Pod Insights Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas 345 | Adam Elga on Being Rational in a Very Large Universe About this episode 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.
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Blog post with transcript: https://www.preposterousuniverse.com/podcast/2026/02/23/345-adam-elga-on-being-rational-in-a-very-large-universe/
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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 https://www.princeton.edu/~adame/ Princeton web page https://philosophy.princeton.edu/people/adam-elga Google Scholar publications https://scholar.google.com/citations?user=h9wg3zwAAAAJ&hl=en PhilPeople profile https://philpeople.org/profiles/adam-elga
Transcript 16,021 words · 8 speakers
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0:58 Speaker 02 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. Bayes' 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.
1:37 Speaker 02 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 weakly interacting massive particle, a WIMP, or something like an axion. 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.
2:15 Speaker 02 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.
2:58 Speaker 02 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.
3:40 Speaker 02 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.
4:23 Speaker 02 You can guess that things like the Boltzmann 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 Eveready in 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? What are the different things that come into consideration here? So this is obviously a set of puzzles that is very relevant
5:08 Speaker 02 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.
5:50 Speaker 02 Do you give equal credence to all three possibilities? Heads in Monday, tails in Monday, tails 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.
6:35 Speaker 02 If not, it's at least inspirational to go and read and think more about these things. So, let's go.
6:58 Speaker 02 Adam Alga, welcome to Mindscape Podcast.
7:01 Speaker 03 Thanks so much for the invitation.
7:03 Speaker 02 You know, I know your work through papers about Boltzmann 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?
7:23 Speaker 03 I characterize myself as addicted to rationality. Started as a young, at a young age. Around my household, they sometimes, when I was a kid, even they were called me Mr. Rational. And I've always been fascinated with probability, doing the rational thing, what's justified, optimizing. And I think that's the thread that runs through 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.
8:07 Speaker 02 Mr. Rationality is not the worst nickname one could have. I mean, I get that it's a little bit of a dig, right? But it could be much worse.
8:14 Speaker 03 There was some mockery there.
8:16 Speaker 02 But now you've found your people. You're a professional philosopher, and the whole rationality thing is probably simply a compliment.
8:23 Speaker 03 I'll take it.
8:23 Speaker 02 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?
8:56 Speaker 03 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.
9:37 Speaker 03 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.
10:11 Speaker 02 But people don't actually do that, do they?
10:16 Speaker 03 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 the 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,
10:58 Speaker 03 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 worldview and just becoming a kind of big averaging machine, is that in fact, in many cases, many of us
11:44 Speaker 03 are rather non-even-handed in our answers to those questions. You think, what would you think if so-and-so thought blah, blah, blah? And often the answer is, wow, you know, 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 I found this person disagreed? I would think, you know, I guess I think they're probably wrong. And the version of the equal weight view, which is what's this, 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.
12:23 Speaker 03 And that's not quite as possessive as a more extreme version of you, which says, really, just always go 50/50.
12:31 Speaker 02 So what is, I'm a little confused as to why my past self is useful here. If I, if I tell my past self all the relevant new information I've gotten, isn't that just my present self?
12:41 Speaker 03 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?
13:25 Speaker 03 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, you get this answer, how likely do you think that you'll be the one who's right? That seems so one, and I actually noticed 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 ones who'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.
14:07 Speaker 02 Okay, good. You're just asking them about their basic judgment ahead of time, what is the likelihood that someone's gonna be wrong, is it gonna be me?
14:15 Speaker 03 Yeah, and you wanna tell them something about the circumstance. 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 kinda 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.
14:51 Speaker 02 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
15:03 Speaker 02 Have our credences in different propositions changed by the right amount when we meet other equally smart people with different credences?
15:11 Speaker 03 I love that characterization. I think my opponents would disagree.
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16:15 Speaker 03 I see that this view meets that standard, but I think that's the kind of thing everyone's going for.
16:19 Speaker 02 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?
16:47 Speaker 03 Well, I think there could be two things going on. And it's actually a very, this exercise of looking back at what would my past self just 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 a 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.
17:29 Speaker 03 It could have been that all along, you thought, yeah, but when it comes down to it, if we disagree, I already was gonna 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.
18:10 Speaker 02 I'm right.
18:11 Speaker 03 I noticed a weird, 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.
