Two different arguments that try to show that a "brain in a vat"-type situation is more likely than a common sense realism situation. Unlike traditional skeptical arguments, these don't simply try to argue that a situation like this is possible, but that we could have more reason to believe that a situation like this is in fact the case. — Michael

The problem is in a sense that our reasoning and beliefs are the same for a common sense universe and the two other possibilities. In other words we have no ability to decide what is the case, because our reasoning and beliefs are the same in all variants.
So it leads to the 3rd answer as being the most reasonable:
1: We live in a common sense universe.
2. We live in a non- common sense universe.
3. We don't know.

I remember thinking about Bostrom's argument while puzzling over the Sleeping Beauty Problem, which grew into a massive thread on PhysicsForums before the mods got sick of it and locked it up.

I can't remember the details but I think I came to the conclusion that the 'two-thirds' position in that problem uses the same sort of argument as Bostrom, and that the argument makes unjustified assumptions. The problem is in how one determines what are 'equally likely events'. If one just assumes that whatever events one is talking about are equally likely then one can change the probabilities just by subdividing events and declaring the subdivided events equally likely to the un-subdivided alternative events. eg subdivide the event 'I am in a simulation' into events E(1), E(2), E(3) etc where E(k) is the event 'I am in a simulation and the program has between k thousand and (k+1) thousand lines of code'.

If I can recall my thoughts a bit more clearly I'll try to post them here because it was a bit of an epiphany for me. Up till then I had considered Bostrom's argument strong and could not point to a reason to reject it, even though I felt intuitively it was flawed. But I think I managed to convince myself that there was an explicit flaw one could point to, along the lines of my rather vague preceding paragraph.

I'm a 'halfer', by the way. Or at least I recall that I was, the last time that I understood the question.

I don't see why (1. or 2. or 3.) have to be almost certainly true. Can you explain it in a few words?

A posthuman stage is one that is capable of running high-fidelity ancestor simulations. So there are three options: 1. we never get to that stage, 2. we get to that stage but decide not to run them, or 3. we get to that stage and decide to run them.

Given 3., "... if even a tiny percentage of them were to run 'ancestor simulations' (that is, 'high-fidelity' simulations of ancestral life that would be indistinguishable from reality to the simulated ancestor), the total number of simulated ancestors, or 'Sims', in the universe (or multiverse, if it exists) would greatly exceed the total number of actual ancestors."

If there are more Sims than there are non-Sims then we're more likely to be Sims.

I suppose the only "out" here is to claim that there wouldn't be enough computing power to simulate that many ancestors, and so that the number of actual ancestors is still greater.

Or you could reject a physicalist account of consciousness and argue that simulated consciousness is impossible.

I get it now, thanks!

Any thoughts — Michael

There are vastly more Boltzmann brain scenarios that are inconsistent than there are that are consistent. Tegmark suggests for example, if our memories recall a famous piece of music as white noise then we might well have reason to believe we are a BB. Otherwise, not.

There are vastly more Boltzmann brain scenarios that are inconsistent than there are that are consistent. — Jake Tarragon

Sure, but then there are vastly more Boltzmann brain scenarios that are consistent than there are non-Boltzmann brain scenarios (or at least that the former are more likely).

Tegmark suggest, for example, if our memories recall a famous piece of music as white noise then we might well have reason to believe we are a BB. Otherwise, not.

Given the above, I don't think so. We only have reason to believe that we are not in an inconsistent Boltzmann brain scenario.

Given the above, I don't think so. We only have reason to believe that we are not in an inconsistent Boltzmann brain scenario. — Michael

Given you agree that an inconsistent BB is far more likely than a consistent one, we should expect to live in an inconsistent one rather than a consistent one, regardless of whether BBs outnumber non BBs.

Given you agree that an inconsistent BB is far more likely than a consistent one, we should expect to live in an inconsistent one, regardless of whether BBs outnumber non BBs. — Jake Tarragon

Sure, but we can see that we don't. So we can rule out that possibility. That then leaves us with a consistent BB or no BB, with the former more likely.

Sure, but we don't. — Michael

And there's the rub ....
I think you would struggle to show that there are more consistent BB scenarios than there are suitable variant universes of all the physical constants, say....

