Do you really think the assumption that the external world (as we conceive it) is real can’t be used in reasoning and decision making performed by us? It is used all the time, to great effect.
Do you think Bostrom is wrong to use anthropic reasoning as a basis for disconfirming Boltzmann’s model? After all, the assumption there is that Boltzmann’s model goes wrong in predicting that we are Boltzmann brains. We know that we’re not, so this is a bad prediction. This piece of reasoning seems to be something you deny we can actually do.
Given any particular instantaneous brain state, later evidence consistent with that brain state is evidence against the Boltzmann model, since random fluctuation is vastly more likely to generate something subjectively incoherent. With a sufficient volume of such evidence I’d feel comfortable concluding that we reside in an environment or simulation consistent with our subjective perceptions. But that doesn’t actually work: we only have access to an instantaneous brain state, not a reliable record of past experience, so we can’t use this reasoning to discredit the Boltzmann model. In a universe big enough to include Boltzmann brains a record of a causal history appears less complex than an actual causal history, so we should favor it as an interpretation of the anthropic evidence.
I’ll admit I find this subjectively hard to buy, but that’s not the same thing as finding an actual hole in the reasoning. Starting with “we know we’re not Boltzmann brains” amounts to writing your bottom line first.
Given any particular instantaneous brain state, later evidence consistent with that brain state is evidence against the Boltzmann model
It is evidence that the model of the past brain state being a Boltzmann brain is incorrect. It unfortunately can’t tell you anything about whether you are a Boltzmann brain now who just thinks that he had a past where he thought he might have been a Boltzmann brain.
Starting with “we know we’re not Boltzmann brains” amounts to writing your bottom line first.
Only if my argument is intended to refute radical skepticism. It’s not. See the update to my post. It’s true that the argument, like every other argument in science, assumes that external world skepticism is false. But I guess I don’t see that as a problem unless one is trying to argue against external world skepticism in the first place.
This seems confused. Boltzmann’s model only has any interesting consequences if you at least consider external-world skepticism; if you use a causal history to specify any particular agent and throw out anything where that doesn’t line up with experiential history, then of course we can conclude that Boltzmann brains (which generally have a causal history unrelated to their experiential history, although I suppose you could imagine a Boltzmann brain with a correct experiential history as a toy example) aren’t in the right reference class. But using that as an axiom in an argument intended to prove that Boltzmann brains don’t pose a problem to current cosmological models amounts to defining the problem away.
Here’s the structure of the purported problem in cosmology:
(1) Model X predicts that most observers with subjective experience identical to mine are Boltzmann brains.
(2) I am not a Boltzmann brain.
(3) The right way to reason anthropically is the SSA.
(4) The appropriate reference class used in the SSA must include all observers with subjective experience identical to mine.
CONCLUSION: Model X wrongly predicts that I’m a Boltzmann brain.
I am not attacking any of the first 3 premises. I am attacking the fourth. Attacking the fourth premise does not require me to establish that I’m not a Boltzmann brain. That’s a separate premise in the original argument. It has already been granted by my opponent. So I don’t see how assuming it, in an objection to the argument given above, amounts to writing my bottom line first.
Your objection assumes that we can distinguish observers by their causal history rather than their subjective experience, and that we can discard agents for whom the two don’t approximately correspond. This is quite a bit more potent than simply assuming you’re not a Boltzmann brain personally: if extrapolated to all observers, then no (or very few) Boltzmann brains need be considered. The problematic agents effectively don’t exist within the parts of the model you’ve chosen to look at. Merely assuming you’re not a Boltzmann brain, on the other hand, does lead to the apparent contradiction in the parent—but I don’t think it’s defensible as an axiom in this context.
Truthfully, though, I wouldn’t describe the cosmological problem in the terms you’ve used. It’s more that most observers with your subjective experience are Boltzmann brains under this cosmological model, and Boltzmann brains’ observations do not reliably reflect causal relationships, so under the SSA this cosmology implies that any observations within it are most likely invalid and the cosmology is therefore unverifiable. This does have some self-reference in it, but it’s not personal in the same sense, and including “I am not a Boltzmann brain” in the problem statement is incoherent.
