 As we have just seen, the proponent of possible worlds semantics must take the italic sentences, (13) and (14), to have the same content; hence, it seems, the proponent of possible worlds semantics must take (15) and (16) to be a pair of sentences which differ only in the substitution of expressions with the same content. But then it seems that the proponent of possible worlds semantics must take this pair of sentences to express the same proposition, and have the same truth-value; but (15) and (16) (like  (7) and (8))  seem to differ in truth-value, and hence seem not to express the same proposition. For an influential extension of this argument, see Soames (1988).