That's a very strong argument, thanks! You've changed my mind! I hadn't appreciated how big the multiverse in MWI could be, but that's a good way to think about it.

In terms of what impact this has on my arguments here, I still think it doesn't affect them, except that I now hope that Ignore-Copies and MWI are not both true at the same time. So in that sense I guess it's a good thing that they seem to be in conflict with one another!

The mangled worlds hypothesis is also extremely interesting! I hadn't come across it before, but as derivations of the Born rule go, this reads as the closest thing I've seen to the possible approach I was trying to describe at the end of my conclusion, where I speculated on what a derivation of the Born rule that did not clash with Ignore-Copies might look like. I'll have to go and read Hanson's papers on that. Thanks again! (may update the post to reference this)

(And if the mangled worlds derivation did turn out to be a way to reconcile Ignore-Copies with MWI, it's also nice you think it might change the conclusion on nothing mattering as well, so that we still wouldn't be forced to conclude that!)

This is a really interesting point! If the universe is big enough that any concievable computer program exists somewhere, and all conscious experience can be represented as a classical computer program, then yes, under what I'm calling the "Ignore Copies" view, nothing would matter. Nick Bostrom makes this point in the "Quantity of experience" paper I referenced (I think it's an important reason why he doesn't like the "Ignore Copies" view).

But I don't think MWI on its own necessarily predicts that the universe is this big. In fact, I'm fairly sure that it doesn't (why would it? Edit: Now think Donald is right here and I was wrong) So I think in principle someone could accept both Ignore-Copies and MWI without being committed to the view that nothing matters.

In any case, I think this point is essentially independent of the point I'm trying to make in this post, which is just that there is a tension between the logic underpinning the "Ignore Copies" ethical view, and the supposed origin of probabilities in MWI.

Thank you!

I have added a list of references, collecting all of the links from the post, and I also added a couple I found helpful but did not end up explicitly referring to in the text.

That's a nice way of looking at it. It's still not very clear to me why the SIA approach of apportioning among possible observers is something you should want to do. But it definitely feels useful to know that that's one way of interpreting what SIA is saying.

That's a fair point! I have probably undersold the idea here. I've edited the post to add a comment about this.

You raise lots of good objections there. I think most of them are addressed quite well in the book though. You don't need any money, because it seems to be online for free: https://www.stafforini.com/docs/Parfit%20-%20Reasons%20and%20persons.pdf And if you're short of time it's probably only the last chapter you need to read. I really disagree with the suggestion that there's nothing to learn from ethical philosophy books.

For point 1: Yes you can value other things, but even if people's quality of life is only a part of what you value, the mere-addition paradox raises problems for that part of what you value.

For point 2:That's not really an objection to the argument.

For point 3: I don't think the argument depends on the ability to precisely aggregate happiness. The graphs are helpful ways of conveying the idea with pictures, but the ability to quantify a population's happiness and plot it on a graph is not essential (and obviously impossible in practice, whatever your stance on ethics). For the thought experiment, it's enough to imagine a large population at roughly the same quality of life, then adding new people at a lower quality of life, then increasing their quality of life by a lot and only slightly lowering the quality of life of the original people, then repeating, etc. The reference to what you are doing to the 'total' and 'average' as this happens is supposed to be particularly addressed at those people who claim to value the 'total', or 'average', happiness I think. For the key idea, you can keep things more vague, and the argument still carries force.

For point 4: You can try to value things about the distribution of happiness, as a way out. I remember that's discussed in the book as well, as are a number of other different approaches you could try to take to population ethics, though I don't remember the details. Ultimately, I'm not sure what step in the chain of argument that would help you to reject.

On the non-transitive preferences being ok: that's a fair take, and something like this is ultimately what Parfit himself tried to do I think. He didn't like the repugnant conclusion, hence why he gave it that name. He didn't want to just say non-transitive preferences were fine, but he did try to say that certain populations were incomparable, so as to break the chain of the argument. There's a paper about it here which I haven't looked at too much but maybe you'd agree with: https://www.stafforini.com/docs/Parfit%20-%20Can%20we%20avoid%20the%20repugnant%20conclusion.pdf

Is that definitely right? I need to have an in-depth read of it, which I won't have time for for a few days, but from a skim it sounds like they admit that FNC also leads to the same conclusions as SIA for the presumptuous philosopher, but then they also argue that isn't as problematic as it seems?

Thanks, I'll check it out!

Thanks for the comment! That's definitely an important philosophical problem that I very much glossed over in the concluding section.

It's sort of orthogonal to the main point of the post, but I will briefly say this: 10 years ago I would have agreed with your point of view completely. I believed in the slogan you sometimes hear people say: "we're in favour of making people happy, and neutral about making happy people." But now I don't agree with this. The main thing that changed my mind was reading Reasons+Persons, and in particular the "mere-addition paradox". That's convinced me that if you try to be neutral on making new happy people, then you end up with non-transitive preferences, and that seems worse to me than just accepting that maybe I do care about making happy people after all.

Maybe you're already well aware of these arguments and haven't been convinced, which is fair enough (would be interested to hear more about why), but thought I would share in case you're not.

I agree that skepticism is appropriate, but I don't think just ignoring anthropic reasoning completely is an answer. If we want to make decisions on an issue where anthropics is relevant, then we have to have a way of coming up with probabilistic estimates about these questions somehow. Whatever framework you use to do that, you will be taking some stance on anthropic reasoning. Once you're dealing with an anthropic question, there is no such thing as a non-anthropic framework that you can fall back on instead (I tried to make that clear in the boy-girl example discussed in the post).

The answer could just be extreme pessimism: maybe there just is no good way of making decisions about these questions. But that seems like it goes too far. If you needed to estimate the probability that your DNA contained a certain genetic mutation that affected about 30% of the population, then I think 30% really would be a good estimate to go for (absent any other information). I think it's something all of us would be perfectly happy doing. But you're technically invoking the self-sampling assumption there. Strictly speaking, that's an anthropic question. It concerns indexical information ("*I* have this mutation"). If you like, you're making the assumption that someone without that mutation would still be in your observer reference class.

Once you've allowed a conclusion like that, then you have to let someone use Bayes rule on it. i.e. if they learn that they do have a particular mutation, then hypotheses that would make that mutation more prevalent should be considered more likely. Now you're doing anthropics proper. There is nothing conceptually which distinguishes this from the chain of reasoning used in the Doomsday Argument.