  Which is metaphysically basic, the tense calculus or the \(l\)-calculus? Prior issued a warning against regarding the above interpretation of the tense calculus within the \(l\)-calculus as ‘a metaphysical explanation of what we mean by is, has been and will be’: the \(l\)-calculus, he said, is not ‘metaphysically fundamental’. His reason is that ‘\(F\)(Socrates is sitting down)’ means ‘It is now the case that it will be the case that Socrates is sitting down’, whereas there is no genuine way of representing the indexical ‘now’ in the \(l\)-calculus (the date variable \(z\) is not an indexical, any more than ‘21 January 1954’ is an indexical). Prior continued: ‘If there is to be any “interpretation” of our calculi in the metaphysical sense, it will probably need to be the other way round; that is, the \(l\)-calculus should be exhibited as a logical construction out of the \(PF\)-calculus rather than vice versa.’ This idea of the primacy of the tense calculus over the \(l\)-calculus (or, as he later put it, of McTaggart’s \(A\)-series over the \(B\)-series) became a central and distinctive tenet of his philosophy. Prior took a similar metaphysical position with respect to ordinary modal logic, arguing that the language of possible worlds is to be interpreted in terms of a language with modal operators and not, as is popularly held, vice versa. These issues form the theme of his final book Worlds, Times and Selves. (For more about the world–time parallel, see Rini and Cresswell 2012.)