Testimonial authority and knowledge transmission

Is speaker knowledge necessary or sufficient for enabling hearers to know from testimony? Here, we offer a novel argument for the answer no, based on the systematic effects of partial belief and the hearer’s view prior to hearing testimony. Modelling partial belief by credence, we show that a requirement entailed by the principles of necessity and sufficiency apparent in the literature is inconsistent with Bayesian updating. Consequently, even when the other grounds of knowledge are in place, the audience correctly updating their partial belief can block the transmission of speaker knowledge, so it need not be sufficient. Nor need speaker knowledge be necessary, because the hearer correctly updating their partial belief can put them in the position to know even though no one in the speaker’s chain knows. We articulate the correct principles of testimonial knowledge transmission. The first supports a shared-credit view of transmission. The second gives a novel and systematic argument for testimony sometimes being a generative (not transmissive) source of knowledge, an argument that makes Lackey’s statement account of testimony otiose. Finally, on at least one account of causation, the two amended principles together show how speaker knowledge can be a cause whose effect is hearer’s knowledge.