Suppose you take S to be a competent inquirer, and you take S to have engaged in competent inquiry concerning p. Suppose as well that these attitudes by you are rational ones. The, if S sincerely tells you that p is true. Some take this to be evidence regarding p, but the most obvious and straightforward account takes it to be evidence for you that there is good evidence for p. That is, the immediate and direct effect of a sincere witness to p is metaevidence regarding p. After all, the point of taking S to be a competent inquirer who has engaged in competent inquiry is that whatever they tell you will be something they have evidence for. So when they tell you that p is true, you obtain evidence that they have evidence for p, which is to say that you get evidence that there is evidence for p.
When such a sincere witness counts as testimony, then metaevidence for p is also evidence for p. That’s why testimony makes one justified in believing what is reported. As noted before, however, having metaevidence for p is no guarantee of having evidence for p. Jamie’s example here is a good refutation:
Suppose that 55% of young guckies are males. And, 60% of young male guckies are aggressive, but only 1% of young female guckies are aggressive, while half of all adult male guckies and half of all adult female guckies are aggressive.
I have Pat, a guckie, in my terrarium. Here is some evidence, which I will call Y: Pat is young. Y is evidence for the following, which I will call YM: Pat is a young male guckie. And YM, in turn, is evidence that Pat is aggressive (which conclusion I call A).
Y is evidence for YM, and YM is evidence for A, but Y is not evidence for A (indeed, Y is evidence against A). So, Y is evidence that there is evidence for A, without being evidence for A. So the meta-evidence principle is false.
I think a similar restriction on the metaevidence principle applies to the word of our peers, both when they agree with us and when they disagree. In fact, the metaevidence principle gives a nice way to characterize the difference betweeen cases of agreement and cases of testimony. When we have a case of testimony, the metaevidence principle is true, assuming that the testimony generates metaevidence; when we have only a case of simple agreement, the metaevidence principle is false. Moreover, this account helps explain the attraction of always taking agreement and disagreement to be epistemically relevant. Since justification by testimony is so pervasive in our rational conception of the world, this pervasiveness leads some to overgeneralize here and take the word of others always to constitute testimony (remember, I’m stipulating that a case of testimony is one where the metaevidence principle is true).
None of this constitutes an argument for my view that simple agreement and disagreement by peers (or superiors) does not count as evidence. But maybe the metaevidence principle helps explain why one would be tempted toward the view I think is mistaken, if you’re convinced that it is mistaken.