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Marcus Arvan

Hi Moti: congrats on the paper! I just skimmed it and have a few questions.

You say that you're concerned with (A) arguments from expert opinion, not (B) arguments from *agreement* amongst experts. Maybe informal logic textbooks aren't clear about this (the two you quote at the beginning certainly aren't), but I wager most people would agree with you that the first type of argument is bad, but hold that the latter is good. Here's why.

I think few people would say (e.g.) a physicist's opinion on a physics matter should be trusted *just* because they're an expert at physics, or similarly that my opinion on some matter of political philosophy should be trusted because I specialize in it. After all, when it comes to many issues, different experts disagree. For example, there were many in the particle physics community that thought supersymmetry was a promising way to resolve certain problems, whereas others didn't. When it comes to these issues -- issues where experts disagree -- it seems plainly silly (to me, at any rate) to trust any one such expert (or even a majority of them, given that minority figures have turned out right many times!).

But is this really the most charitable way to understand the notion of "appeal to expert opinion"? I would think that most people would say something like: "Clearly, we should only take expert opinion to be good evidence when there's an empirically well-supported expert *consensus.*" I respect what physicists say about relativity or the Standard Model because they *all* agree that those theories are the best supported by numerous strands of evidence. In contrast, no matter what hedge fund managers say, I don't trust any of them because there is no evidence that they, as a group, have any demonstrable expertise at all!

Anyway, I think this is a good issue, and I thank you for investigating it so carefully. Should informal logic textbooks perhaps hold that appeal to expertise is strong when and only when (1) experts achieve something like a consensus on an issue, and (2) there are demonstrable empirical grounds that they are genuine experts in the relevant area (i.e physics works, stock market predictions don't)?

Moti Mizrahi

Thanks for your thoughtful comments, Marcus.

I agree that appealing to expert opinion when there is no agreement among the relevant experts is unconvincing. In the paper, however, I discuss a slightly different sort of “appeal to expertise.” Suppose that you are considering whether or not p is worthy of belief. Should you count the fact that E says that p as a reason to believe p? I argue that you shouldn’t because the fact that E says that p does not make p more likely to be true. In other words, “E says that p” is statistically irrelevant to “p.”

Your point, I take it, is that you should count the fact that most experts agree that p as a reason to believe p. In this case, too, the crucial question is whether “most experts agree that p” makes it more probable that p. To answer this question I think we need evidence of the sort I cite in the paper. We can easily come up with examples where consensus turned out to be wrong (e.g., http://www.youtube.com/watch?v=BYgLAPhbyKg ) but we need evidence that this is the case more often than not, or at least that consensus does no better than chance in terms of getting it right.

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