Firstly, I’d say that the most common point of disagreement between you and the people who have responded in the thread is not that they take skepticism more seriously than you, it is that they disagree with you about the implications of semantic externalism. You say “Subjective indistinguishability does not entail evidential indistinguishability.” I think most people here intuitively disagree with this, and assume your “evidence” (in the sense of the word that comes into Bayesian reasoning) includes only what you “know” in the subjective, introspective sense, not the externalist sense. (E.g. I interpret in this way the comments of Jack, Vladimir and ryan_sandwich.) It might be more clear if you had made two posts, or one divided in two parts, one explaining clearly your take on externalism and its conception of evidence, and one explaining its relevance to the Boltzmann Brains (BB) problem.
Secondly, I think the BB problem is a more serious argument for skepticism than the standard evil demon or brain-in vat scenarios. It allows us (which the other scenarios don’t) to make an argument of this form: “Either current physics as I understand it is essentially correct, or it is not. If not, I know nothing about the universe. If it is, it implies that I am very likely to be a BB, in which case I know nothing about the universe. So I know nothing about the universe.” I’m not defending this argument as valid, just saying that it seems to be in a different class from saying “my experience is indistinguishable from that of a brain-in-vat, so I don’t know if I am one or not”.
I think most people here intuitively disagree with this, and assume your “evidence” (in the sense of the word that comes into Bayesian reasoning) includes only what you “know” in the subjective, introspective sense, not the externalist sense.
Yeah, I’m getting this now, and I must admit I’m surprised. I had assumed that accepting some form of semantic externalism is obviously crucial to a fully satisfactory naturalistic epistemology. I still think this is true, but perhaps it is less obvious than I thought. I might make a separate post defending this particular claim.
You’re right that the BB-based skeptical argument you offer is a different argument for skepticism than brains-in-vats. I’m not sure it’s a more serious argument, though. The second premise in your argument (“If current physics is not essentially correct, I know nothing about the universe.”) seems obviously false. Also the implication that I am very likely to be a BB does not come just from current physics. It comes from current physics in conjunction with something like SSA. So there’s a third horn here, which says SSA is incorrect. And accepting this doesn’t seem to have particularly dire consequences for our epistemological status.
Two points, in response to your update:
Firstly, I’d say that the most common point of disagreement between you and the people who have responded in the thread is not that they take skepticism more seriously than you, it is that they disagree with you about the implications of semantic externalism. You say “Subjective indistinguishability does not entail evidential indistinguishability.” I think most people here intuitively disagree with this, and assume your “evidence” (in the sense of the word that comes into Bayesian reasoning) includes only what you “know” in the subjective, introspective sense, not the externalist sense. (E.g. I interpret in this way the comments of Jack, Vladimir and ryan_sandwich.) It might be more clear if you had made two posts, or one divided in two parts, one explaining clearly your take on externalism and its conception of evidence, and one explaining its relevance to the Boltzmann Brains (BB) problem.
Secondly, I think the BB problem is a more serious argument for skepticism than the standard evil demon or brain-in vat scenarios. It allows us (which the other scenarios don’t) to make an argument of this form: “Either current physics as I understand it is essentially correct, or it is not. If not, I know nothing about the universe. If it is, it implies that I am very likely to be a BB, in which case I know nothing about the universe. So I know nothing about the universe.” I’m not defending this argument as valid, just saying that it seems to be in a different class from saying “my experience is indistinguishable from that of a brain-in-vat, so I don’t know if I am one or not”.
Yeah, I’m getting this now, and I must admit I’m surprised. I had assumed that accepting some form of semantic externalism is obviously crucial to a fully satisfactory naturalistic epistemology. I still think this is true, but perhaps it is less obvious than I thought. I might make a separate post defending this particular claim.
You’re right that the BB-based skeptical argument you offer is a different argument for skepticism than brains-in-vats. I’m not sure it’s a more serious argument, though. The second premise in your argument (“If current physics is not essentially correct, I know nothing about the universe.”) seems obviously false. Also the implication that I am very likely to be a BB does not come just from current physics. It comes from current physics in conjunction with something like SSA. So there’s a third horn here, which says SSA is incorrect. And accepting this doesn’t seem to have particularly dire consequences for our epistemological status.