And there's the rub ....
I think you would struggle to show that there are more consistent BB scenarios than there are suitable variant universes of all the physical constants, say.... — Jake Tarragon

That's what Boltzmann does: "The number of self-aware brains that spontaneously and randomly form out of the chaos, complete with memories of a life like ours, should vastly outnumber the brains evolved from an inconceivably rare local fluctuation the size of the observable universe."

His argument (from physics) isn't just that spontaneous brains are more likely, but that spontaneous brains with memories of a life like ours are more likely.

the size of the observable universe." — Michael

yeah, but that's titchy!

As philosophical arguments for skepticism, the two arguments are plainly incoherent. It doesn't make sense to doubt our knowledge of the external world (or its existence) by appealing to aposteriori premises that themselves could be known only if we presuppose that we do have knowledge of the external world (or that it exists).

His argument (from physics) isn't just that spontaneous brains are more likely, but that spontaneous brains with memories of a life like ours are more likely. — Michael

Is this spontaneity a form of last Thursdayism? Also, the spontaneous brains would involve the formation of an environment I can survive in long enough to remember, and will likely involve interaction with other people and technology, such as what I'm using right now.

Or you could reject a physicalist account of consciousness and argue that simulated consciousness is impossible. — Michael

You could reject that consciousness could be emulated (not just simulated where the Sims act as if they were conscious) and still be a physicalist about it. Physicalism doesn't commit one to functionalism about subjectivity. Searle would probably reject the simulation argument, for example.

As philosophical arguments for skepticism, the two arguments are plainly incoherent. It doesn't make sense to doubt our knowledge of the external world (or its existence) by appealing to aposteriori premises that themselves could be known only if we presuppose that we do have knowledge of the external world (or that it exists). — Fafner

They say nothing about doubt or knowledge. They only say that it's more likely that we're simulations or Boltzmann brains. Doubt may follow from recognising this, but that's just a side effect of the arguments.

Regarding the Boltzmann brain paradox, that it depends on a posteriori premises is probably why it's considered a paradox. If our knowledge of the external world is genuine then it's more likely that we're Boltzmann brains.

And what a posteriori premise(s) does the simulation argument use?

They say nothing about doubt or knowledge. They only say that it's more likely that we're simulations or Boltzmann brains. — Michael

But what does a 'simulation' mean? Doesn't it entail that all your experiences are illusory?

As philosophical arguments for skepticism, the two arguments are plainly incoherent. It doesn't make sense to doubt our knowledge of the external world (or its existence) by appealing to aposteriori premises that themselves could be known only if we presuppose that we do have knowledge of the external world (or that it exists). — Fafner

Yes, that is the point for all aposteriori premises including your version, that we can trust our experiences. They are all begging the question and so are you. All claims to what the world really is as far as claims of knowledge beg the question. They all show that the trust in reason and the experience are limited.

It is funny though that you can't spot the problem in your own claim to knowledge.

But what does a 'simulation' mean? Doesn't it mean that all your experiences are illusory? — Fafner

If all your experiences are illusory, are they then real and how the world works?

It is funny though that you can't spot the problem in your own claim to knowledge. — Mikkel

Where did I make such claims?

What post-human civilization would create simulations of waiting in a doctor's office for hours before being called into the exam room? or of waiting for hours in a waiting room while your vehicle was being repaired? or the hours it takes to drive cross-country? or any other menial, boring job or event that takes hours when we usually play simulations on our phones or in our heads to pass the time during these events in order to escape the boredom and pass the time.

So what the OP is telling us is that these events are boring to us but meaningful and enjoyable to a post-human civilization? Why would the post-human civilization have simulations inside simulations to pass the time in the boring parts of the first simulation? That doesn't sound very efficient to me.

Supposedly a Jupiter-sized computer could perform one million ancestor simulations a second, so I guess all the boring detail isn't that big a deal, and probably would just be for research purposes, such as alternate histories or figuring out things traditional history lacks the data for.

But whether a Jupiter-sized computer could actually be built and perform such incredible feats is pure speculation. And whether an advanced civilization would be motivated to build it, who knows.