This is quite a bit more potent than simply assuming you’re not a Boltzmann brain personally: if extrapolated to all observers, then no (or very few) Boltzmann brains need be considered. The problematic agents effectively don’t exist within the parts of the model you’ve chosen to look at.
I’m not sure what you mean by this. I’m claiming we need not consider the possibility that we are Boltzmann brains when we are reasoning anthropically. I’m not claiming that Boltzmann brains are not observers (although they may not be), nor am I claiming that they do not exist. I also think that if a Boltzmann brain were reasoning anthropically (if it could), then it should include Boltzmann brains in its reference class. So I don’t think the claims I’m making can be extrapolated to all observers. They can be extrapolated to other observers sufficiently similar to me.
Your objection assumes that observers’ subjective experience is generally a more or less reliable record of their causal history.
I hope this is not the case, since I don’t believe this. I think it’s pretty likely that our universe will contain many Boltzmann brain type observers whose subjective experience is not a reliable record of their causal history (or any sort of record at all, really). Could you clarify where my objection relies on this assumption?
Truthfully, though, I wouldn’t describe the cosmological problem in the terms you’ve used.
The problem is often presented (including by Bostrom) as a straight Bayesian disconfirmation of models like Boltzmann’s. That seems like a different argument from the one you present.
including “I am not a Boltzmann brain” in the problem statement is incoherent.
Why? The other three premises do not imply that I am a Boltzmann brain. They only imply that model X predicts I’m a Boltzmann brain. That doesn’t conflict with the second premise.
I hope this is not the case, since I don’t believe this
That was poorly worded. I’d already updated the grandparent before you posted this; hopefully the revised version will be clearer.
Why? The other three premises do not imply that I am a Boltzmann brain. They only imply that model X predicts I’m a Boltzmann brain. That doesn’t conflict with the second premise.
I was talking about my formulation of the problem, not yours. Assuming you’re not a Boltzmann brain does lead to a contradiction with one of my premises, specifically the one about invalid observations.
After all, the assumption there is that Boltzmann’s model goes wrong in predicting that we are Boltzmann brains. We know that we’re not, so this is a bad prediction. This piece of reasoning seems to be something you deny we can actually do.
That’s because it is. We DON’T know that we’re not Boltzman brains. There would be no possible way for us to tell.
Do you really think the assumption that the external world (as we conceive it) is real can’t be used in reasoning and decision making performed by us? It is used all the time, to great effect.
Do you think Bostrom is wrong to use anthropic reasoning as a basis for disconfirming Boltzmann’s model? After all, the assumption there is that Boltzmann’s model goes wrong in predicting that we are Boltzmann brains. We know that we’re not, so this is a bad prediction. This piece of reasoning seems to be something you deny we can actually do.
Given any particular instantaneous brain state, later evidence consistent with that brain state is evidence against the Boltzmann model, since random fluctuation is vastly more likely to generate something subjectively incoherent. With a sufficient volume of such evidence I’d feel comfortable concluding that we reside in an environment or simulation consistent with our subjective perceptions. But that doesn’t actually work: we only have access to an instantaneous brain state, not a reliable record of past experience, so we can’t use this reasoning to discredit the Boltzmann model. In a universe big enough to include Boltzmann brains a record of a causal history appears less complex than an actual causal history, so we should favor it as an interpretation of the anthropic evidence.
I’ll admit I find this subjectively hard to buy, but that’s not the same thing as finding an actual hole in the reasoning. Starting with “we know we’re not Boltzmann brains” amounts to writing your bottom line first.
It is evidence that the model of the past brain state being a Boltzmann brain is incorrect. It unfortunately can’t tell you anything about whether you are a Boltzmann brain now who just thinks that he had a past where he thought he might have been a Boltzmann brain.
