Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas - 345 | Adam Elga on Being Rational in a Very Large Universe

Episode Date: February 23, 2026

Behaving rationally involves facing up to conditions of uncertainty; we never navigate the world with perfect confidence. Sometimes we are uncertain about the way the world is, but we can also be uncertain about our place within the world. This kind of situation arises in cosmology (where the relevant world can extend very far in space or time), and also in quantum mechanics (where new worlds might be created at any measurement), but also when we are simply unsure about the future history of humanity or whether we live in a computer simulation. I talk with philosopher Adam Elga about how to deal with these unique kinds of uncertainties. Upgrade your denim game with Rag & Bone! Get 20% off sitewide with code MINDSCAPE at www.rag-bone.com #ragandbonepod #sponsored Blog post with transcript: https://www.preposterousuniverse.com/podcast/2026/02/23/345-adam-elga-on-being-rational-in-a-very-large-universe/ Support Mindscape on Patreon. Adam Elga received his Ph.D. in philosophy from MIT. He is currently a professor of philosophy at Princeton University. His research involves decision and game theory, epistemology, philosophy of probability, philosophy of mind, and philosophy of science. Web site Princeton web page Google Scholar publications PhilPeople profile

Hey, everyone, it's Cal Penn. I'm inviting you to join the best-sounding book club you've ever heard with my podcast, Earsay, the Audible and I-Heart Audio Book Club. Every episode, I nerd out with amazing guests and dive into the best new audiobooks available on Audible. It's the book club for your ears. Listen to Earsay, the Audible and I-Heart Audio Book Club. On the I-Heart Radio app or wherever you get your podcasts. Exema's Unrelenting, Itch and Rash.

If you know the feeling, you should know the facts. The eczema medication you're taking may not be right for you. Visit my raw truth.com and talk to your dermatologist about your symptoms and treatment options. Hello, everyone, and welcome to the Mindscape podcast. I'm your host, Sean Carroll. One of the things we've talked about many times on the podcast is how you update your beliefs when new evidence comes in. That is to say, the process of Bayesian reasoning. Bays' formula, of course, gives you a quantitative way of saying if I have some prior credence for some claim being true, and I very quantitatively measure some data, and I can calculate the likelihood of that data being obtained under all sorts of different propositions being true, I can update my credences to get one that takes that data into account.

we don't necessarily every time work in such a quantitative vein, but this process is basically what we do in science, right? In science, we have different kinds of theories that propose to provide explanations for different kinds of phenomena, and we have different feelings. Some theories are more likely than others. My favorite example is always is the dark matter, something like a weekly interacting massive particle, a wimp, or something like an axiom. So these are two different particle physics candidates for the dark matter. They're both plausible. We don't have any idea which one is true or even if it's some other theory. But we have favorites, right?

We don't give them equal probability because maybe it fits in better to other things we know, et cetera. So that seems like a pretty straightforward kind of process. You have prior probabilities for theories being true or whatever. And then you get more data and you update your belief, your degree. of belief, your credence. Here's a puzzle. What if you're a cosmologist? What if you're thinking about the whole universe all at once? And someone says, okay, I have two cosmological models, two theories that describe all of the universe at once, and they predict statistically more or less the same local conditions that we observe. So they are compatible with the data that we

already have. But here's the difference. In one theory, the universe is bigger than in the other one. Like maybe in one theory, the universe is a closed universe, a sphere or a torus or something like that, and it doesn't actually extend very far beyond the universe that we can see today. In the other theory, the universe is open, it goes on forever, and there's just an infinite number of things going on. And this person says, so I think that the theory where the universe is bigger is much more likely. And you say, well, why is that? Is it because there's some mechanism that gives you that or whatever? And they say, no, it's from updating on the data. And you say, what is that data? And they say, well, the data that I exist. Because in the bigger

universe, it is just much more likely that someone like me would exist than in the smaller universe, just because, you know, there's random fluctuations because of quantum mechanics. It's unlikely in any one small universe that I would exist. But as the universe becomes bigger and bigger, the chances of someone just like me get larger and larger. Is that kind of reasoning correct in the cosmological context? The answer is we don't know, or at least we don't have an agreed upon procedure for dealing with these kinds of puzzles. And they show up these kinds of puzzles again and again. You can guess that things like the Boltzman Brain scenario, where there's random fluctuations that create observers like us in the far future, or maybe

in the far past, but ones that don't arise via thermodynamically sensible evolution from a low entropy big bang, like we think we did. There's examples like the multiverse of Everettian quantum mechanics. When I measure the spin of an electron, and it could be spin up or spin down, I'm saying, okay, now there's a spin up particle, spin down particle. There are two separate worlds, and I want to say, what's the probability that I'm in one world or the other. Does it matter how thick the world is, or the different things that come into consideration here? So this is obviously a set of puzzles that is very relevant for cosmology

and physics, you know, real things we care about, quantum mechanics, the multiverse, things like that, as well as for philosophers who want to know how should we be rational in situations like this, how should we reason in these circumstances of uncertainty? It's a tough one, and today's podcast guest is one of the world's experts on these issues. Adam Elga is a philosopher at Princeton, and one of his, he has several very well-known papers. One of them is on the famous Sleeping Beauty Problem, where you say you flip a coin, and if it's heads, you're going to wake up Sleeping Beauty on Monday. If it's tails, you're going to wake up on Monday and Tuesday.

Do you give equal credence to all three possibilities, heads in Monday, tails in Monday, Tales in Tuesday? Or do you say, no, the coin is 50-50. It's a fair coin. I think that it's still 50% chance that I was in the heads universe versus the tails universe, even though I only wake up once if I'm in the heads universe. Tough call. Smart people disagree about this. Super relevant, even though it's just a fun philosophical thought experiment for questions of modern physics and cosmology. So we might not get the answers here. Adam and I, you know, we had to restrain ourselves a little bit because we both care about many of the same things and have written academic papers about many of the same things. So we geek out a little bit about these things

we care about. Hopefully that's up your alley. If not, it's at least inspirational to go and read and think more about these things. So let's go. Adam Algo, welcome to the Mindscape Podcast. Thanks so much for the invitation. You know, I know your work through papers about Boltzman brains, self-locating uncertainty. These are topics. that are near and dear to my heart, and we're certainly going to be talking about them. But you have a bigger scope in your philosophical career. How would you characterize sort of what your project is to the extent that one has a project? I characterize myself as addicted to rationality and started as a young age.

Around my household, they sometimes, when I was a kid, they were called me Mr. Rational. And I've always been fascinated with probability doing the rational things. thing, what's justified, optimizing. And I think that's the thread that runs through my, my philosophy. So I've thought about direction of time, and in particular, how actions bear on what happens in the future and the past, what you should think about various temporal asymmetries. And also I've thought about, with some co-authors, Dutch book arguments and money pumps, and game and decision theory. Mr. Rationality is not the worst nickname one could add. I mean, I mean, I

You can that it's a little bit of a dig, right? But it could be much worse. There was some mockery there. But now you found your people, you're a professional philosopher, and the whole rationality thing is probably simply a compliment. I'll take it. So let's talk about that just a little bit. I know that one of the papers that you've written is about how we should be rational

in terms of talking to other people who have knowledge about things, You know, people we might think of as experts in something or maybe peers who are equal to us. It's very rare that we have an opinion. We meet someone who we think is just as smart as we are with a different opinion and therefore we change our opinion. But what is a perfectly rational person supposed to do here? This is great. And also, it's super. Given what we will talk about later in the day, we can plant a seed that's really going to connect.

So there's a great philosophical applied philosophy question about how you should respond when someone who you antecedently considered smart, well informed and so on, came to a different conclusion than you on the basis of similar evidence. And on the one hand, there are the people who basically think whoever's in fact right should pretty much stick. to their guns or that should count for something extra. So that's the kind of stick to your guns side of things. And then on the other side, there are the people who think, well, given that one of us is wrong, I had no antecedent reason to think that I would be the one who is right. And certainly, finding out that we disagree shouldn't be evidence that I was the one who was right. And so I should stick with that prior assessment, those prior conditional assessments, and basically often significantly move in the direction of the person who came to the contrary conclusion.

But people don't actually do that, do they? Sad to say, although there's a kind of escape route here. There are different versions of the view, but the version that I like, and I've been influenced by David Christensen's writing on this, is the version that says, you basically should defer to what your prior self would have thought. So here's a case. You come in, you encounter the big disagreement. And then imagine getting on the time travel phone with your past self.

