Before I present a more detailed account of ad hominem arguments, I will say a few words about social epistemology and the position that is presupposed in this commentary. Jennifer Lackey’s (2006) account of social epistemology relies upon a kind of dualism, in which she combines anti-reductionism and reductionism. According to her, social epistemology has made the mistake of addressing the debate between reductionism and non-reductionism unilaterally. Reductionism takes epistemic responsibility and the rationality of the hearer far too seriously, because the hearer has to rely upon other sources of knowledge like perception, memory, deductive inferences, etc. The claim here is that testimony is not a source of knowledge in the first place, because a hearer could never know the intentions of a speaker who held accidently or intentionally false beliefs. In contrast, anti-reductionism always focuses on the speaker’s perspective and her propensity for credible testimonials. Proponents of anti-reductionism claim that a large amount of our knowledge depends on testimonials. We would know almost nothing if we were as restrictive as the reductionist claims (Coady 1992). Lackey wants to combine these two accounts in a kind of dualism. Her dualism contains the presupposition of the reliability of the speaker along with positive reasons to accept the speaker’s testimony, evaluated from the hearer’s perspective. If the speaker utters a true sentence and the hearer has positive reasons to trust the speaker, then knowledge from testimony is possible. Lackey argues for the following conditional, which contains three necessary conditions:[5]
For every speaker A and hearer B, B justifiedly believes that p on the basis of A’s testimony that p only if: (1) B believes that p on the basis of the content of A’s testimony that p, (2) A’s testimony that p is reliable or otherwise truth conducive, and (3) B has appropriate positive reasons for accepting A’s testimony that p. (2006, p. 170)
For the present account it is important that a testimony, given by A, qualifies as a source of knowledge that depends on the hearer having positive reasons to think that A’s testimony is reliable. Recall the informative intention and the mind-to-world-direction of fit, which is fulfilled if the speaker causes a new belief in the hearer. The direct proper function of the cooperation between speaker and hearer in Millikan’s (1984, 2005) account would be fulfilled. But according to Lackey’s condition (3), the achievement of a new belief is only justified if there are various positive reasons that account for the reliability of the speaker’s testimony. Consider the account of Sperber et al. (2010, p. 379) that “the filtering role that epistemic vigilance […] in the flow of information in face-to-face interaction” is an important feature of communicative agency. But which kind of filtering do they mean? In other words, what are positive reasons, exactly? Could they be past experiences about the reliability of the speaker or even a group to which the speaker belongs?