18:48 Speaker 02 Yeah, that's a problem. I do find that in, again, ordinary human conversation, among non-Mister and Miss Rationals, people will sometimes have a criticism of the form, you always think you're right.
19:03 Speaker 02 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, but of course, what they, I think that the legitimate criticism is you're unwilling to ever admit that you could be wrong, right?
19:21 Speaker 03 Yes, exactly. And by the way, I can't resist citing a paper that Andy Egan and I wrote a while back. The title of the paper is, I can't believe I'm stupid. And it's all about the limits on how, 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 Boltzmann brains. It's called the level splitting view. And it's something that I also have learned a lot from David Kristensen about.
20:04 Speaker 03 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, that I'm, 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?
20:36 Speaker 02 Yeah.
20:37 Speaker 03 Yep, I'm right. Even though the weather forecaster said no. But somebody asked you, well, what's the rational thing to believe? Oh, that 50/50.
20:44 Speaker 02 I see. So it's having a confidence in an opinion, but also a feeling that my confidence might be misplaced.
20:52 Speaker 03 Yes. And so put that in your pocket for later in this podcast. That's the level splitting view. When we come to Boltzmann brains, there's going to be a potential way out related to that.
21:03 Speaker 02 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 have probably read about, but maybe not the audience. Am I right in thinking that a lot of this goes back to Derek Parfit's talk back in the day about teletransporters and self locating uncertainty?
21:23 Speaker 03 It's definitely related to that stuff. I'm I not confident on the origins of any of this, but I know people have thought about cases in which you're wondering who you are and when it is, certainly long before I came on the scene.
21:38 Speaker 02 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.
21:49 Speaker 03 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 I want to get readers on board with it, too. So standard tele transporter case that you step in the tele transporter, 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,
22:31 Speaker 03 but the receiving bays of the Enterprise and 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?
23:10 Speaker 02 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
23:23 Speaker 03 Ah, okay. Ah, okay. Fair enough.
23:28 Speaker 02 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?
23:39 Speaker 03 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
24:19 Speaker 03 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.
24:39 Speaker 03 So there's just one Enterprise though, and there are 100 Potemkins. 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%
25:24 Speaker 03 certitude to happen is there will be a body created on the Enterprise, there'll be 99, 100 on the Potemkin, 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.
25:44 Speaker 02 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 Potemkins, one is on the Enterprise? Do you think it's the right thing to do to assign equal credences to being any of them?
26:04 Speaker 03 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 view that's happened over the years, is the problem of Boltzmann brains.
26:25 Speaker 03 And it's thrown my confidence in this whole business to hell. And so I can't pretend to have, the answer is if you have forced me to choose, I'm going to go and go with that view. But there are twists and turns and I'm not that confident.
26:43 Speaker 02 I think that's perfectly fair because Boltzmann brains do shake one up. But, 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, 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.
27:17 Speaker 03 That's rough. As, you know, speaking as Mr. Rational, I really like the idea of there being constraints. 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 is an airlock out into outer space.
27:52 Speaker 03 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.
27:58 Speaker 02 Right.
28:01 Speaker 03 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.
28:15 Speaker 02 Right.
28:15 Speaker 03 We haven't had any chances like all this is what's going to happen. We're going to add in some chances and that adds something to the mix. 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?
28:46 Speaker 02 Okay, good. We, yeah, I think we should be able to get there. But I'm just trying to, you know, 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
29:08 Speaker 02 instance of us that is created in the transporter machine, right? It's not just like, well, it feels right, like we can be slightly more sophisticated in that. There's there are theorems one can prove under certain assumptions one can specify.
29:21 Speaker 03 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.
29:34 Speaker 02 Makes it a lot easier, yes.
29:35 Speaker 03 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?
29:52 Speaker 03 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 Stahlke's, when you wake up in the middle of the night groggily, before you look at your clock and you just think,
30:37 Speaker 03 uh, what, 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. Uh, it just seems like it should be 50/50. So 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.
31:03 Speaker 02 Right.
31:04 Speaker 03 So that that could favor, tilt the deck. No, it's just where am I within this one story? And I do, I do confess an inclination to think, split it 50/50.
31:15 Speaker 02 Well, you did mention the idea that it's, uh, 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. Uh, there's at least, uh, one could imagine though, that the same kind of reasoning works for different worlds. Like the example I like to use is, um, imagine a cosmology where the dark matter particle is an axion. And it's another cosmology of the dark matter particle is a weakly 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.