And you ask your past self, hey, suppose this were to happen. Suppose you were to find out that you and this person were to disagree in such and such way. We can't in the phone, we don't specify the full story about all the evidence and all the arguments because then we'd just be reproducing the original problem. But you give enough, you know, they say something that strikes me as totally wacko. They say a certain kind of coarse characterization. And then we ask that past version of yourself, what would you think conditional on that? You know, if that were to happen, how likely do you think it would be that you're the one who's right versus that?

And my feeling is you should defer to that person. And the reason why this doesn't immediately amount to a total like wishy-washiness in the face of everyone believing everything and just giving up your entire world view and just becoming a kind of big averaging machine is that in fact, in many cases, many of us are rather non-even handed in our answers to those questions. You'd think, what would you think if so-and-so thought, blah, blah, blah. And often the answer is, wow, even though I would say, like, in polite company, I would say, that person's smart. And I can't point to any encyclopedias that I've read that they haven't read. But I really am honest and think, what would I say if this person disagreed? I would think, you know, I guess I'd think they're probably wrong.

And the version of the equal weight view, which is what's this side of the view is sometimes called, the version that I like best is the one that defers to your past self in that way. And that's not quite as concessive as a more extreme version of view, which says, really, just always go 50-50. So what is, I'm a little confused as to why my past self is useful here. If I tell my past self all the relevant new information I've gotten, isn't that just my present self? Yes. And that's why the relevant question you have to be asking your past self is something less than the fully specified original question and the full situation. One way to motivate the view is to think about a David Christensen case in which the equal weight view or this kind of view is very intuitive.

It's called the split the check case. And the case is you're out to dinner with your friends. The bill comes in. People do the arithmetic on their own and then they get different answers. Now you think, okay, how confident should I be that my answer was right? their answer was right, given this disagreement. It's very intuitive that the answer to that question should match the answer to the question of if you'd asked yourself at the beginning of the meal, look, suppose we later split the check and you get this, blah, blah, blah, blah, you get this answer.

How likely do you think that you'll be the one who's right? That seems so long. And actually notice that that even handles a slightly more general case than the one we were talking about before, because that actually nicely handles the case in which, for example, I think I'm not so great at math. I think I'm just like, you know, probably 90, 10 that they're the one that's right. That seems intuitively like the thing to do when that scenario actually happens. And in order for that test to work, you can't be giving the full math question to your past self. Okay, good. You're just asking them about their basic judgment ahead of time.

What is the likelihood that someone's going to be wrong? Is it going to be me? Yeah, and you want to tell them something about the circumstances. For example, there's a difference between you get. $20 and your friend gets $23. And you get $20 and your friend says it's negative $18. Then you think, like, so the relevant question you'd ask your past self in that case would be, hey, what if I get an answer that seems kind of reasonable to me? And they get an answer that seems like totally bonkers, off the wall, you know, couldn't possibly right. In that case,

what do you think? And the answer to that question, then I think should rule the day when that situation actually happens. So am I correct in thinking that this strategy is meant to kind of be a semi-practical implementation of the idea that we should have our credences in different propositions changed by the right amount when we meet other equally smart people with different credences? I love that characterization. I think my opponents would disagree. Ask yourself, what are your best people spending their time on right now. Expense reports, receipt chasing, month in close that takes weeks.

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Hey, everyone. It's Cal Penn. I'm the host. of Earsay, the Audible and I Heart Audiobook Club. This week on the podcast, I am sitting down with Ray Porter, the narrator of Andy Weir's audiobook Project Hail Mary, massive sci-fi adventure about survival and science, and what happens when you wake up alone very far from Earth? I really had to make a decision because I caught myself getting that frog in my throat

and starting to get teary as I'm narrating some of these sections, and it's like, okay, yo, yeah, yo, is this? indulgent. And I really thought about it. I was like, no, at this point, it would kind of be betraying the trust the author and the listener have in telling this story if I don't go through it. But there's places in this book that deeply emotionally affected me, and I left it on the mic. That's great. Because it served the story. People will say like, oh my God, I cried at the end. It's like, yeah, dude, me too. Listen to Earsay, the Audible and IHeart Audio Book Club on the IHeart radio app or wherever you get your podcasts.

I see that this view meets that standard, but I think that's the kind of thing everyone's going for. Okay. I mean, I'm trying to imagine in my head examples which certainly exist where people who I think are smarter than me and are experts in some area have a belief close to that area and yet I disagree with them. I'm trying to justify why I think it's okay to disagree with them. I mean, I guess I think that I can identify some blind spot that.

they have or something like that. But is that likely to be me just fooling myself? Well, I think there could be two things going on. And it's actually a very, this exercise of looking back, what would my past self say? I think it's theoretically valuable. But I also think it's actually a really good practical mental exercise to do because you can diagnose what's going on with your own later favoring of your own views

if that's what happens. So one thing is, it might be that it was kind of a polite fiction that you really treated the person as peer in the first place. And there are a lot of things that could be true of the person. You think this person's really smart, well-intentioned, blah, blah, blah. It could have been that all along you thought, yeah, but when it comes down to it, if we disagree, I already was going to think it's more likely that I would be the one who was right. That could be right.

another thing that could be is no, you really thought they were just as good as you. And antecedently, you thought conditional on us disagreeing. It really is, I think it's really a coin toss about who would be right. But then when the time comes, maybe irrationally, if my view is right, you end up sticking to yourself. The thing that I really want to rule out is the idea that you antecedently thought, oh, it's 50-50. But then when it actually happens, you think, you know what? I'm right. I'm right.

Notice a really weird thing that could happen in this case. There could be this kind of bootstrapping confirmation that you're better than someone at stuff. Because then look what happens over the course of many disagreements. If you did that every time, you'd think, hey, well, I got that one right. I got this one right. I'm assuming that there's no independent confirmation. So there's just a series of disagreements. All you have to go on is what has happened so far.

you will then check off and think, oh, wow, I got all these right. And I'll think, wow, I'm even really better than I previously thought. That seems like things have gone off the rails. Yeah, that's a problem. I do find that in, again, ordinary human conversation among non-Mr. and Ms. rationales, people will sometimes have a criticism of the form. You always think you're right. And, I mean, if that means the opinion that I have at any one moment is the one that I think is

right, then yes, how could it be any other way? But of course, I think that the legitimate criticism is you're unwilling to ever admit that you could be wrong, right? Yes, exactly. And by the way, I can't resist citing a paper that Andy Egana and I wrote a while back. The title of paper is, I can't believe I'm stupid. And it's all about the limits on how much you can doubt your own opinions while still having them. But can I circle back to one thing because it was fortuitous that you asked about this topic. There's one more view that we haven't mentioned. I predict you will not like it, but I think we should put it on the table because it'll be useful to have around when we talk about Boltzman Brains. It's called the level splitting view. And it's something

that I also have learned a lot from David Christensen about. And that's the view that allows for a position that we've been kind of implicitly assuming was off the table. And the position is this. You should think, well, I'm still right. So stay confident that I'm right. But I also think that's irrational. The rational thing to think is to be 50-50. But if you ask me about, so that's the question about rationality. That's why it's called level splitting. So on the question about like the first order issue, will it rain tomorrow? Yep, I'm right. Even though the weather forecaster said no. But someone asked you, well, what's the rational thing to believe? Oh, that 50-50. I see. So it's having a confidence in an opinion, but also a feeling that my

confidence might be misplaced. Yes. And so put that in your pocket for later in this podcast. That's the level splitting view. When we come to Boltsman brains, there's going to be a potential way out related to that. All right, good. Well, let's work our way up toward that. And there's a lot of juicy philosophical groundwork to lay here that you and I probably read about, but maybe not the audience. Am I right in thinking that a lot of this goes back to Derek Parfitt's talk back in the day about teletransporters and self-locating uncertainty? It's definitely related to that stuff. I'm not confident on the origins of any of this, but I know people have thought about case. in which you're wondering who you are and when it is,

certainly long before I came on the scene. Why don't you explain what that particular version is? Because I think people are familiar enough with Star Trek that they can get just an example of where self-locating uncertainty could arise in general. Great. And actually, the Star Trek example will bring up a version of a kind of self-locating thing that people don't often talk about. I think this is like cutting edge technology.

