 Hintikka 1969, Stalnaker 1984, and Lewis 1986 are the foremost proponents of unstructured-propositionalism. (See the entry on  structured propositions  for further details.) Briefly, the idea is that propositions are sets of possible worlds. The proposition that Bush is president, for example, just is the set of all possible worlds in which Bush instantiates the property being president. (Or, to remain neutral on issues of the metaphysics of transworld identity, all the worlds in which Bush or a counterpart of Bush instantiates that property.)