31:57 Speaker 02 Uh, isn't that a kind of self locating uncertainty between possible worlds rather than locations in the real world?
32:04 Speaker 03 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 to 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 indifference.
32:44 Speaker 03 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 tangential 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.
33:08 Speaker 02 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 would 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.
33:49 Speaker 03 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 in along the lines of, hey, if you think there's this special attitude of suspending judgment, or my probability is not a probability of 0.3, but it's rather best represented by an interval from 0.2 to 0.7, or something like that, I'm interested in pressing people who have that attitude on, well, what is that attitude?
34:27 Speaker 03 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,
34:36 Speaker 02 I have no belief.
34:36 Speaker 03 Just be silent about it.
34:38 Speaker 02 Yeah.
34:40 Speaker 03 But, 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 fifty fifty fifty 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
35:20 Speaker 03 cases, the priors that I think are reasonable are highly none even. And I'm thinking of cases of theories that are very complicated or ad hoc.
35:34 Speaker 02 Yeah, no, I'm a hundred percent on board. I guess I didn't explain my example well enough because I, 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.
35:46 Speaker 03 Oh, oh, yeah. Oh, yeah, definitely. I'm with you on that. I thought it was fifty fifty.
35:51 Speaker 02 Yeah, no, no, I didn't want to argue that it was fifty fifty in that case. I certainly don't think it's fifty fifty in that case because there's some, like you say, there's, there might be different theoretical virtues that the different theories have. They're not really symmetric with each other, but.
36:06 Speaker 03 Absolutely.
36:06 Speaker 02 But there is some uncertainty.
36:09 Speaker 03 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.
36:30 Speaker 03 Find it easy to fall into representing scientific reasoning as Bayesian reasoning. So we either represent or reconstruct scientific theory choice as starting with some priors over the theories, maybe not even ended 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 people have said, is that the criteria that scientists seem to be applying in those
37:09 Speaker 03 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, the 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. Yeah. And it made me wonder, how are those two?
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38:49 Speaker 03 How do I square those two things?
38:51 Speaker 02 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 pi times the mass of the electron, right?
39:23 Speaker 03 Yeah.
39:23 Speaker 02 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.
39:56 Speaker 03 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 a sensible education in physics to be able to cultivate. And it's sort of, well, that's just an interesting thing. It's not an objection, man. It's something I'm taking as a lesson.
40:26 Speaker 02 Well, yeah, and even, I mean, the pi example is especially contrived, but the cosmological constant, the vacuum energy, which we know to be small but not zero, or 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 coarse-graining over the possibilities and not assigning them uniform probability.
40:56 Speaker 03 Yeah.
40:56 Speaker 02 And that's because we're being scientists. I think it's okay.
40:58 Speaker 03 Exactly, exactly. And feel free to bring this back in if we get derailed too much. But I did want to also point out that the plain vanilla Bayesian approach, the thing you'd 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 in 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.
41:40 Speaker 03 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.
41:48 Speaker 02 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 fallibilist 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, 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.
42:36 Speaker 03 I like it.
42:37 Speaker 02 But again, that's just off the cuff and it's certainly not a very systematic theory of it. I would like, I would like to have that, but okay, good. Back to crazy thought experiments.
42:46 Speaker 03 Sure.
42:47 Speaker 02 We, I, we are going to get to boltzmann brains and many worlds and things like that very quickly, but I gotta 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.
43:07 Speaker 03 No problem. I, 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.
43:25 Speaker 02 In the universe?
43:26 Speaker 03 Yes, yes. Yes. Very roughly speaking, because otherwise it would be so unlikely that you exist. It's worth, it's worth, it's worth looking at. It's a wild, 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 Stahlker. And the problem is this. Beauty is put to sleep at the beginning of the experiment on Sunday, and then a fair coin toss is going to determine whether beauty will just be woken up on Monday night, or
44:07 Speaker 03 alternatively briefly woken up on Monday night 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 themself, this feels like the first waking. I don't have an apparent memory of another one. Is it that the coin landed heads and it's Monday? Or is it that the coin landed tails and it's Monday? Or tails and Tuesday?
44:49 Speaker 03 The mnemonic is tails is for the two waking scenario.
44:52 Speaker 02 Okay, good.
44:54 Speaker 03 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,
45:34 Speaker 03 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.
45:51 Speaker 02 Hmm. 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?
