Some hold that propositions lack constituents altogether, and so are unstructured.  If propositions are unstructured, then if they are sets, they inherit the identity conditions for sets: sameness of members. Thus, if a proposition is the set of worlds in which it is true (as in Stalnaker 1976), then P=Q iff P and Q have the same worlds as members iff P and Q are true in the same worlds. As is well-known, this theory leads to a very coarse individuation of propositions, too coarse, arguably, to handle propositional attitudes. (See Soames (1987) for a discussion of this theory as well as the theory of propositions as sets of concrete situations or facts.