 One consequence of Stalnaker’s view is that, contra semantic accounts of presupposition, presupposition failure need not produce a semantic catastrophe. There are, however, two weaker types of failure that can occur: (i) a speaker uttering some sentence S can fail to assume that some proposition P is in the common ground, even though most utterances of S would be accompanied by the presupposition that P; and (ii) a speaker can presuppose something that is not in the common ground. We see these two types of failure respectively in (15a), where there is no presupposition that Mullah Omar is alive, and (15b) where it is presupposed that Luke was alive.