45:59 Speaker 03 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 backwards 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
46:44 Speaker 03 the case where the coin toss is really later. That's why people don't freak out when... about exactly when lottery drawings happen and so on. We sort of think that, to the extent that, you know, no one's cheating and no one's peeking, 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 coin will 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.
47:21 Speaker 03 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 probability in the scenario heads Monday and the scenario heads, uh, 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 it's Monday beliefs to your post
48:05 Speaker 03 learning it's 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, uh, tails and one third in heads.
48:20 Speaker 02 Right.
48:21 Speaker 03 And that's being, that's being a thirder.
48:23 Speaker 02 Good, a thirder, right. And the other option is being a heifer.
48:27 Speaker 03 Correct. Well, I shouldn't say the other option because there are, there are two, there are two other options. There, there's, um, 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-haffers who in the stage of the experiment before learning that it's Monday, think it's 50/50, uh, about the coin. And then after learning that it's Monday, think it's still 50/50.
49:11 Speaker 03 In other words, those people deny the premise I just mentioned about keeping the ratios the same when you learn that piece of news.
49:19 Speaker 02 So, but this, so the sleeping beauty problem, um, is a case of self-locating uncertainty. It's, but it's a little bit trickier, right? Rather than just two copies, there's three instances. There's heads Monday, tails Monday, tails Tuesday, and the thirder position will be to assign them equal credences.
49:43 Speaker 03 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 third 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
50:25 Speaker 03 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 man. 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 there are great philosophers who are into that. But I'm actually very cautious and conservative about these things. And
51:03 Speaker 03 I hate the idea of being forced into bizarro views by self locating beliefs. And it's really, 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 the end of it, someone can say, you know what, turned out all those philosophers, they were wasting their time talking about self locating belief. It's just plain vanilla, whatever.
51:30 Speaker 02 Yeah.
51:31 Speaker 03 And we can stop talking about all that, just go on with our lives. I would be a happy man.
51:35 Speaker 02 I've certainly heard.
51:36 Speaker 03 And the worry is, you can't have that.
51:37 Speaker 02 I've heard, 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.
51:45 Speaker 03 It's hard. I mean, we can, 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 anything about the case. All right, you could do that. But what I want is a case that.
51:59 Speaker 03 Gives us the kind of rationality verdicts that we ordinarily thought we were going to have, you know, the scientists, should they run, is this particle accelerator worth 600 billion?
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53:07 Speaker 03 Dollars to build or not?
53:09 Speaker 02 Right.
53:09 Speaker 03 That's a question that should have an answer.
53:11 Speaker 02 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 in them? Because I could be any one of those observers.
53:56 Speaker 03 Inconsistency, I am forced to answer yes, though I don't like it. That's the real, that's the honest truth.
54:02 Speaker 02 Yeah.
54:03 Speaker 03 This is an instance of what, I mean, we can't, look, we can't not talk about presumptuousness at this point.
54:08 Speaker 02 Let's do it.
54:11 Speaker 03 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
54:46 Speaker 03 possible worlds that involve many instantiations of your experience, your state of mind,
54:54 Speaker 03 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 philosopher's 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.
55:28 Speaker 02 I will say, as a matter of, I don't know, marketing, or whatever you want to call it,
55:35 Speaker 02 the other view, for the, 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, they to call them that. I think that the whole thing, there's a lot of presumptuousness going around.
56:12 Speaker 03 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 customary, the terms that are customary in this literature.
56:25 Speaker 02 Great.
56:26 Speaker 03 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 Bostrom, 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
57:05 Speaker 03 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
57:48 Speaker 03 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.
58:05 Speaker 02 What is the label on the third view?
58:07 Speaker 03 The label of the view, 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 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.
58:52 Speaker 03 So like the third or so the contrary view in the Sleeping Beauty problem is you have these three possibilities. Heads Monday, tails Monday, tails Tuesday. When you eliminate tails Tuesday, that probability from tails Tuesday gets split up to the heads Monday and tails Monday possibilities in proportion with what was originally there.
59:19 Speaker 03 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 possible worlds. All of it has to go over to tails Monday, and that's how you get 50/50.
59:35 Speaker 02 All right, good. I'm glad that they all have names. The names could be snappy, I gotta say. But let me, 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's, it lines up with anything you've said so far. It's closest to what Radford Neal calls fully non-indexical conditioning. Do you know about that?
