  What, then, is the semantic value of an expression replacing \(p\) in a sentence of the general modal form ‘It is — that \(p\)’? Certainly not a truth value, as is the case with the standard extensional propositional calculus, for substituting a different expression with the same truth value into the sentence of the form ‘It is — that \(p\)’ may alter the truth value of the latter sentence. Prior’s answer—and in a sense it amounts to a rejection of the question—is that modal functions take propositions as arguments, but propositions are logical constructions. All sentences containing the word ‘proposition’—including such sentences as ‘A modal operator expresses a function from propositions to truth values’—mean no more and no less than sentences which contain neither the word ‘proposition’ nor an equivalent. In essence, Prior’s view is that there are intensional contexts but no intensions. For the last six years of his life he worked on a book that was to give systematic expression to his views on propositions. The incomplete manuscript, which Prior had entitled Objects of Thought, was published posthumously in 1971.