  What of the meaning of the binary relation \(U\)? As previously remarked, the idea that the variables of quantification of the calculus should range over possible states of affairs or possible worlds is present in The Craft. In 1960, following a suggestion by Geach, Prior began thinking of \(U\) as a relation of accessibility between worlds. Prior tells us that Geach cashed out the notion of ‘reaching’ one world from another in terms of ‘some dimension-jumping vehicle dreamed up by science fiction’ (1962b: 36; see also 1962a: 140). (Geach referred to the whole business as ‘Trans World Airlines’.) With this interpretation of \(U\) to hand, the property calculus can be viewed as treating \((\Box p)a\) — or ‘Necessarily\(-p\) in world \(a\)’ — as short for ‘\(p\) is true in all worlds accessible from \(a\)’.