You’re right of course that Bostrom is not engaging with the problem you’re focusing on. But the context for discussing Boltzmann’s idea seems different from what he says about “freak observers”—the former is about arguing that the historically accepted objection to Boltzmann is best construed as relying on SSA, whereas the rationale for the latter is best seen in his J Phil piece: http://www.anthropic-principle.com/preprints/cos/big2.pdf ). But I’ll grant you that his argument about Boltzmann is suboptimally formulated (turns out I remembered it being better than it actually was).
However, there is a stronger argument (obvious to me, and maybe charitably attributable to Bostrom-sub-2002) that you seem to be ignoring, based on the notion that SSA can apply even if you can (based on your evidence) exclude certain possibilities about who you are. The argument doesn’t need the absurd inference from “lives in higher entropy region” to “observes higher entropy region” but rather needs Boltzmann’s model to suggest that “very few observers observe large low-entropy regions”. Since most Boltzmann brains have random epistemic states and can’t be described as observing anything (much less large low-entropy regions), the latter sentence is of course true conditional on Boltzmann’s model. Most observer’s don’t observe anything (and even the lucky ones who do tend to have tiny-sized low-entropy surroundings) so virtually all observers do not observe large low-entropy regions.
Anyhow, if a Boltzmann-brain-swamped scenario is true, a very tiny fraction of observers make our observations, whereas if the world isn’t Boltzmann-brain-swamped (e.g. if universes reliably stop existing before entering the de Sitter phase), a much less tiny fraction of observers make our observations. The SSA takeaway from this is that our observations disconfirm Boltzmann-brain-swamped scenarios.
You can of course exclude Boltzmann brains from the reference class, but this doesn’t get you very far. Much more rarely but still infinitely often, there will arise observers with some surrounding environments and veridical observations about those surrounding environments. Still, it seems very plausible that the typical observations of such observers are different from the expected observations of evolved beings, so they dominate the reference class and again the SSA argument goes through. For your move against this argument to be successful, you have to have good grounds for excluding all de Sitter phase observers. And based on my rudimentary understanding it seems that the variety of de Sitter phase observers won’t allow you to do that with general considerations like externalism. Hope this helps.
A question: why do you think that your non-qualitative evidence should be taken into account differently than qualitative evidence if SSA holds? At least for observers subjectively indistinguishable to ourselves, your choice is to exclude the one’s who don’t have our presumed external evidence from the reference class. Why not just include everyone in the reference class and then update based on both qualitative and non-qualitative/external evidence?
You’re right of course that Bostrom is not engaging with the problem you’re focusing on. But the context for discussing Boltzmann’s idea seems different from what he says about “freak observers”—the former is about arguing that the historically accepted objection to Boltzmann is best construed as relying on SSA, whereas the rationale for the latter is best seen in his J Phil piece: http://www.anthropic-principle.com/preprints/cos/big2.pdf ). But I’ll grant you that his argument about Boltzmann is suboptimally formulated (turns out I remembered it being better than it actually was).
However, there is a stronger argument (obvious to me, and maybe charitably attributable to Bostrom-sub-2002) that you seem to be ignoring, based on the notion that SSA can apply even if you can (based on your evidence) exclude certain possibilities about who you are. The argument doesn’t need the absurd inference from “lives in higher entropy region” to “observes higher entropy region” but rather needs Boltzmann’s model to suggest that “very few observers observe large low-entropy regions”. Since most Boltzmann brains have random epistemic states and can’t be described as observing anything (much less large low-entropy regions), the latter sentence is of course true conditional on Boltzmann’s model. Most observer’s don’t observe anything (and even the lucky ones who do tend to have tiny-sized low-entropy surroundings) so virtually all observers do not observe large low-entropy regions.
Anyhow, if a Boltzmann-brain-swamped scenario is true, a very tiny fraction of observers make our observations, whereas if the world isn’t Boltzmann-brain-swamped (e.g. if universes reliably stop existing before entering the de Sitter phase), a much less tiny fraction of observers make our observations. The SSA takeaway from this is that our observations disconfirm Boltzmann-brain-swamped scenarios.
You can of course exclude Boltzmann brains from the reference class, but this doesn’t get you very far. Much more rarely but still infinitely often, there will arise observers with some surrounding environments and veridical observations about those surrounding environments. Still, it seems very plausible that the typical observations of such observers are different from the expected observations of evolved beings, so they dominate the reference class and again the SSA argument goes through. For your move against this argument to be successful, you have to have good grounds for excluding all de Sitter phase observers. And based on my rudimentary understanding it seems that the variety of de Sitter phase observers won’t allow you to do that with general considerations like externalism. Hope this helps.
A question: why do you think that your non-qualitative evidence should be taken into account differently than qualitative evidence if SSA holds? At least for observers subjectively indistinguishable to ourselves, your choice is to exclude the one’s who don’t have our presumed external evidence from the reference class. Why not just include everyone in the reference class and then update based on both qualitative and non-qualitative/external evidence?