Guest Post: Peer Disagreement

[The following is a guest post from Barry Maguire, discussing the view that one ought to 'split the difference' upon disagreeing with an epistemic peer, giving 'equal weight' to your own judgment and theirs...]

Here is something of a response to an objection to the equal weight view, based on a suggestion by Adam Elga last week. Consider first the following distinction: between epistemically identical agents, who have identical background information, local evidence, computational faculties, dispositions to err, and so on; and seemingly equivalent agents, who have equivalent epistemic capacities with respect to some issue as far as they both can tell.¹

Two epistemically identical agents could not come to differing conclusions about some issue. Our world does not contain any epistemically identical agents, for any candidates would at least differ in some small degree in their background information and dispositions. Two seemingly equivalent agents obviously could differ in their conclusions. Suppose you are one of two seemingly equivalent agents, and you have come to different conclusions with respect to the some question. In such a case, one of the following sorts of things has gone wrong: either one of you has made a computational error of some kind (perhaps simply making a faulty inference, perhaps failing properly to attent to the evidence), or one of you has better evidence than the other. I want to make a claim about the latter sort of case. So suppose, for now, that you are the two most pedantic and punctilious persons you know. Suppose, further, that you have widely divergent backgrounds and perspectives on the question. It seems reasonable for you both to conclude on the basis of your divergent results that one of you has better evidence than the other. Suppose you both do so conclude.

Now lets call the two agents under discussion ‘you’ and ‘Sally.’ Suppose you are an equal weight view theorist. Then you should split the difference between your conclusion and Sally’s. Well and good. But: now suppose that Sally invites ten friends to reflect on the question. All twelve of you are seemingly equivalent epistemic agents. Suppose all ten of Sally’s friends independently come to the same conclusion as Sally. Should you move further towards her view?

Here is reason why you might not. You previously considered yourself and these eleven others to be seemingly equivalent epistemic agents. This allowed room for the possibility that some of the twelve had evidence or capacities that the others lacked. But the point was that no-one had any discernible epistemic advantage. Now you find that you come to one conclusion, and the other eleven come to some different conclusion. If you were epistemically identical, apart from your disposition to err, this result would almost certainly be reason for you to move more than half way to the other conclusion. But this case is different. Here there is the possibility that you have evidence that all the others lack, or on the other hand that all eleven of the others share exactly the same piece of evidence that you lack. Moreover, it is possible that you have some higher-order piece of evidence which indicates the likelihood of one of these results. And if so – and here’s the claim – you should move no further in your view for the eleven than you did for Sally. For you moved towards Sally on the basis of your assessment of the relevance of the evidence you judged that she had and you lacked. By hypothesis, her ten friends have come to their conclusions on the basis of this same piece of evidence. So if you changed your position on the basis of the indication from Sally that you were lacking this evidence, you need make no further change when you find others indicating that you were lacking this evidence. So, further, if Sally had introduced a hundred other friends, or a million, and they all agreed with each other and disagreed with you, still: if you judge it likely that either you have some evidence they all lack or they all share the same piece of evidence that you lack, then you should move no further from your view for the hundred, or the million, than for the one.

Now there is a further wrinkle here. We are talking about the significance of higher-order evidence – evidence that there is evidence – and it itself comes in degrees. Suppose that knowledge of the missing evidence would move you 10 points. Presumably the higher-order evidence from Sally is defeasible: it does not itself entail the first-order evidence. So it might be that Sally’s higher-order evidence only moves you 7 points. Then her friends would make a difference, for they would provide further higher-order evidence. If there were a million of them, you might be moved the full 10 points. This seems right: more higher-order evidence is more evidence of a sort. However note two things. Firstly, the missing evidence may be rather paltry. Perhaps the difference between your conclusion and Sally’s was 100 points. Then even all her friends’ conclusions would not move you more than 10% towards her view. Secondly, if you were certain of the relation between Sally’s higher-order and lower-order evidence then, as before, her million friends would move you not at all.

1 If you like we can further distinguish fine and coarse seemingly equivalent agents: fine agents are seemingly equivalent on each assessable parameter – background evidence, local evidence, etc.; these themselves can admit of more or less fine-grained interpretations – coarse agents are equivalent across the range. I don’t think this distinction affects the current point.

- Barry.