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Jack comments on Boltzmann Brains and Anthropic Reference Classes (Updated) - Less Wrong
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Of course we are. That's the big scary implication of the Boltzmann brain scenario. If you know a priori that you can't be a Boltzman brain then it is easy to exclude them from your reference class. You're entire case is just argument from incredulity, dressed up.
No, that is not the big scary implication. At least not the one physicists are interested in. The Boltzmann brain problem is not just a dressed up version of Descartes' evil demon problem. Look, I think there's a certain kind of skepticism that can't be refuted because the standards of evidence it demands are unrealistically high. This form of skepticism can be couched in terms of an evil demon, or the Matrix, or Boltzmann brains. However you do it, I think it's a silly problem. If that was the problem posed by Boltzmann brains, I'd be unconcerned.
The problem I'm interested in is not that the Boltzmann brain hypothesis raises the specter of skepticism; the problem is that it, in combination with the SSA, is claimed to be strong evidence against our cosmological models. This is an entirely different issue from radical skepticism. In fact, this problem explicitly assumes the falsehood of radical skepticism. The hypothesis is supposed to disconfirm cosmological models precisely because we know we're not Boltzmann brains.
Right. The argument is a modus tollens on the premise that we could possibly be Boltzmann brains. It's, a) we are not Boltzmann brains, b) SSA, c) cosmological model that predicts a high preponderance of Boltzmann brains: PICK ONLY TWO. Now it's entirely reasonable to reject the notion that we are Boltzmann brains on pragmatic grounds. It's something we might as well assume because there is little point to anything if we don't. But you can't dissolve the fact that the SSA and the cosmological model imply that we are Boltzmann brains by relying on our pragmatic insistence that we aren't (which is what you're doing with the externalism stuff).
My externalism stuff is just intended to establish that Boltzmann brains and actual humans embedded in stable macroscopic worlds have different evidence available to them. At this point, I need make no claim about which of these is me. So I don't think the anti-skeptical assumption plays a role here. My claim at this point is just that these two systems are in different epistemic situations (they have different beliefs, knowledge, evidence).
The rejection of skepticism is a separate assumption. As you say, there's good pragmatic reason to reject skepticism. I'm not sure what you mean by "pragmatic reason", but if you mean something like "We don't actually know skepticism is false, but we have to operate under the assumption that it is" then I disagree. We do actually know there is an external world. To claim that we do not is to raise the standard of evidence to an artificially high level. Consistent sensory experience of an object in a variety of circumstances is ordinarily sufficient to claim that we know the object exists (despite the possibility that we may be in the Matrix).
So now we have two premises, both arrived at through different and independent chains of reasoning. The first is that subjective indistinguishability does not entail evidential indistinguishability. The second is that I am not a Boltzmann brain. The combination of these two premises leads to my conclusion, that one might be justified in excluding Boltzmann brains from one's reference class. Now, a skeptic would attack the second premise. Fair enough, I guess. But realize that is a different premise from the first one. If your objection is skepticism, this objection has nothing to do with semantic externalism. And I think skepticism is a bad (and somewhat pointless) objection.
That's fine. But what matters is that they can't actually tell they are in different epistemic situations. You've identified an objective distinction between Boltzmann brains and causally-embedded people. That difference is essentially: for the latter the external world exists, for the former it does not. But you haven't provided anyway for a Boltzmann brain or a regular old-fashioned human being to infer anything different about the external world. You're confusing yourself with word games. A Boltzman brain and a human being might be evidentially distinguishable in that the former's intentional states don't actually refer to anything. But their subjective situations are evidentially indistinguishable. Taboo 'beliefs' and 'knowledge'. Their information states are identical. They will come to identical conclusions about everything. The Boltzmann brain copy of pragmatist is just as confident that he is not a Boltzmann brain as you are.
This statement is only true if you reject either the SSA or a cosmological model that predicts most things that are thinking the same thoughts I am are Boltzmann brains. Which is, like, the whole point of the argument and why it's not actually a separate assumption. The Boltzmann brain idea, like the Simulation argument, is much stronger than typical Cartesian skepticism and they are in no way identical arguments. The former say that most of the things with your subjective experiences are Boltzmann brains/in a computer simulation. That's very different from saying that there is a possibility an evil demon is tricking you. And the argument you give above for knowing that there is an external world is sufficient to rebut traditional, Cartesian skepticism but it is not sufficient to rebut the Boltzmann brain idea or the Simulation argument. These are more potent skepticisms.
Look at it this way: You have two premises that point to you being a Boltzmann brain. Your reply is that the SSA doesn't actually suggest you are a Boltzmann brain because your intentional states have referents and the Boltzmann brain's do not. That's exactly what the Boltzmann brain copy of you is thinking. Meanwhile the cosmological model you're working under says that just about everything thinking that thought is wrong.