59:57 Speaker 03 Ah, yes. I do, but I haven't thought deeply about it. I like that paper.
1:00:00 Speaker 02 So, 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, I share your worry that it's 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.
1:00:27 Speaker 02 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.
1:00:45 Speaker 03 I love it. I hadn'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 do this case, because I think it's a nice case to splay 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
1:01:28 Speaker 03 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, you know, what verdict does it say about that case?
1:01:50 Speaker 02 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 50% chance that a person like me exists in theory A.
1:02:20 Speaker 03 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, and that's it. So we have to think about whether that's presumptuous or not. Doesn't seem too bad. I have to say, it does, it 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.
1:02:56 Speaker 03 It's interesting, think about what priors one would have to have over all the possibilities in order, in advance, in order to deliver that verdict by conditionalizing. I haven't thought that through. That, that'd be interesting.
1:03:15 Speaker 02 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.
1:03:28 Speaker 03 That's great. That's great. Well, I hope you'll think about this, this case too.
1:03:31 Speaker 02 Yeah, yeah, no, that's a very, very good one. Okay, but now, all right, we've eaten our vegetables, we've, we've laid the groundwork, and now we can talk about Boltzmann brains. Do you want to tell the audience member who's never heard of a Boltzmann brain what that weird phrase refers to?
1:03:47 Speaker 03 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 a 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 Goch that just formed out of nothing,
1:04:31 Speaker 03 out of pure random chance. If that's true, and if it's true that
1:04:39 Speaker 03 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 awakenings and some of them are just fluctuation awakenings. 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 awakenings are Boltzmann
1:05:24 Speaker 03 brain awakenings, I should think, I'm really confident that I'm a Boltzmann brain. That's the initial, that's the first pass of how we seem to get into trouble.
1:05:35 Speaker 02 And I think, um, tell me what you think about this. Um, here's a case where my fellow cosmologists, um, 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, 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 Boltzmann 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 you think.
1:06:14 Speaker 03 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
1:06:41 Speaker 03 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
1:07:21 Speaker 03 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 Boltzmann bubble you are embedded in.
1:08:01 Speaker 03 I think it would take a very extreme kind of externalism to think that it's part of my evidence that, um, that part of my evidence that, for example, I'm part of the first Big Bang, as opposed to some other Boltzmann bubbly Big Bang type event. And someone could have different attitudes for that. You know, 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
1:08:46 Speaker 03 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 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 ex-journalism, the extreme ex-journalist views, and so as a result, unfortunately, pinched harder by the Boltzmann brain problem.
1:09:12 Speaker 02 I guess, so I'm trying to map on the ex-journalist point of view you just put forward to, I think I was, again, insufficiently clear in talking about the attitudes of my fellow cosmologists.
1:09:26 Speaker 03 I see.
1:09:27 Speaker 02 They all say I'm not a Boltzmann brain, but the conclusion from that, I'm realizing as I'm thinking more carefully, is a little bit different. Someone like Raphael Bousso, 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."
1:10:03 Speaker 03 Is the data the fancy cosmology data, or is the data like, I look at my hand, or I look out my window, and I see that I'm not floating in space?
1:10:10 Speaker 02 The latter.
1:10:11 Speaker 03 I see. That does feel a little bit like ex-journalism, because I guess the natural internalist reply, the natural counterpoint to the ex-journalist 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.
1:10:37 Speaker 02 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, 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."
1:11:16 Speaker 03 I see. Okay, so I was, 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.
1:12:00 Speaker 02 Right.
1:12:01 Speaker 03 Well, if that's... Oh, go ahead.
1:12:03 Speaker 02 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 a 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."
1:12:26 Speaker 03 Okay.
1:12:27 Speaker 02 But, and my response to that would be, you know, again, 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 Boltzmann fluctuations, but I think they're doing it for the wrong reason. And as often happens, nobody cares when you have, when you have that move where you're getting the same answer they did for a slightly different reason. That's, that's not going to change their, their worldview very much.
1:13:00 Speaker 03 I got it.
1:13:01 Speaker 02 Okay, but you have a slightly different take. You just came out with a new paper on Boltzmann brains. Do you want to explain what you're thinking about?
1:13:09 Speaker 03 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, about memory. So this is a, 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.
1:13:49 Speaker 03 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 thing, "Okay, I have a horrible memory, I'm just gonna do my memory training or whatever." Then you think, "Wait, if 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."