Where did I make such claims? — Fafner

You claim by extension of your demands for logic and e.g. coherence in regards to knowledge, that knowledge is possible with logic and e.g. coherence, unless your point is that there is no knowledge, which abides to logic.
So I suspect that you believe that there is such a sensible concept as knowledge and that it must abide to reason, logic and so on.
So if knowledge must abide to reason, logic and so on, then that is also the case for your claims to knowledge. So what do you claim about knowledge?
That there is a world, which is non-solipsistic and for which our experiences (in general) match the world. The problem is that is begging the question and thus not logical.
So for once, try to give reason, logic and evidence for the fact, that there is a screen in front of you, which you read this on and don't follow into the following trap:
It is absurd, if we can't trust our senses, therefore we can trust our senses.

P1: We can't trust our senses.
P2. That is absurd.
Conclusion: Therefore we can trust our senses.
As a deduction it is invalid.

PS: "How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?" Sir Arthur Conan Doyle

You completely misunderstood my argument (from that other thread on skepticism). I didn't assume that we have knowledge, or that there is a world, but I made an internal criticism of the skeptical argument, and that's a different thing. But I don't want to go into details since I already explained the main idea in my OP, and I don't think it's appropriate to discuss it here.

You completely misunderstood my argument (from that other thread on skepticism). I didn't assume that we have knowledge, or that there is a world, but I made an internal criticism of the skeptical argument, and that's a different thing. But I don't want to go into details since I already explained the main idea in my OP, and I don't think it's appropriate to discuss it here. — Fafner

You in this thread dismiss an argument, because it is incoherent, That is fair and well, but it misses the following possibility: That all claims with knowledge in mind about what the world is, is not possible with reason and/or logic, because all such claims run into Agrippa's Trilemma.
In other words, it only matters that a specific argument is incoherent if you can make an argument for a given area which is within reason and/or logic. So I am trying to get you to understand that all strong claims to metaphysics and knowledge run into Agrippa's Trilemma.

In general words as in the regards to regards to knowledge within methodological naturalism, then we, humans, live inside a cognitive bubble. We can't know what the world is, we just hold differently beliefs about what the world is.

because all such claims run into Agrippa's Trilemma. — Mikkel

What's "Agrippa's Trilemma"? Would you mind explaining?

What's "Agrippa's Trilemma"? Would you mind explaining? — Fafner

Agrippa's Trilemma is a variant of skepticism about knowledge.
It can be explained in the following manner:
When you claim something, you ask yourself how you know that. Then you make a reasoned argument about that and repeat - "How do I know that?". When you continue to do that, you realize that you run into these 3 problems. You will do one of the following:
#1: Run into an infinite regress turning to ground your knowledge in something, for which you can stop asking how do I know that.
#2: You dogmatically declare that it is so.
#3: You beg the question.

So as a Skeptic, I have stopped claiming I have knowledge and just explain what I believe in.
Relevant to this thread, I believe that there is a world like you believe, but I don't know that nor do I claim that I know it. I believe it.

When you claim something, you ask yourself how you know that. Then you make a reasoned argument about that — Mikkel

The obvious answer to this is to say that not all knowledge is based on arguments with premises as you claim. You can know many things non-inferentially, say by basing your beliefs on a perceptual experience which you take to reveal to you directly how things are in the world, and not by virtue of being part of some argument.

The obvious answer is to say that not all knowledge is based on arguments with premises. You can know many things non-inferentially, say by basing your beliefs on a perceptual experience which you take to reveal to you directly how things are in the world (and that means that there's another horn to the trilemma). — Fafner

That is a variant of a dogmatic claim. There is a screen in front of me, because that is how it is and it is not worth questioning and anyway it is absurd, illogical, meaningless, wrong and what not to question if there is a world as I know there to be. THERE IS A SCREEN IN FRONT OF ME - PERIOD!!! Stop asking silly questions. :)

But that's not what I'm saying. I'm not saying that your argument is wrong because I know that there is such and such in front of me because I can see. My point is that you argument makes an unwarranted assumption, that all reasons to believe must take the form of an argument. Of course arguments have to stop somewhere (as the regress problem shows), and my claim is that sense experience would be a very plausible stopper. It does not follow however that stopping at sense experience deprives you of the right to claim that you know (or that it would be dogmatic to do so), since again, I don't see any reason to assume in advance that arguments are the only legitimate grounds for knowledge.