 These examples involve a subclass of factive verbs called "semifactives," which Karttunen (1971b) concluded only trigger a presupposition in some person and tense forms. As Karttunen himself realized, such a stipulation is unmotivated. For Stalnaker’s pragmatic account of presupposition, these examples are not problematic; the verb "know" need not presuppose that its complement is true. When an addressee hears the first sentence of (15a), he will realize that if it were in the common ground that Mullah Omar was alive, then the speaker would know this, and so the speaker’s claim would be false. Therefore the hearer can reason that the speaker is not presupposing the complement of “know” to be true. On the other hand, when a hearer is confronted by (15b), it is consistent to assume that Luke was alive. Since speakers using “know” typically presuppose the truth of the complement, we can assume that this is the case here.