  So the Master Argument is indeed valid. The fallacy, Prior tells us, lies with the second ‘broad assumption’ \((\neg p \amp \neg Fp) \rightarrow P\neg Fp\) (which says: when anything neither is nor will be the case, it has been the case that it will not be the case). This, Prior tells us, is not true if \(p\) refers to a future contingency, and thus has the truth value ½ or ‘indeterminate’. Where \(p\) is indeterminate, both \(Fp\) and \(\neg Fp\) are indeterminate, so the consequent of the disputed formula, \(P \neg Fp\), is false. \(\neg p\) must also be indeterminate (for if the negation of \(p\) were determinate, \(p\) could not be indeterminate). Thus the antecedent of the ‘broad assumption’, \(\neg p \amp \neg Fp\), is indeterminate, since both its conjuncts are indeterminate.