In comments on the recent discussion of epistemic peers, I gave an argument that I want to give in a slightly cleaner form. It's not a hugely earth-shattering argument, but it's a nice little guideline in this sort of discussion.
Definition of 'Epistemic Peer' (more or less)
Two people are epistemic peers precisely when they are equal in such background, ability, inquiry-relevant virtues, and means as are relevant for evaluating evidence and drawing correct conclusions about a given topic.
: NB! This means that 'being an epistemic peer of' just means 'being equal to, in epistemic matters'. Thus the relation 'being an epistemic peer of' has all the basic features of the relation of equality:
It is reflexive: Everyone is epistemic peer of themselves.
It is symmetric: If you are epistemic peer of anyone on a given topic, they are an epistemic peer of you on that same topic.
It is transitive: If A is an epistemic peer of B on a given topic, and B is an epistemic peer of C on that same topic, then A is an epistemic peer of C on that same topic.
It is antisymmetric: If A and B are epistemic peers, they are for all relevant purposes not distinct -- you can interchange them without changing anything essential for evaluating evidence and drawing correct conclusions.
: Also, it is worthwhile to keep in mind that, despite analytic tendencies to put everything in alphanumeric symbols, all accounts and definitions of epistemic peers are left somewhat vague, since what makes someone your epistemic peer is merely whatever is relevant for evaluating evidence and drawing correct conclusions about the topic at hand.
Definition of Equal Weight View
If a person, A, comes to a conclusion, and another person, B, comes to a contrary conclusion on the same topic, and A and B are epistemic peers, and A discovers that B is drawing a contrary conclusion, A, knowing that B is his or her epistemic peer, should give B's conclusion equal weight with his or her own.
: There are complications in making this very precise, as well. Jehle and Fitelson have a paper (PDF) in which they consider various versions of what one might mean by 'equal weight'.
A Simple Argument
(1) Epistemic peers are epistemically indistinguishable from oneself.
: From the more-or-less defintion of what it is to be an epistemic peer.
: Epistemically indistinguishable, of course, meaning that they are equal in all relevant epistemic respects.
(2) Therefore any position your epistemic peer holds that is contrary to your own is a position you yourself could hold, whether by mistake or by chance differences in assessment of evidence or reasoning used.
: Conceivably there might be other ways in which deviation could occur, but they would just make the premise more complicated without changing anything fundamental.
(3) Therefore any disagreement between you and an epistemic peer is possible solely to the extent that you are able to come to either the position you hold or the contrary position your epistemic peer holds.
: And therefore talk of epistemic peers is not fundamentally different from talking about your own ability to come to different conclusions than you actually do. Indeed, it can't be, by the properties of the relation: for all relevant purposes, your epistemic peers are just like you, and could as easily be considered duplicates of you. If they can draw a conclusion, so can you.
(4) Thus if you should give equal weight to the conclusions of disagreeing epistemic peers, you should give equal weight to any contrary conclusions you could draw.
: From (3), adding the hypothetical of giving equal weight to disagreeing epistemic peers.
(5) Therefore the equal weight view implies that you should suspend judgment about any matter on which you might come to a contrary conclusion, whether through mistake or chance differences in reasoning.
: Strictly speaking, the equal weight theorist could get around this by saying that all your possible selves get a vote, and the majority wins. So, for instance, if there are 100 possible versions of you and 99 come to one conclusion and only 1 deviates, giving them all equal weight makes you, in a sense, 99% certain of the position. But since we have no way of identifying how many discrete possible selves you have -- indeed, the whole question may be nonsensical -- and since, epistemic peers being interchangeable, all of your possible selves that accept the same conclusion can always be treated as one, this doesn't seem to be a viable way out.
: Again, if there are other ways you might come to a contrary conclusion without changing your background, abilities, virtues, and means, feel free to add them.
(6) It is absurd to suspend judgment about everything about which one could be mistaken or about which one cannot be perfectly certain.
: Even Descartes didn't go so far: although he thinks you can only legitimately do it a certain way, he does salvage certain kinds of belief in uncertain matters (in Meditation VI).
: It's arguably not even psychologically possible to do such a thing.
: We can have quite good reasons for accepting a conclusion even when we know we might have made a mistake somewhere, and even when we know we can only establish it with some probability.
(7) Therefore the equal weight thesis is absurd.
: Of course, given the vagueness besetting the whole discussion, there might be variations that avoid this conclusion. But an argument of this sort gives us a reference point for discussion.