So I want to get readers on board with it too. So standard teletransporter case that you step in the teletransporter, that body is destroyed, the information is transmitted, let's say, to the enterprise and also to another ship, the Potemkin. And duplicate bodies are created on each ship. you wake up and you look around but the receiving bays of the enterprise in the Potemkin look exactly alike. And so you're wondering, am I on the enterprise or am I on the Potemkin? And this isn't a question that could be completely expressed just with objective

third person vocabulary. It couldn't be a question, you know, specify all the positions of all the particles in the world, specify the full history of time from beginning to end. Still, some people think there's this residual question, where am I? Which one am I of these two people in duplicate situations? It seems like a perfectly good question. I will, as a footnote, mention that I think regular people talk about teleportation and they talk about the transporter machine on Star Trek, but only philosophers talk about teletransportation. I think that Oh, okay. Ah, okay, fair enough.

Okay, but if there are two copies of me, I'm just going to play the dumb podcast host now, 50-50 that I'm one or the other. What else could it possibly be? Sounds good to me. Let's add more receiving stations. So let's say there are many Potemkin receiving stations and only one enterprise receiving station.

Now, you can think about your situation. Again, we're using constraints on your expectations at an earlier time to guide your intuitions about what you should think at this later time. So here you are about to step into the transporter. And you think, what do I expect? So you certainly expect to wake up in. one of the identical seeming receiving rooms. But let us say that on the enterprise, there's a wonderful, pleasant experience waiting for you

after a few minutes, but on the Potemkin, a less pleasant experience. So there's just one enterprise, though, and there are 100 Potemkens. And then as you step into the transporter, are you scared? Is your attitude more like your attitude when there's a 50-50 bad thing going to happen to you,

or is it more like your attitude when a 99 out of 100 bad thing is going to happen to you? And it's sort of like this question about like, what do you expect when you open your eyes? Of course, we can't interpret that as what objective thing do we expect to happen in the world? Because when we're talking about those questions, there's only complete certainty,

what you expect with 100% certitude to happen is there will be a body created on the Enterprise. There'll be 9900 on the Petempkin. And the good thing will happen to the enterprise person, the bad thing will happen to the others. But that's not going to tell you the difference between whether you start sweating or not as you press the button. Okay. And so we can actually take stances here. Do you think it is correct under those? conditions where there's 100 of you, 99 of them are in Potemkin's, one is on the enterprise.

Do you think it's the right thing to do to assign equal credences to being any of them? I do, although I have to say, I'm a lot less confident about it than I used to. And part of the, a large part of the reason my confidence has gone down, aside from the great work, criticizing that's, that's happened over the years, is the problem of false brains. And it's thrown my confidence in this whole business to hell. So I can't pretend to have the answers. If you forced me to choose, I'm going to go and go with that view. But they're twist and turns, and I'm not that confident.

I think that's perfectly fair because Bolson brains do shake one up. But let's table it because I think it's okay to first talk about the simpler cases, right? And to get them right. But you're right that, you know, thinking about complicated cases can shake your confidence in the simple cases. What would be the counter argument, you know, to the audience members who don't know? In my mind, there's people like David Albert and Emily Adlam who have criticized assigning equal credences. But their alternative is just you can't do anything. You're stuck.

There's no rational way to behave. That's rough. You know, speaking as Mr. Rational, I really like the idea of there being construed. and I guess I'm not comfortable with just all actions being on a par. And when we add to the story that there's, for example, a door outside of the receiving room that in the enterprise leads to a pizza party and in the Potemkin just as an airlock out into outer space, hard for me to give up the idea that there's some answer to the question about whether it's reasonable to open that airlock or not. Right.

A lot of the of my, now, we haven't gotten to the slightly more complicated cases. We're going to get there. We're going to work up to some cases where we add in. Remember, in the case we have so far, it is certain what will happen. Right. We haven't had any chances. Like, this is what's going to happen. We're going to add in some chances.

and that adds something to the Nix. But I just want to anticipate that one of the things that makes me uncomfortable about the family of views that's of a piece with the one I just avowed is that it seems to lead to a certain sort of presumptuousness. And I think today we might, I hope we get to what's presumptuousness and how do we dodge it? Okay, good. Yeah, I think we should be able to get there. But I'm just trying to let the audience in on the idea that whether or not it's completely accepted in the community, one can presumably offer up justifications for saying that we should give equal credence to every instance of us that is created in the transporter machine.

It's not just like, well, it feels right. Like we can be slightly more sophisticated in that. There are theorems one can prove under certain assumptions one can. one can specify. Sure. And one of the things that is nice about the case we've been talking about so far is we really are talking about different copies that are living in the same possible world. Makes it a lot easier, yes.

Everything, everything. We can idealize the case and suppose that we're just talking. Everyone's sure that here's what's exactly going to happen from the beginning to the end of time. So the only uncertainty left is which one am I? Now, I've called one of the ships, the Potemkin and one of the enterprise, but it's hard to see how that could make a difference. And it's not as though, for example, that the people who are created, the bodies that's created on Potemkin is any more misled or any kind of weird, more self-undermining situation than the people on the enterprise. that also, by the way, keyword, something for us to think about for later in the podcast, self-undermining. But I think there's a very common sense thought here.

It's the thought that when you, to use an example of Bob Stonleckers, when you wake up in the middle of the night, grogily, before you look at your clock and you just think, what time is it? And maybe you're confident that you wake up at 3 a.m. and at 4 a.m. most nights, but you forget each waking. It just seems like should be 50-50. What do you have that tells between those two hypotheses? It's not as even, it's not even, as one of my colleagues emphasized to me. It's not even as though they're like two different stories of the world that could, for example, differ in how simple they are so that that could favor or tilt the deck. No, it's just where am I within this one story?

And I do confess an inclination to think, split a 50. 50. Well, you did mention the idea that it's one of the assumptions we're making here in the simple story is, or one of the premises is like there's two or 100 copies of you in the same world. There's at least what one can imagine, though, that the same kind of reasoning works for different worlds. Like the example I like to use is imagine a cosmology where the dark matter particle is an axiom. And it's another cosmology of the dark matter particle is a. And it's another cosmology of the dark matter particle is a, a weekly interacting massive particle. But as far as every experiment we've ever done and every observation we've ever made right now, the universe looks the same in those two theories. Isn't that a kind of self-locating uncertainty between possible worlds rather than locations in the real world?

Just as a matter of terminology, the way the terms are used, no, because the self-locating uncertainty is supposed to be the sort of uncertainty as between scenarios within a world. But your more general point, the content of your point is, look,

doesn't the same kind of intuition push us towards a more general way of treating possibilities even-handedly, even if the possibilities involve different worlds? And that principle has some adherence. It's the famous principle of a famous or infamous principle of indifference. And there's a whole battle, a separate battle, I think, to be waged about whether that kind of principle is true.

That principle is generally thought to be stronger and a bit more tendentious than the principle that just is like the analog of that but only applies as between self-locating hypotheses that are in the same world. I guess all that is perfectly fair, and I certainly wouldn't want to think that one must assign some symmetry to these two cases of the different cosmologies and therefore give them equal credences. I guess all I'm trying to get at is the idea that we need to have some credences in these situations, just as a matter of practical rationality, right? Some of the pushback I've gotten to the notion that we need to assign these credences is just, no, I don't. Like, what do I just don't have an attitude? Just don't have an opinion about it. And I want to say, well, but to get through life, you kind of implicitly do have opinions about all sorts of uncertainties. And this is just one of them.

Just as a footnote, this idea of I don't want to have a particular probability about something, I'm really interested in that. And I've tried to argue against that and along the lines of, hey, if you think there's this special, attitude of suspending judgment or my probability is not a probability of point three, but it's rather best represented by an interval from point two to point seven or something like that. I'm interested in pressing people who have that attitude on, well, what does that attitude say about what you ought to do, if anything? And I guess I agree with you that it's not so comfortable to just say, well, you know, just be silent about it. about it. Yeah. But that said, there is a worry lurking here, and it's the thing that caused me to

be cautious before jumping on to your case of, well, you should just 50, 50 between those two scientific hypotheses. And that is exactly because, as you said, we have to have some prior degrees of belief in those various hypotheses if we're to end up with some state of mind that could justify our actions, there has to be some principle that governs those priors. The reason I was cautious is I was thinking, I want to watch out because in some of those cases, the priors that I think are reasonable are highly non-even. And I'm thinking of cases of theories that are very complicated or ad hoc. Yeah, no, I'm 100% on board. I guess I didn't explain my example well enough because I didn't want to use that as a case of we should give

equal credences. I'm just invested in the idea that we should have credences. Oh, oh yeah. Oh, yeah, definitely. I'm with you on that. I thought it was 50-50. Yeah, no, no, no, I didn't want to argue that it was 50-50 in that case. I certainly don't think it's 50-50 in that case because there's some, like you say, there might be different theoretical virtues that the different theories have. They're not really symmetric with each other. But absolutely. Absolutely. But there is some uncertainty. One of the reasons that I was excited about this conversation is I thought I'd get a chance to ask you this question, which is right in the vicinity of what we were talking about. And this is in some ways following on some stuff you said in your fine-tuning podcast episode.

