  In a matchless piece of philosophical reconstruction—in ‘Diodoran Modalities’—Prior expresses the conclusion of the Master Argument, that what neither is nor will be true is not possible, as \((\neg p \amp \neg Fp) \rightarrow \neg \Diamond p\), and derives this in his calculus from Diodorus’ premisses, \(Pp \rightarrow \neg \Diamond \neg Pp\) and \(\neg\Diamond q \rightarrow (\Box(p \rightarrow q) \rightarrow \neg\Diamond p),\) together with two ‘broad assumptions about time, likely to have been taken for granted both by Diodorus and by his main opponents’, viz. \(p \rightarrow HFp\) and \((\neg p \amp \neg Fp) \rightarrow P\neg Fp.\)