 Because the structured proposition view arose in large part due to dissatisfaction with the then prevailing view of propositions, discussing this other account of propositions will help to illuminate structured proposition views. The late 1950s and 1960s saw the development of a new sort of model theory, “possible worlds semantics”, for systems of modal logic. In the framework of possible world semantics, linguistic expressions are assigned extensions “at” possible worlds. Thus, e.g. names, n-place predicates and sentences are assigned individuals, sets of n-tuples of individuals, and truth values, respectively, at different possible worlds. Intuitively, possible worlds are to be thought of as “ways things could have been”, and the assignment of (possibly different) extensions to expressions at different possible worlds is part of capturing this intuition. Thus there might have been more or fewer cows, and this is reflected in the fact that the extension of ‘cow’ (intuitively, the set of things that are cows) can vary from possible world to possible world.