 The first version of monism has been developed by David Chalmers and Frank Jackson (Chalmers 1996, 1999, 2002a,b, 2004, 2006a,b; Chalmers and Jackson 2001; Jackson 1998, 2004, 2011), who build on earlier work by David Kaplan (1989a,b), Gareth Evans (1979) and Martin Davies and Lloyd Humberstone (1980), and others. On Chalmers’s and Jackson’s view, what explains the phenomena uncovered by Kripke is not a difference between two spaces of possible worlds. There is only a single space of possible worlds: the metaphysically possible worlds—the ways the world could have been—just are the epistemically possible worlds: the ways the world might be for all we can know independently of empirical evidence. What explains the data is a difference between two different ways in which sentences can be used to describe the worlds in that space, i.e., between two different notions of a sentence’s being true in a world. The distinction can be illustrated by appeal to our example of the proper name ‘Phosphorus.’ Suppose that we have just introduced this name by using the description ‘the last celestial body visible in the morning’ to fix its reference. Consider a possible world \(w\) where the description singles out, not Venus (as in our world), but Saturn. Assume further that in \(w\) (as in the actual world), Venus is the second planet from the sun, but Saturn is not. Consider: