  The most popular conceptions of unstructured propositions are those that take them to be either sets of possible worlds or functions from possible worlds to truth-values (Lewis 1986a, 53; Stalnaker 1987, 2). On these theories a proposition is the set of possible worlds in which it is true, or a function that has the value True when it takes as argument a world where the proposition is true and has the value False when it takes as argument a world where the proposition is false.