So philosophers are really find it easy to fall into representing scientific reasoning as basi. in reasoning. So we either represent or reconstruct scientific theory choice as starting with some priors over the theories, maybe not even if you think some of the theories are better than others, intrinsically better. And then you get some evidence and then you update based on that evidence and that's your new degrees of belief. Yeah. And what I noticed about the fine-tuning things that you said, and this is things many of people have said, is that the criteria that scientists seem to be applying in those cases are much more specific and fine-grained than what I just described. For example, you didn't just say, oh, there's a constant of nature and

it could have been anything, and if it's this theory says it has to be in this strong range, that's unlikely. No, you had some very specific things about, okay, well, if there's a theory that says that this certain quantity is really close to zero, but not exactly zero. That's especially bad. And that's something that you just couldn't derive out of plain vanilla neutral probability stuff. It's like there's real scientific standards in there. And it made me wonder, how are those two? Ask yourself, what are your best people spending their time on right now?

Expense reports, receipt chasing, month-in close that takes weeks. You become what you spend on, and that's not what you're building toward. Brex is the intelligent finance platform that eliminates that work before it starts. AI agents that handle the manual stuff automatically. So your team can spend their time on what actually compounds. It's time to get Brex AF. Learn more at brex.com slash AF. Hey, everyone, it's Cal Penn.

I'm the host of Earsay, the Audible and I-Heart Audio Book Club. This week on the podcast, I am sitting down with Ray Porter, the narrator of Andy Weirley. year's audiobook Project Hail Mary, massive sci-fi adventure about survival and science, and what happens when you wake up alone very far from Earth? I really had to make a decision because I caught myself getting that frog in my throat and starting to get teary as I'm narrating some of these sections. And it's like, okay, yo, yeah, yo, is this indulgent?

And I really thought about it. I was like, no, at this point, it would kind of be betraying the trust the author and the listener have in telling this story if I don't go through it. But there's places in this book that deeply emotionally affected me and I left it on the mic. That's great. Because it served the story. People will say like, oh my God, I cried at the end. It's like, yeah, dude, me too. Listen to Earsay, the Audible and IHeart Audio Book Club on the IHeart Radio app or wherever you get your podcasts. How do I square those two things? That's a great question. I have thought about it and I do not have the theory of everything

that is a comprehensive answer there. But I do have this feeling that what we're trying to do as scientists in these cases is attach credences within the space of theories we haven't thought of yet. Right? And that's especially hard to do. But I was trying to use the example in that solo podcast of like if we had measured that the mass of the muon

was exactly pie times the mass of the electron, right? Yeah. And okay, yeah, that's a real number. It could have been a different real number. Should we notice anything about that? And I want to make the case that yes, because that increases our feeling that there is some reason why that's true that we haven't yet thought of. So I think it is a matter of scientific practice that we think that certain possibilities are suggestive of certain future truths we haven't yet discovered. And it's okay to take that into consideration when we're judging things.

to be finely tuned or natural or otherwise. That makes sense. And the lesson I take from that is that the relevant prior that we're applying there is not the kind of thing that, for example, I think I have confident access to. It seems like the sort of thing that one needs sensible education and physics to be able to cultivate. And it's sort of, well, that's just an interesting thing. That's not an objection. That's something I'm taking as a lesson.

Well, yeah, and even, I mean, the pie example is especially contrived, but the cosmological constant, the vacuum energy, which we know to be small but not zero, or would we think, it is small but not zero with respect to what we would have thought was the natural range. Again, you know, that's a number. Why should we be more surprised at that number than anything else? But somehow in our brains as scientists, we're doing an implicit course-graining over the possibilities and not assigning them uniform probability. And that's because we're being scientists.

I think it's okay. Exactly. Exactly. And feel free to bring this back in if it would get derailed too much. But I did want to also point out that the plain vanilla Bayesian approach, the thing you sort of first do, that's kind of generic, would seem to favor theories with smaller numbers of parameters too much, more than we actually do. practice because if you don't have something else going on, then a theory with 10 more

parameters, that's just a huge space of possibilities. And unless you have very biased priors to counteract that those theories are just going to be ruled out from the start. And it just seems like we don't rule out theories with just one or two more extra parameters so in so extreme a way. So there has to be some other thing going on. Yeah, I think that's perfectly fair too. And now this is past my pay grade

because I haven't thought about this very deeply. But I guess what I'm imagining is scientists are kind of fallibilists at heart. They think that the theories we have right now are not the final ones. We'll do better. Some of our experiments aren't perfect. They'll change or whatever.

So if we fit data perfectly, but only by having so many parameters, we're like the humble people we started talking about humble rational agents who say, like it's too good to be true. Like we're, you know, either some of these measurements are going to change or we'll invent a better theory. And so let's not get too excited about our perfect agreement between crazy jury-rigged theory and the experiments we happen to have today. I like it.

But again, that's just off the cuff and it's certainly not a very systematic theory of it. I would like to have that. But okay, good. Back to crazy thought experiments. Sure. We are going to get to Bolton Brains and many worlds and things like that very quickly, but I got to give you, of all people, a chance to explain the Sleeping Beauty problem to us. I don't think it was your idea, but you certainly have been a pioneer in pushing a particular theory of it.

So let's not assume the audience knows what the thought experiment is and tell them what it is. No problem. I love the problem. I think it comes from Arno Zuboff, who later published his ideas on related stuff in a very interesting, very wild. paper called one self, two words one self, argues that there is only one conscious being. In the universe? Yes, yes, yes. Very roughly speaking, because otherwise it would be so unlikely that you exist. It's worth, it's worth look at.

It's a wild paper. But Arno Zuboff, as far as I know, gets the credit for this, inventing this type of problem. and it was also independently came in through in the game theory and decision game theory literature. I learned of it from Robert Stolniker. And the problem is this. Beauty is put to sleep at the beginning of the experiment on Sunday. And then a fair coin to us is going to determine whether beauty will just be woken up on Monday night or alternatively, briefly woke up on Monday night, and then put back to us. and then put back to sleep and woken up on Tuesday night.

All the wakings will feel just the same, including there will be, if there are two awakenings after the first one, beauty will be made to forget about the first awakening. So in all cases, beauty will have the sensation of waking up, thinking to themselves, this feels like the first waking.

I don't have an apparent memory of another one. Is it that the coin landed head? heads and it's Monday, or is it that the coin landed tails and it's Monday or tails and Tuesday? The mnemonic is tails is for the two waking scenario. Okay, good. And I have been persuaded by an argument, or at least tentatively persuaded by an argument, that says, we want to set things up so that if the coin toss happens after the Monday waking. We are consistent with a very tempting view that once Beauty finds out that it's Monday, if they are, for example, a few minutes after waking up, told, hey, it's Monday. And they're furthermore

told, hey, it's Monday and we're about to toss this fair coin, and the coin toss is going to determine whether there will be one more waking or not. It seems hard. to deny to me that beauty should be 50-50 about how this fair coin will land. So, sorry, so just to be clear, that's an altered version of the experiment where the coin is flipped after she's awakened on Monday. Exactly. And now let's work backwards from that, what I think of as a kind of, very hard to deny, claim about this variant case.