Yeah, that’s what I was trying to convey with the second half of that paragraph. I probably could have organized it better.
Only if my argument is intended to refute radical skepticism. It’s not. See the update to my post. It’s true that the argument, like every other argument in science, assumes that external world skepticism is false. But I guess I don’t see that as a problem unless one is trying to argue against external world skepticism in the first place.
This seems confused. Boltzmann’s model only has any interesting consequences if you at least consider external-world skepticism; if you use a causal history to specify any particular agent and throw out anything where that doesn’t line up with experiential history, then of course we can conclude that Boltzmann brains (which generally have a causal history unrelated to their experiential history, although I suppose you could imagine a Boltzmann brain with a correct experiential history as a toy example) aren’t in the right reference class. But using that as an axiom in an argument intended to prove that Boltzmann brains don’t pose a problem to current cosmological models amounts to defining the problem away.
Here’s the structure of the purported problem in cosmology:
(1) Model X predicts that most observers with subjective experience identical to mine are Boltzmann brains.
(2) I am not a Boltzmann brain.
(3) The right way to reason anthropically is the SSA.
(4) The appropriate reference class used in the SSA must include all observers with subjective experience identical to mine.
CONCLUSION: Model X wrongly predicts that I’m a Boltzmann brain.
I am not attacking any of the first 3 premises. I am attacking the fourth. Attacking the fourth premise does not require me to establish that I’m not a Boltzmann brain. That’s a separate premise in the original argument. It has already been granted by my opponent. So I don’t see how assuming it, in an objection to the argument given above, amounts to writing my bottom line first.
Your objection assumes that we can distinguish observers by their causal history rather than their subjective experience, and that we can discard agents for whom the two don’t approximately correspond. This is quite a bit more potent than simply assuming you’re not a Boltzmann brain personally: if extrapolated to all observers, then no (or very few) Boltzmann brains need be considered. The problematic agents effectively don’t exist within the parts of the model you’ve chosen to look at. Merely assuming you’re not a Boltzmann brain, on the other hand, does lead to the apparent contradiction in the parent—but I don’t think it’s defensible as an axiom in this context.
Truthfully, though, I wouldn’t describe the cosmological problem in the terms you’ve used. It’s more that most observers with your subjective experience are Boltzmann brains under this cosmological model, and Boltzmann brains’ observations do not reliably reflect causal relationships, so under the SSA this cosmology implies that any observations within it are most likely invalid and the cosmology is therefore unverifiable. This does have some self-reference in it, but it’s not personal in the same sense, and including “I am not a Boltzmann brain” in the problem statement is incoherent.
I’m not sure what you mean by this. I’m claiming we need not consider the possibility that we are Boltzmann brains when we are reasoning anthropically. I’m not claiming that Boltzmann brains are not observers (although they may not be), nor am I claiming that they do not exist. I also think that if a Boltzmann brain were reasoning anthropically (if it could), then it should include Boltzmann brains in its reference class. So I don’t think the claims I’m making can be extrapolated to all observers. They can be extrapolated to other observers sufficiently similar to me.
I hope this is not the case, since I don’t believe this. I think it’s pretty likely that our universe will contain many Boltzmann brain type observers whose subjective experience is not a reliable record of their causal history (or any sort of record at all, really). Could you clarify where my objection relies on this assumption?
The problem is often presented (including by Bostrom) as a straight Bayesian disconfirmation of models like Boltzmann’s. That seems like a different argument from the one you present.
Why? The other three premises do not imply that I am a Boltzmann brain. They only imply that model X predicts I’m a Boltzmann brain. That doesn’t conflict with the second premise.
That was poorly worded. I’d already updated the grandparent before you posted this; hopefully the revised version will be clearer.
I was talking about my formulation of the problem, not yours. Assuming you’re not a Boltzmann brain does lead to a contradiction with one of my premises, specifically the one about invalid observations.
That’s because it is. We DON’T know that we’re not Boltzman brains. There would be no possible way for us to tell.