 (Kaplan goes on to attribute this “metaphysical picture” of structured propositions to Russell.) Adopting this structured proposition account makes it simple to distinguish between directly referential expressions and other expressions, rigid or not. Directly referential expressions contribute their referents (in a context) to the propositions expressed (in that context) by the sentences containing them. Non-directly referential expressions contribute some complex that may or may not determine the same individual in all possible circumstances. Thus the desire to distinguish clearly between directly referential expressions and other rigid designators prompted Kaplan to re-introduce the Russellian notion of a structured proposition into the philosophical literature (see the discussion of Russell below). However, Kaplan [1977] tends to treat the notion of a structured proposition as a heuristic device. He repeatedly calls it a picture, explicitly says that it is not part of his theory, and in his formal semantics he adopts the possible worlds account of propositions (contents of formulae), taking them to be functions from worlds (and times) to truth values.