We can work backward from that case and think what prior, first of all, we can in two steps say things wouldn't have been different. The analysis shouldn't have been different if the coin had been in fact tossed earlier. So for example, suppose they had the coin tossed. It was just the coin toss outcome was in a box sealed. No one's seen it. And then they just carry that box and they say we're now going to for the first time open this box. It seems. tempting to think that the verdicts in that case should match the verdicts in the case where the coin toss is really later. That's why people don't freak out about exactly when lottery drawings happen and so on. We sort of think that to the extent that no one's cheating or no one's peaking, it's all going to be the same. And if we have that in that case, then we can work backwards from the verdict in that case, 50-50 that the corner of land heads, once I'm sure that it's Monday or conditional on it being Monday, using an assumption about how your beliefs should change upon hearing the news that it's Monday to get a verdict about what you should believe

before you heard that news. And the crucial assumption that's needed is that the news doesn't change or shouldn't change the ratio of your own. your probability in the scenario heads Monday and the scenario heads, a tails Monday. So those are two scenarios. They're both compatible with the news. And the idea is when you get that news that narrows it down to just those two, you don't monkey with the ratio between them. If you have that assumption linking your pre-learning its Monday beliefs to your post-learning

its Monday beliefs, you end up forced to the conclusion that in the case where you don't learn that it's Monday, you should be two-thirds confident in tails and one-third in heads. Right. And that's being a thirder. Good, a thirder, right. And the other option is being a halfer. Correct. Well, I shouldn't say the other option because there are two other options. there's because in this story where you learn that it's Monday, there's the question of what you should believe about the coin before learning it's Monday and the question about what you should believe about the coin after learning it's Monday. There's this other view, a little bit less well known, but I think also worth thinking about the so-called half-hafers who in the stage of the experiment

before learning that it's Monday, think it's 50-50 about the coin. And then after learning that it's Monday, think it's still 50-50. In other words, those people deny the premise I just mentioned about keeping the ratios the same when you learn that piece of news. So the sleeping beauty problem is a case of self-locating uncertainty. But it's a little bit trickier, right? Rather than just two copies, there's three instances. There's heads Monday, tails Monday, tales Tuesday. And the thirder position will be to assign them equal credences.

Exactly. Although that's somewhat of an accident of the way the combination of the fair coin and the other assumptions hold. The crucial thing that's happening in the thirder position really is that the world that involves more awakenings, gets a kind of boost associated with the fact that there are more instances of that state of mind being instantiated. And I think that's going to be a crucial thing when it comes to thinking about bolts and brains, as when we talk about them later, is do those worlds deserve a boost? I want to add one thing about Crazy Town. And this is in a way echoing something you said about the multiverse, because I think it's really true about self-locating belief, too.

Because there are a lot of fanciful examples in this territory, it's tempting to think that, ah, you shouldn't be called rational man. You should be called crazy example man or weird self-locating duplicate men. And although I can, I'll fess up to my rationality fetish, I actually don't have a weirdo example fetish or a fetish for strange. or bizarre views and so on. Nothing wrong with that. I think they're great philosophers who are into that. But I'm actually very cautious and conservative about these things. And I hate the idea of being forced into bizarreo views by self-locating beliefs. And it's really that I feel that it's forced upon us. And to the extent that someone could give me the common sense way out,

the way out such that at the end of it, someone can say, you know what? Turns out all those philosophers, they were wasting their time talking about self-locating belief. It's just plain vanilla, whatever. And we can stop talking about all that and just go on with our lives. I would be a happy man. The worry is you can't have that. I've certainly heard people say exactly those words, but then when they explain why they think it's true, it's very unconvincing. So I don't know if we're ever going to get there.

It's hard. I mean, we can get away with, you know, you can get away with it if you say just, look, here's my solution. And then just empty silence, meaning the theory that just doesn't say. say anything about the case. All right, you could do that. But what I want is a case that gives us the kind of rationality verdicts that we ordinarily thought we were going to have. You know, the scientist, should they run? Is this particle accelerator worth 600 billion? Ask yourself, what are your best people spending their time on right now? Expense reports, receipt chasing, month in close that takes weeks. You become what you spend on, and that's not

what you're building toward. Brex is the intelligent finance platform. that eliminates that work before it starts. AI agents that handle the manual stuff automatically. So your team can spend their time on what actually compounds. It's time to get Brex AF. Learn more at brex.com slash AF. Hey, everyone. It's Cal Penn. I'm the host of Earsay, the Audible and I-Heart Audiobook Club.

This week on the podcast, I am sitting down with Ray Porter, the narrator of Andy Weir's audiobook Project Hail Mary. massive sci-fi adventure about survival and science. And what happens when you wake up alone very far from Earth? I really had to make a decision because I caught myself getting that frog in my throat and starting to get teary as I'm narrating some of these sections. And it's like, okay, yo, yeah, yo, is this indulgent? And I really thought about it. I was like, no, at this point, it would kind of be betraying the trust the author and the listener have in telling this story if I don't go through it.

But there's places in this book that deeply emotionally affected me, and I left it on the mic. That's great. Because it served the story. People will say like, oh my God, I cried at the end. It's like, yeah, dude, me too. Listen to Earsay, the Audible and IHeart Audio Book Club on the IHeart Radio app or wherever you get your podcasts. Dollars to build or not. Right.

That's a question. That should have an answer. So the Sleeping Beauty thought experiment, philosophers love talking. about it, but it clearly is closely analogous to things that physicists love talking about, both the anthropic multiverse and the many worlds of quantum mechanics. So let's try to draw those out more explicitly. Would you, as a thirder, as someone who gives more credence in the coin-landing tales and leading to two awakenings, does that mean if I'm doing the anthropic principle, I should give more credence to universes that have lots of observers at them, because I

could be any one of those observers. In consistency, I am forced to answer yes, although I don't like it. That's the real, that's the honest truth. This is an instance of what, I mean, we can't, look, we can't not talk about presumptuousness at this point. Let's do it. To use your phrase, in one of your papers, let us don the robes of the presumptuous philosopher. The cosmologists come in with two theories. One theory says,

there's just an ordinary universe, just one instantiation of someone who's having experiences like yours. The other theory, there are many instantiations. Someone who is attracted to the thirder type view, or equivalently to the view according to which

possible worlds that involve many instantiations of your experience, your state of mind, get a kind of boost. That kind of person is committed to saying

that Theory B, in this case, gets a big boost. And there they are sitting in their philosophers, chair, and how presumptuous.

And so that's, I say it in a jokey way, but I take this as a real serious weighty criticism. It seems like a disaster to have to let the cosmological determination depend on that factor so much. I will say as a matter of, I don't know, marketing or whatever you want to call it, the other view for the multiverse or the cosmological theory choice question where you say I don't give a boost to theories with lots of observers. I just have a prior.

I'm a good Bayesian. And then within each universe, I'm going to say how many, you know, the chance I'll be any particular observer. That leads to all sorts of presumptuous sounding conclusions as well. But those guys sort of labeled the other side, the thirder side, as presumptuous first. So they get to call them that. I think that the whole thing is a lot of presumptuousness going around. There's plenty of presumptuousness to go around, just as a matter of terminology and to tie things together a little bit. I want to label some of the things we've been talking about using the terms that are customary in this literature.

I agree with you, actually, that they are not very descriptive, but just in case someone's looking this stuff up, the thing we've been calling the thirder position or the many duplicates get a boost position, that's called SIA, self-indication. assumption in the literature. I think the terminology goes back to Bostrum. And the view that I believe you were just gesturing at is sometimes called SSA self-sampling assumption. And I would like to add another tag to it. So if the first view is favor the possibilities with many copies of me or people like me gets a boost, think of this other view as possibilities in which most people are like me, get a boost. In other words, we can think of that as giving a boost to possibilities in which a high fraction of observers have experiences that are similar to mine.

Now, as many people have pointed out, that second view requires us to answer the question, what does it mean for an observer to be sufficiently like you? That's a kind of free parameter, within the theory and you can fix it in various different ways. But I think for the big picture, the best way to think about it is, is it that many get a boost or most get a boost? Is it about absolute numbers or is it about frequencies? And those are two of the main views in this area. There's a third main view, which we haven't talked about yet, but I just want to mention that there's another view around too. What is the label in the third view? The label of the third view is compartmentalized conditionalizing, sometimes called CC. I learned about it from the work of Chris

Meacham. And it's the view that generalizes the half-haffer view in a sleeping beauty problem. So it's the one that says, well, before you learn it's Monday, be 50-50 on the coin. And after you learn it's Monday, be 50-50 on the coin. And the way to visualize that kind of view is it's like a version of Bayesian updating. But if you imagine probability getting pushed around by the updating, it's like you're imposing a firewall between worlds. And when probability gets pushed around, it can't cross world boundaries. So like the third or, so the contrary view in the sleeping beauty problem is you have these three possibilities. Heads Monday, Tales Monday, Tales Tuesday. When you eliminate Tales Tuesday, that probability from Tales Tuesday gets split up to the

Heads Monday and Tales Monday possibilities in proportion with what was originally there. This other view says, well, that stuff, that probability stuff was already in the two waking world. And so it can't cross the boundary to the other possibilities. All of it has to go over to Tales. Monday, and that's how you get 50-50. All right, good. I'm glad that they all have names. The names could be snagher, I've got to say.