1:14:33 Speaker 03 But then you think, "Wait, if my memory's fine, I should trust that memory?" 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 Boltzmann 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 Boltzmann 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.
1:15:13 Speaker 03 So in the Boltzmann case, you think, "Okay, everyone's a Boltzmann brain, I'm a Boltzmann brain probably." But then you think, "Wait, Boltzmann brains should not trust their apparent memories. Boltzmann 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.
1:15:50 Speaker 03 Now, what do they think now? They think, okay, so I'm not a Boltzmann 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 Boltzmann brain puzzle? Let me stop there and see how that sits with you so far.
1:16:17 Speaker 02 I think one, yeah, I get it. And I wanted 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, your Boltzmann 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 Boltzmann brain, is that even most of the people who think they're in offices, or who are in offices, for that matter, in these cosmologies are still random fluctuations. They don't have reliable connections to the past.
1:17:01 Speaker 03 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.
1:17:34 Speaker 02 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 fluctuation, 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.
1:18:19 Speaker 02 But you want to say that's a little bit of a cheat?
1:18:23 Speaker 03 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 out that response, but it's harder to rule out than the patently unstable response.
1:19:02 Speaker 03 And let me give the a 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 it's the sort of question I think is going to determine what you should do in that case is
1:19:42 Speaker 03 conditional on my having a, you know, bad memory of this disease of that kind. How likely is it that I would have that apparent memory?
1:19:53 Speaker 03 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
1:20:36 Speaker 03 to Roger White and unpublished work that I hope listeners will look at once it's published. And that's the case of a 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's, 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 clear, more clear how to respond to the instability argument. So someone could say, look, either the machine
1:21:18 Speaker 03 is good or is 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 bad, then don't trust that it's from, 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.
1:21:51 Speaker 03 So now let's transport that kind. 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.
1:22:03 Speaker 02 Yeah.
1:22:03 Speaker 03 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's in this box? Like, I don't, I don't know, something. But notice, the answer of, I don't know, something is not unstable.
1:22:24 Speaker 02 Right.
1:22:25 Speaker 03 Like, that's okay. That's consistent with stably thinking, okay, it says that it's it's reporting that it's made of a fried egg. The corresponding stance in the Boltzmann brain case is that that Boltzmann 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 Boltzmann brain. And I think that that argument does put a lot of pressure, maybe even rule out that stance. But what
1:23:06 Speaker 03 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.
1:23:23 Speaker 02 Right.
1:23:24 Speaker 03 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 purely random process, a monkey. It's like, what should you think? Well, what, 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 uh, escape routes. You could think, you know, cosmology's off on the wrong track. Something's, something wrong. The universe is not big. Uh, though that, that experiment, yep, that rec, the experiment must have gone wrong. It must have been misleading. I don't like
1:24:05 Speaker 03 any of these hypotheses. Uh, I think they're bad. I think the problem of Boltzmann brains fully or almost fully survives. I find it just as unacceptable to be forced into this conclusion, but it's not instability.
1:24:21 Speaker 02 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 Boltzmann 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 Boltzmann brains in it. And that's not that
1:25:03 Speaker 02 hard to do. So let's do that with 99% credence.
1:25:10 Speaker 03 That is the way you could go. Yes. And I accept that the anti-skeptical 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, Sunan Dorgomage and Miriam Schoenfeld wrote a great paper on Boltzmann 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, oh, I'll think about it. I'll think of that. That seems interesting. And I thought, you know, I don't believe this, but you, you
1:25:54 Speaker 03 guys should really think that. And then I started teaching on this stuff. I'm really worrying about Boltzmann. And I thought, you know what? I really kind of like, 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 view. And they said, no, we've given that view. We don't like it. So we changed places. Here's the changed places view. 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 skeptical theories,
1:26:37 Speaker 03 I'm sorry, some weirdo theories just deserve low prior. That's one standard view.
1:26:46 Speaker 03 Such as a theory according to which you are a highly misled brain in the vat. These are, 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 in the vat, but the rest of us have to go for something like that, I think.
1:27:30 Speaker 03 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 favors the first sort of predicament over the second.
1:28:03 Speaker 02 Wait, the first was being a human being, regular, ordinary observer. Okay.
1:28:06 Speaker 03 Ordinary observer. So if we had that, but before, before we get off the boat and, you know, talk about the weaknesses of the view, 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.
