The knowledge norm of assertion (Unger 1975: 256–60; Williamson 2000: ch. 11) is mainly in competition with a high probability or rational credibility norm. (‘One should assert only what is rationally credible to one.’ Douven 2006: 458–59.) The argument for the knowledge norm that I offer turns on cases in which a hearer responds to a speaker's assertion by asserting another sentence that would lower the probability of the speaker's assertion, were its probability less than one. In cases like this, though with qualifications, is the hearer's contribution a challenge to the speaker's assertion or complementary to it? My answer is the latter, and that only the knowledge norm yields that answer.

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The cases that I rely on follow from an elementary probability relation,...

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