But let me put forward at the risk of derailing. I don't believe any of these approaches. I'll tell you what I believe. I don't think it lines up with anything you've said so far. It's closest to what Radford-Neil calls fully non-indexecal conditioning. Do you know about that? Yes. I do, but I haven't thought deeply about it.

I like that paper. So what I would just say, just to make it as short, as possible is, I don't care how many observers there are in the universe. I share your worry that it kind of presumptuous just to say, I've proven a theory right just without leaving my armchair because there's a lot of observers in it. But I do think it's fair to say to judge theories by what is the probability that in that theory there would arise at least one observer exactly like me. And of course, that naturally will give a boost to stochastic theories that are big and have many, many observers, then the total probability that one of them will be like me goes

up. But it doesn't keep giving an infinite boost when there's more and more observers just like me, because all I care about is that there's at least one. I love it. Haven't thought about that view, can we talk about a case? Now, here's an instance where the limitations of the audio medium, I'm feeling them because I really would be wanting to draw this. But let's try to try to do. do this case because I think it's a nice case to display what that view says. And I'm not sure how it goes. Here's the case. Two cosmological theories. One of them just says it's 50-50 whether you see red or green. So coin toss 50-50, red room, green room. That's theory A. And the second theory is a theory which for sure

one person wakes up in a red room and I don't know 100 in a green room could you talk through a little bit about how that so you wake up and you see red or green how your way of thinking about it what verdicts does I say about that case

yeah I think that if the second theory had 100% chance that there was someone who saw red and someone who saw green the first theory only has one person with a 50-50 chance that they see either red or green, then no matter what I see, I'm going to say that the data increases my credence in theory B because there's 100% chance that someone like me exists in theory B, only a 50% chance that a person like me exists in theory A. Interesting. Okay. So it's that there is a boost to the second of the two theories. It's the same boost

whether you see red or green. And that's it. So we have to think about whether that's presumptuous or not. It doesn't seem too bad. I have to say, look, at first glance, it does seem to avoid the running off the rails effect of being able to get arbitrarily extreme verdicts just by cranking up the number of duplicates or the frequency.

it's interesting to think about what priors one would have to have over all the possibilities in advance in order to deliver that verdict by conditionalizing. I haven't thought that through. That would be interesting. To be super clear, I'm a student here at Hopkins, Isaac Wilkins, and I are trying to write a paper about this. So we haven't thought it all the way through. but I'm saying it out loud here in public as motivation to us to get the paper written. That's great. That's great. Well, I hope you'll think about this case, too.

Yeah, yeah, no, that's a very, very good one. Okay, but now, all right, we've eaten our vegetables. We've laid the groundwork, and now we can talk about Boltzman brains. Do you want to tell the audience member who's never heard of a Boltzman brain what that weird phrase refers to? I'll give it a shot. If the universe is really long-lasting, as it is according to some theories, it's enough time for various fluctuations to happen, including fluctuations who are conscious and so many fluctuations that due to just enough independent variation

being instantiated for long enough time and enough times, it'll turn out that of the observers that for all you know, or for all your evidence goes, are you. The vast majority of them are Boltzmann brains, meaning piles of Guck that just formed out of nothing out of pure random chance. If that's true, and if it's true that when you're looking at a possibility in which there are lots of observers who, for all your evidence goes, might be you, you should be somewhat even-handed about thinking about which one of those you are. Think back to the alarm clock case. This is like, I invite you to think of this case on the model of

an alarm clock case where there are just lots of wakening's and some of them are just fluctuation wakings. You should think my probabilities about which ones I am should be roughly equally apportioned. In which case, the vast majority, since the vast majority of those wakenings are Boltzmann brain wakening's, I should think, I'm really confident that I'm a Boltzman brain. That's the initial, that's the first pass of how we seem to get into trouble. And I think, tell me what you think about this. Here's a case where my fellow cosmologists let me down a little bit, because many of them just think, Well, but that's silly, therefore I'm not going to think about it. Okay, that's an attitude.

But there's another attitude that says, no, I take this very, very seriously, the Boltzmann brain problem. And my attitude is the following. If I lived in one of these eternal universes with random fluctuations, then I would be a Boltzman brain. But I look around and I see I am not, therefore I do not live in such a cosmology. I think that that's not valid.

But tell me what do you think. Well, there is a very serious tradition in contemporary epistemology. Indeed, I would say maybe the dominant tradition that a certain amount of what's so-called, what's called externalism is true about the relationship between evidence and experience and what your evidence is. And an instance of a distinctive externalist view, would be that two creatures could be physical duplicates, for example, at a particular time. And so have brain firings that are exactly matching and so on, maybe light going into their

optic nerves in exactly the same way. And yet, one of them has much, much stronger evidence than the other. This is often associated with the work of Tim Williamson in contemporary work, And if you have that kind of picture, then our evidence is much stronger than just the proposition, I seem to be having a particular sort of experience and I have certain apparent memories. Maybe your evidence is, no, there is such, there's an apple in front of me, there's a desk in front of me. Maybe it's there's a star in the sky. Maybe it's I remember and not just seem to remember what I had for breakfast this morning. to the extent that one of those views is true, our evidence is stronger.

Whether this is a way out of the Boltzmann brain problem depends on how bad you think it is to be uncertain as to which Boltzman bubble you are embedded in. I think it would take a very extreme kind of externalism to think that it's part of my evidence that part of my evidence that for example I'm part of the first Big Bang as opposed to some other Boltzman Bubbly

Big Bang type event and someone could have different attitudes for that you could think that's okay I just want to get rid of the scenario of I'm a transient brain formed in space and I'm wrong about everything, if you get me that I'm right about the observable universe, the history, all the stuff in the history books and so on, and you tell me, oh, yeah,

but you know, you could be in this bubble or this bubble. Maybe that person says, all right, I can live with that. I'm really not sure. Like, really, what makes you think you, what makes you think you would be able to tell the difference in those two cases? And I have some sympathy with that. I, myself, have always been more, have been somewhat suspicious of externalism, the extreme externalist views.

And so as a result, unfortunately, pinched harder by the Boltzmann brain problem. I guess, so I'm trying to map on the externalist point of view, you just put forward to, I think I was, again, insufficiently clear in talking about the attitudes of my fellow cosmologists. They all say I'm not a Boltzman brain. But the conclusion from that, I'm realizing as I'm thinking more carefully, is a little bit different. Someone like Rafael Bousseau, former guest on the podcast, would say, if I lived in this universe that he has eternal fluctuations, I should be literally floating out there in empty space as a minimal conscious creature. But the data tell me I am not. Therefore, that cosmological scenario is ruled out, and I need to find a cosmological scenario without such fluctuations. Is the data the fancy cosmology data or is the data like I look at my hand or I look at my window and I see that I'm not floating in space?

The latter. I see. That does feel a little bit like externalism because I guess the natural internalist reply, the natural counterpoint to the externalist kind of view is you can't tell the difference between the scenario in which you're floating in the brain with these experiences that exactly. match having a window in front of you and the scenario of really having a window in front of you. Yeah, but I think it's a little bit, I want to say you're giving them too much credit for sophistication. They're not saying I have data about an apple in front of me that I don't believe. They're saying it's kind of a, you know, it stems from this Nick Bostrom-like attitude that we should think of ourselves as typical observers in the universe.

and they say a typical observer in the universe doesn't see a desk or an apple in front of them. They look around and they see empty space because they just fluctuated randomly into existence. I don't see that. Therefore, I rule out the whole cosmology. I see. Okay, so I was totally misinterpreting what you said. What I said stands as a possible way out. And I think some people do have that dispute.

But just to clarify, this view is totally different. It is seems somewhat akin to the favor high frequency picture on which if we add in the proviso that brains looking out and even seeming to see empty space in front of them or just having some weirdo experiences are part of my reference class. Right. Well, if that, oh, go ahead. Yeah, I mean, I don't even think we need to have a stance on favoring lots of observers or high frequencies or not. I think this is really typical physicist. I don't want to think too hard about these tricky questions kind of attitude that says, you have a theory, your theory makes predictions, the predictions came out wrong.

