 Robert Stalnaker (1978, 2001) has given a detailed account of the pragmatic mechanism by which a contingent proposition comes to be asserted by the utterance of a sentence that semantically expresses a necessary proposition. On his account, linguistic communication evolves in a context characterized by background assumptions that are shared between the participants. These assumptions can be represented by the set of worlds at which they are jointly true, which Stalnaker calls the ‘context set.’ The point of assertion is to add the proposition asserted to the set of background assumptions and thereby eliminate worlds where it is not true from the context set. To achieve this, every assertion needs to conform to the rule that the proposition asserted is false in some of the worlds that were in the context set before the utterance (otherwise there are no worlds to eliminate) and true in others (since the audience cannot eliminate all worlds from the context set). Now consider a context where the shared background assumptions include the proposition that the references of ‘\(A\)’ and ‘\(B\)’ were fixed by certain descriptions but leave open whether the two descriptions single out the same object. Suppose that someone says ‘\(A\) is \(B\).’ In every world in the context set, the sentence semantically expresses either a necessary truth (if the two descriptions single out the same object in the world) or a necessary falsehood (if they don’t). If the proposition that the speaker intends to assert were the one that is semantically expressed by the sentence, the aforementioned rule would be  violated.[7]  To avoid attributing this rule violation to the speaker, the audience will construe the utterance as expressing a different proposition, and the most natural candidate is the proposition that the sentence uttered semantically expresses a true proposition. (Stalnaker calls this the ‘diagonal proposition.’) By exploiting this mechanism of reinterpretation, a speaker can use the sentence to express the diagonal proposition. This proposition is true in just those worlds in the context set where the two descriptions single out the same object. It is clearly a contingent proposition, and empirical evidence is required to know it. Stalnaker suggests an analogous explanation of Kripke’s proposed cases of contingent a priori truth (1978, 83f.).