You forgot about MetaOmega, who gives you $10,000 if and only if No-mega wouldn't have given you anything, and O-mega, who kills your family unless you're an Alphabetic Decision Theorist. This comment doesn't seem specifically anti-UDT -- after all, Omega and No-mega are approximately equally likely to exist; a ratio of 1:1 if not an actual p of .5 -- but it still has the ring of Just Cheating. Admittedly, I don't have any formal way of telling the difference between decision problems that feel more or less legitimate, but I think part of the answer might be that the Counterfactual Mugging isn't really about how to act around superintelligences: It illustrates a more general need to condition our decisions based on counterfactuals, and as EY pointed out, UDT still wins the No-mega problem if you know about No-mega, so whether or not we should subscribe to some decision theory isn't all that dependent on which superintelligences we encounter.

I'm necroing pretty hard and might be assuming too much about what Caspian originally meant, so the above is more me working this out for myself than anything else. But if anyone can explain why the No-mega problem feels like cheating to me, that would be appreciated.

Counterfactual Mugging

by Vladimir_Nesov 1 min read19th Mar 2009295 comments


Related to: Can Counterfactuals Be True?, Newcomb's Problem and Regret of Rationality.

Imagine that one day, Omega comes to you and says that it has just tossed a fair coin, and given that the coin came up tails, it decided to ask you to give it $100. Whatever you do in this situation, nothing else will happen differently in reality as a result. Naturally you don't want to give up your $100. But see, Omega tells you that if the coin came up heads instead of tails, it'd give you $10000, but only if you'd agree to give it $100 if the coin came up tails.

Omega can predict your decision in case it asked you to give it $100, even if that hasn't actually happened, it can compute the counterfactual truth. Omega is also known to be absolutely honest and trustworthy, no word-twisting, so the facts are really as it says, it really tossed a coin and really would've given you $10000.

From your current position, it seems absurd to give up your $100. Nothing good happens if you do that, the coin has already landed tails up, you'll never see the counterfactual $10000. But look at this situation from your point of view before Omega tossed the coin. There, you have two possible branches ahead of you, of equal probability. On one branch, you are asked to part with $100, and on the other branch, you are conditionally given $10000. If you decide to keep $100, the expected gain from this decision is $0: there is no exchange of money, you don't give Omega anything on the first branch, and as a result Omega doesn't give you anything on the second branch. If you decide to give $100 on the first branch, then Omega gives you $10000 on the second branch, so the expected gain from this decision is

-$100 * 0.5 + $10000 * 0.5 = $4950

So, this straightforward calculation tells that you ought to give up your $100. It looks like a good idea before the coin toss, but it starts to look like a bad idea after the coin came up tails. Had you known about the deal in advance, one possible course of action would be to set up a precommitment. You contract a third party, agreeing that you'll lose $1000 if you don't give $100 to Omega, in case it asks for that. In this case, you leave yourself no other choice.

But in this game, explicit precommitment is not an option: you didn't know about Omega's little game until the coin was already tossed and the outcome of the toss was given to you. The only thing that stands between Omega and your 100$ is your ritual of cognition. And so I ask you all: is the decision to give up $100 when you have no real benefit from it, only counterfactual benefit, an example of winning?

P.S. Let's assume that the coin is deterministic, that in the overwhelming measure of the MWI worlds it gives the same outcome. You don't care about a fraction that sees a different result, in all reality the result is that Omega won't even consider giving you $10000, it only asks for your $100. Also, the deal is unique, you won't see Omega ever again.