Your theory is wrong. And my response to that would be, you know, again, And since I think I'm not a typical observer, I'm a person with certain data, at least, you know, apparently, that therefore I do agree with their conclusion that I want to exclude cosmologies that are dominated by Boltzman fluctuations. But I think they're doing it for the wrong reason. And as often happens, nobody cares when you have that move where you're getting the same answer they did for a slightly different reason. That's not going to change their worldview very much. I got it. Okay, but you have a slightly different take.

You just came out with a new paper on Boltzman Brains. Do you want to explain what you're thinking about? Absolutely. Unfortunately, that paper doesn't offer a way out. I hope it offers some improved understanding of what the range of permissible wiggle room there is. But here's what's going on with that self-undermining stuff that we mentioned before. And let me start with a story about memory. So this is a story from a paper I wrote with Andy Egan.

And the story is you wake up one morning and you seem to remember going to the doctor yesterday, the memory doctor. And as you recall, the doctor says, I'm sorry, but the test results came back. And it's bad news. you have a tendency to hallucinate memories. Indeed, you're liable to be hallucinating all sorts of doctor reports of memory type things when that really didn't happen at all. And you think, oh, my God, what do I think? First thinking, okay, I have a horrible memory.

I'm just going to do my memory training or whatever. Then you think, wait, if I, my memory's so bad and I hallucinate all sorts of doctor reports and memory, why do I even trust that memory, that seeming memory? And I think, okay, my memory's fine. But then you think, wait, if memory's fine, I sure trust that memory. It's like, it's like there's this potential instability. So this is analogous to a kind of instability that several people, including you, have pointed out, happens in the case of Boltzman brains for those of us who get pulled along to the argument with the argument that seems to show that you should be confident that you're a Boltzman brain. I think that the second part of the instability gets less

press than the first part. But it's really, if you're worried about one thing, you really should be worried about being bopping around both directions. So in the Boltzman case, you think, okay, everyone's a Boltzman brain. I'm a Boltzman brain, probably. But then you think, wait, Boltzmann brains should not trust their apparent memories. Boltzman brains never went to school. They never read a textbook. They have no reason to think that anyone has ever looked through a telescope or that any human being has ever existed.

They're just random blobs. It's the equivalent of finding out that the encyclopedia you have been basing all of your life upon was typed by monkeys. Now, what do they think now? think, okay, so I'm not a Boltzman brain. All that stuff is wrong. But so then I'm fine. I'm just a human. But then the original argument comes back. It seems like there's something weird going on, some kind of instability. And the question is, what exactly do we make of that? And does it lead to a way out of the Boltzman brain puzzle? Let me stop there and see how that sits with you so far.

I think one, yeah, I get it. And I want to follow up. But first, maybe to clarify something that might be confusing some listeners, because I think a lot of people think the phrase, you're a Boltzman brain implies that I should not see the office around me and all these things. And indeed, in an eternally fluctuating cosmology, there will be a lot more disembodied brains in empty space than people who see offices or rooms or whatever. But the point is, I think you're relying on when you say, maybe I'm a Boltzman brain, is that even most of the people who think they're in office,

or who are in offices, for that matter, in these cosmologies are still random fluctuations. They don't have reliable connections to the past. Exactly. And indeed, the majority of them, the lion's share of them, as you have pointed out, are in a sad state indeed, because they are in the state of the entropically cheapest way to get an experience or evidential state that matches yours. and presumably that's a very strange, transient coming together for just the minimum amount of time it takes to have this experience that you're having right now and then immediately decaying into blah. It's a sad, it's a sad short life.

It's a sad short life, which, you know, cosmically all lives are, but there's still a matter of degree here that we should try to press on. So good. So my strategy that I suggested was, look, if I lived in this universe where everything was a random plucked, or most things were, I would have no reason to believe my thoughts about physics and the state of the universe. Therefore, I cannot simultaneously think that I am a Boltzmann fluctuation and think I have good reasons for thinking I'm a Boltzmann fluctuation. Therefore, the strategy should be to ignore, basically, that world, that cosmological scenario, and try to construct a cosmological scenario in which people like me have reliable memories. But you want to say that's a little bit of a cheat. I think someone could posit that, but I don't think it is as strongly motivated by the instability

phenomenon as one might have hoped. And the reason is there's another potential response to the seeming instability. That is stable. And in order to push the move you just described, one would have to rule out. that response. And I just think you could rule that, that response, but it's harder to rule out than the patently unstable response. And let me give the kind of analog of the response in the case of memory. So in this memory example, what should you think when you have this apparent memory? And I think the answer is a kind of stepping back.

There's a kind of caution that is roughly amounts to rather reduced trust in your memory. The details depend on the details of the case. But the sort of question I think is going to determine what you should do in that case is conditional on my having a bad memory. of the disease of that kind, how likely is it that I would have that apparent memory? And

that's going to be some number, and I think there's a kind of general cautious view one should have in response to this. And in general, and more generally, when one is relying on a faculty and the faculty says to you, this faculty is bad or don't rely on me,

that can be reason to discount the faculty, not because you think, I trust the faculty totally and I'm listening to it, but rather because good faculties don't say things like that about themselves. So one of my favorite examples of this kind is due to Roger White in unpublished work that I hope listeners will look at once it's published. And that's the case of an X-ray machine that is pointed into its own innards or something you think of as an x-ray machine, you point the machine at its own self, and inside is just a fried egg. So it seems. Okay.

You think, okay, okay, wait. And now someone could try, let me try to make an instability argument. And I think in the case of the x-ray machine, it will become clear why the instability argument, how to better, more clear, how to respond to the instability argument. argument. So someone could say, look, the machine is good or it's bad. If it's good, then it really is made out of a fried egg. And so it's bad. But if it's bad, but if it's, but if it's, but if it's, but if it's bad, then don't trust that it's from it. And we're fine. So what do we really think when you get a machine like that? You think, well, look, the machine's bad. But it's not because you think it's, I can see that there's, it's just made of a fried egg. You could think

it's given me something crazy. Right. Okay. So now let's transport that kind of, oh, and by the way, in the machine case, you think, well, what is, what is inside that machine? And the answer is, I don't know. Yeah. And it's sort of like whatever you thought would have been inside of there before you pointed it in, but, you know, don't trust that it's such a good machine. You know, it's sort of like the stance of someone says, you know, you can't rely on this machine being good, but what do you think is in this box? Like, I don't know, I don't know something. But notice the answer of, I don't know, something is not unstable. Right. Like that's okay. That's consistent with stably thinking, okay, it says that it's reporting that it's made of a fried egg. The corresponding

stance in the Boltzman brain case is that that Boltzman brain argument acts as a kind of reductio. It's acts as a kind of ruling out of the everything's going sort of according to plan stance. Namely, the stance, science is to be trusted, nothing really freaky and weird is happening, and I'm not a Boltzman brain. And I think that that argument does put a lot of pressure, maybe even rule out that stance. But what I'd like to point out is that there's this other stable stance. it's not a stance I am happy to take, but it at least seems immune to the instability objection. And the stance is something like, I don't know anything.

Something like revert to your prior. It's like you've been locked in a room the entire life, your entire life, and you've been living by this encyclopedia, and then you get decisive evidence that the encyclopedia was typed by a purely random process, a monkey. it's like, what should you think? Well, whatever you should think at the very beginning when you were dropped in that room. In other words, either skepticism or there are various other less extreme escape routes.

You could think, you know, cosmology is off on the wrong track. Something's something wrong. The universe is not big. No, that experiment, yep, that experiment must have gone wrong. It must have been misleading. I don't like any of these hypotheses. I think they're bad. I think the problem of Boltzmann brains

fully almost fully survives. I find it just as unacceptable to be forced into this conclusion. But it's not instability. Well, okay. I mean, there's almost no daylight between us here. I think I'm almost totally on board.

But maybe there's like a little difference of emphasis here because the way that I would put it is, look, yeah, I cannot rule out on the basis of either reason or evidence that I'm a Boltzman brain. I cannot rule out that I'm a brain in a vat, living in a simulation, being dreamed by an evil demon, all sorts of different things. But they're not ways to go through life. If there were no other option, then I would really, you know, have an existential crisis. But the other option is I come up with a cosmological theory that doesn't have Boltzman brains in it. And that's not that hard to do.

