 When Barwise and Perry started their joint work, a new, more fine-grained, notion of information content seemed to be urgently needed in natural language semantics, because of a known challenge facing possible worlds semantics, which, under the influence of Lewis 1972 and Montague 1974, was the framework of choice for most formal semanticists at the time (see the entry on    possible worlds).   In possible worlds semantics, propositions are identified with the set of possible worlds where they are true (see the entry  propositions).  Consequently, propositions that are true in the same possible worlds are identical, and we seem to predict wrongly that a person who believes a proposition p should also believe any proposition that is true in the same worlds as p (see the entry  propositional attitude reports).  To distinguish logically equivalent propositions, we seem to need a more fine-grained notion of what the information content of a sentence is, and the state-of-affairs or infons of situation semantics were marketed to provide just that.