike Haas-Spohn (1995), and Kai-Yee Wong (1996). However, it is two later theorists—Frank Jackson (1994; 1998a; 1998b; 2004) and David Chalmers (1996; 2002b; 2002c; 2004; 2006a)—who have most systematically developed and defended G2D as a way of reconciling the lessons of semantic externalism with the traditional apriori approach to meaning and modality.