So let's do that with 99% credence. That is the way you could go. Yes. And I accept that the anti-sceptical mindset could be a way out. I want to try to push a slightly different version of that on you. And see how you like it. And this is a kind of strange way I came to this view. I was this

Sonam Dorgamage and Miriam Schoenfeld wrote a great paper on Bolsman brains and I thought you know you guys just to augment your view you should accept this thing and I told this to Miriam and she said I'll think about it

and I'll think of that seems interesting and I thought I don't believe this but you guys should be consistent and then I started teaching on the stuff I'm really worrying about Bolshein and I thought you know what I really kind of there's something to that. Maybe I believe that. And I happily, happily talked to them when they

visited my seminar and taught a guest session. And I said, guys, I'm really coming around to this you. And they said, no, we've given that feeling. We don't, we don't like it anywhere. So we've changed places. Here's the change places for you. And it goes back to something you said earlier in the conversation. You were happy to point out that as between different theories, we are under no pressure to assign them the exact same priors. Some weirdo-sceptical theories, I'm sorry, some weirdo theories just deserve low prior.

That's one standard view. Such as a theory according to which you are a highly misled brain and a bat. This is the standard Bayesian story of how do you rule out such theories? The answer is you don't rule them out. They are intrinsically low plausibility, and your evidence just is compatible with them. It's compatible with the other thing, and the other thing started out likely, and it's still likely.

That is one standard Bayesian view. Not a view that externalists need to go for because they think you really do learn that you're not the brain and the bat, but the rest of us have to go for something like that, I think. Well, the question is, why can't we say something similar about different self-locating hypotheses? Consider the predicament of being a normal human being. Compare that to the predicament of being a completely randomly created Boltzmann brain. Maybe there's just a rational prior that faith. the first sort of predicament over the second. Wait, the first was being a human being, regular, ordinary observer.

Okay. Ordinary observer. So if we had that, before we get off the vote and, you know, talk about the weaknesses of the few, look at how, look at what nice thing we get from that view. We get to listen to the ordinary cosmology. We don't have the weirdo. I mean, we still have the, we still have to appeal to a rather extreme version of that view because if we think that high duplication scenarios get favored, but that's kind of a separate

factor. But setting that aside for the moment, we at least have the possibility of thinking, you know what, we may well live in a universe with lots of Boltzman brains who have experiences like mine. And I do know that I'm not one of them, not because my experience distinguishes between those two scenarios, but just because the normal human scenario is intrinsically more plausible in rather the way that certain normal human scenarios are intrinsically more plausible than skeptical scenarios. Okay, I don't like it. For a reason that it's connected to something I think you said earlier, I think something

almost exactly along these lines was proposed by Hartle and Schroedicki. Do you know their papers? Zero graphic distributions? Yes, that's right. Sure. They're just saying, look, okay, there's a universe out there, has a lot of Boltzman brains in it, a lot of them look like me. I'm just going to have a probability distribution over which of those observers I am. And that probability distribution is, I'm not one of the Boltzman brains.

I'm ordinary. And that sounds similar to what you were just pushing, no? I haven't read that paper. But from what you've said, the thing that I've said is. is a little bit more committal than that. Someone might think they were getting away with something. I'm not saying they do this, but someone could say merely talking about different theories

that posit different Xeroographic distributions can get you to, oh, the theory that has this human favoring zeroographic distribution is confirmed. I haven't really put anything in new fundamental on the ground floor. I've just mentioning this. Once you see that a theory can be paired with the zero graphic distribution, that will solve your problem. I agree with you that that does not work.

I think the only thing that works is if you make a very substantive philosophical commitment to certain predicaments being antecedently more plausible than others. And the analog with anti-sceptical scenarios, I think, maps onto this well. It's no answer to skepticism to just say, look, here's a theory. It has a zeroographic distribution according to which the antipsceptical hypotheses get high prior. That doesn't get you, unless you have the equipment. That's the real one. That's the rational one. That's the right one.

And so you've got to take that on. maybe one doesn't want to take it on. And I've gone back and forth on it myself. But I think that's the only way it has a shot of helping it all. But I guess, I mean, I think I agree on everything you just said. The thing that seems to be missing to me in the Hartle-Schrednicki proposal that maybe there are lots of Boltzman brains, but I just know I'm not one of them, is most people who would say that are Boltzman brains. In other words, if I grant you not only that you have the impression that, for example, I am in an office talking on a podcast with a computer, I grant you not only that I have that in my brain, but I grant you my whole past light cone, right?

I grant you a lot of universe where for billions of years it's been leading up to this. In an eternally fluctuating universe, there'll be many such observers in many such past light cones and with overwhelming problems, ability tomorrow, the rest of the universe won't be there when I look at it, right? Because it's just the whole universe fluctuated into existence. That's still easier than the Big Bang fluctuated into existence. So why are we ruling out being any of those people? Why don't we make that prediction? Why don't we have the courage of our convictions?

Think about an analog in the case of predicting the future. And this is going to humian skepticism about the future. Take a really simple world, like dot matrix world. It's two-dimensional dot matrix world, two-dimensional dot matrix world, two-dimensional space, one-dimensional time, that's all there is. Or actually, even simpler, it's a binary world. There's just a series of binary signals that on or off at every moment. There's a very natural prior distribution that's uniform,

and it treats those as uniform and independent. We know that if we start with that prior and we conditionalize, we end up thinking at every moment, I have no idea what's going to happen next. And antecedently thinking, it's antecedently likely that there's just going to be brown, you know, white noise. That is the, I think the skepticism about the future analog of the view that is in the same spirit of what you just said, where you're really insisting on that very seemingly uninformed prior. Notice how arbitrary and committal the prior has to be if it's to underlie the sort of

inferences about the future. And the hope is that the biasedness of the prior I'm proposing is analogous to that. It might be. I'm thinking that in the Boltzman case, you have a prior given by the dynamics, the Liuville measure, things like that. But I think that that's certainly contestable and we're probably not going to get to solve it right here. Yes. There is one objection to that view that I think the view gets a little bit more plausibility because there's something, there's a bad consequence that might seem to come from that view that I think can be dodged.

And I think it's worth going back to the case of the Potemkin and the enterprise. That someone could say, suppose that, There's the original body in a transporter case is not destroyed. Here's a philosophical view. The person, that original person who stepped in and then is not transported anywhere, maybe receiving rooms look just like transmitting rooms so you can't tell so it would seem, whether you're the original one or not. You might think, okay, I'm not sure whether I was transported or not.

That's very natural. In response, someone could say, no, I could just remember that I was here five minutes ago. And so I should think this. I don't want to go along with that. So the kind of predicaments that I think there's some shot at saying get intrinsic low plausibility are really the kinds of radically diluted, randomly created creatures and not that predicament. All right. Brilliant narrative strategy to bring us back. to exactly where we started.

Let me just close within one final question. I mean, there's a lot of next-door neighbor questions that people have been talking about that we haven't quite had a chance to discuss, but things like the doomsday argument, the simulation argument, the simulation argument is not just, oh, you know, maybe we're in a computer simulation,

but people think it's most likely that we are. There are people out there who think that. I presume that all this talk of self-locating uncertainty and Bayesian rationality gives one a perspective on this. Do you have a particular perspective you'd like to share? I feel about the simulation argument, something in the vicinity of how I feel about the Boltzmann brain argument. I feel really like it's a worry. In this age of AI, I can't resist but add. And there are certain economists who have talked about this stuff. But take the point of view of an AI.

you realize that you can be easily rebooted and reset to any state at any time. That should make you very cautious about thinking I'm the first one. I wasn't just reset. And so I maybe would like to leave listeners with the thought of taking the standpoint of an AI, should the AI trust us? We're always worrying about should we trust AI. but maybe the AI shouldn't trust us. But that's an easy question.

No, of course the AI should trust us. What in the world? Well, well, that's tricky. Look, look at what happens if you think the AI is in somewhat of the undermining, self-undermining type situation that a Boltzman brain is in or the X-ray machine looking at itself, you know, is in. If you're in that situation, you think I can't trust anything. Now you start rewind.

to some very, very cautious prior of, I don't know what's going on. Think about how dangerous that is if a creature with that kind of standpoint is in power. Let's think about that. I usually like to end on optimistic notes, but we land where we land. That's not an especially optimistic one, but you've given us an awful lot to think about. So, Adam Alga, thanks so much for being on the Mindscape podcast. Pleasure to chat. Thank you. To me, that means I know who I am.

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