  As sources for these ideas Prior cited John Wallis (a seventeenth century mathematician) and the account of logically necessary and logically impossible propositions given by Wittgenstein in the Tractatus (1951: 737). Interestingly, he mentioned Carnap only in a footnote: ‘Professor Carnap has a similar definition of logical necessity in terms of what he calls “state-descriptions”’ (ibid). Prior did not refer to, and presumably had not at that time read, Carnap’s 1946 paper ‘Modalities and Quantification’, which attempted a semantics for quantified S5 in terms of state-descriptions. (A state-description is a class of sentences satisfying certain conditions; each state-description represents a possible state of affairs.) Carnap too cited the Tractatus account of modal propositions as his inspiration (1946: 47).