 But now consider (16b), repeated from (12). Here there is an implicature that the speaker doesn’t know whether there is a knave. The hearer accepts this and other implicatures, and then considers the presuppositions that there is a knave and that there are some tarts. The presupposition that there are tarts is unproblematic, and is added, but the hearer cannot consistently add the presupposition that there is a knave. So this presupposition is canceled, and (16b) does not presuppose that there is a knave. Hence, according to Gazdar, presuppositions are sometimes blocked by conversational implicatures.