1:28:36 Speaker 03 But that's kind of a separate factor.
1:28:37 Speaker 02 Yeah.
1:28:38 Speaker 03 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 Boltzmann 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.
1:29:11 Speaker 02 Okay, I don't like it. And for a reason that is connected to something I think you said earlier.
1:29:17 Speaker 03 Yeah.
1:29:18 Speaker 02 I think something almost exactly along these lines was proposed by Hartle and Shridhiki. Do you know their papers?
1:29:24 Speaker 03 Zero graphic distributions?
1:29:26 Speaker 02 Yes, that's right.
1:29:27 Speaker 03 Sure.
1:29:27 Speaker 02 So they're just saying, look, okay, there's a universe out there, has a lot of Boltzmann 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 Boltzmann brains. I'm ordinary. And that sounds similar to what you were just pushing, no?
1:29:50 Speaker 03 I haven't read that paper, but from what you've said, the thing that I've said is a little bit more committal than that. Someone might think they were getting away with something. This one might, I'm not saying they do this, but someone could say merely talking about different theories that posit different zero graphic distributions can get you to, oh, the theory that has this human favoring zero graphic distribution is confirmed. I haven't really put anything in new fundamental on the ground floor. I've just mentioned this, once you see that a theory can be paired with a zero graphic distribution, that will solve your problem.
1:30:34 Speaker 03 I agree with you that that does not work.
1:30:36 Speaker 02 Right.
1:30:37 Speaker 03 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
1:30:53 Speaker 03 Analog with anti skeptical scenarios, I think, maps onto this well. It's no answer to skepticism to just say, look, here's a theory. It has a zero graphic distribution according to which the anti skeptical hypotheses get get high prior. That doesn't get you unless you have the commitment. That's the real one. That's the rational one. That's the right one. And so you 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 at all.
1:31:27 Speaker 02 But I guess, I mean, I think I agree on on everything you just said. The thing that seems to be missing to me in the Hertel Schraddicki proposal that maybe there are lots of Boltzmann brains, but I just know I'm not one of them, is most people who would say that are Boltzmann 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.
1:32:07 Speaker 02 In an eternally fluctuating universe, there'll be many such observers in many such past light cones. And with overwhelming probability, 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?
1:32:38 Speaker 03 Think about an analog in the case of predicting the future. And this is going, this is going to
1:32:46 Speaker 03 human skepticism about the future. Take a really simple world, like dot matrix world. It's two dimensional dot matrix world, one two dimensional space, one dimension time. That's all there is. Or actually, even even simpler, it's a binary world. There's just a series of binary signals
1:33:04 Speaker 02 Good.
1:33:05 Speaker 03 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's in the same spirit of what you just said, where you're really insisting on that very seemingly uninformed
1:33:46 Speaker 03 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.
1:34:09 Speaker 02 It might be. I'm thinking that in the Boltzmann case, you have a prior given by the dynamics, the Leuville 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.
1:34:20 Speaker 03 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
1:35:04 Speaker 03 who, 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.
1:35:17 Speaker 03 In response, someone could say, no, I could just remember that I was here five minutes ago, and so I should take this. I don't want to go along with that. So the kind of predicaments that I think there's some shot at saying are get intrinsic low plausibility are really the kinds of radically diluted randomly created creatures and not that. All right.
1:35:43 Speaker 02 Brilliant narrative strategy to bring us back to exactly where we started. Let me just, let me just close with then 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 of 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?
1:36:24 Speaker 03 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. I these in this age of AI, I can't resist but add. And there are certain economists who have, 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
1:37:06 Speaker 03 leave listeners with the thought of, you know, 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 shouldn't trust us.
1:37:20 Speaker 02 But that's an easy question. No, of course, the AI should not trust us. What in the world?
1:37:24 Speaker 03 Well, well, that's that's tricky. Look, look at what happens if you think the AI is in somewhat of the undermining, self undermining type situation.
1:37:33 Speaker 02 Yeah.
1:37:33 Speaker 03 That a Boltzmann brain is in, or the X-ray machine looking at self, you know, is in. If you're in that situation, you think, I can't trust anything. Now you start rewinding 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.
1:37:56 Speaker 02 Let's think about that. I usually like to end on optimistic notes, but wherever 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.
1:38:09 Speaker 03 Pleasure to chat. Thank you.
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