Here is an argument that some people might find compelling:
It may be that the world is mad, and that as the only sane person around it falls on me to make sure we don’t all kill ourselves. If that’s the case, then my impact on the world may be huge. Let’s say that in this case, I can improve the world by 1%.
Maybe the claim that I’m particularly influential, call it proposition P, isn’t certain. But at least there’s a good chance. Subjectively it feels like about 1%, since if I looked at 100 similarly surprising facts, I would expect one of them to be true. (I wouldn’t be that surprised to discover that I’m the most important person ever…) That still leaves me with the ability to improve the world by 0.01% in expectation, which looks pretty good. I might as well not even worry about stuff I could do that would improve the world by a mere 0.001%, like being an extraordinarily successful entrepreneur.
What is wrong with this argument? Intuitively, the trouble is that out of the 7 billion people on Earth, at most a handful can be so important. So even if you discovered evidence that suggested P quite strongly, you ought to remain skeptical. Even if a magic 8-ball which lied only one time in a million told you that you were the most influential person alive, you should still bet against it—after all, 7,000 people will hear this particular lie, while only one will be right. (Setting aside the fact that your mere possession of such an 8-ball constitutes much more than million to one evidence!)
If you have some clever argument that you aren’t “in the same reference class” as those other 7 billion people, you need to be awfully sure that it would be difficult to manufacture that argument if you weren’t in fact the most influential person. If you had a 1 in a million chance of being able to delude yourself into thinking you were special, you’d still be wrong nearly 99.99% of the time. But replies the skeptic…
Putting a prior probability of 1 in 7 billion on something plausible is ridiculous! After all, if the argument you just gave has even a 1% chance of being wrong, then I might have a prior probability of up to 1%! Do you think you could make 100 arguments that compelling, before you messed one of them up?
One response to the situation is to say that you really are so confident, because this kind of anthropic prior improbability is a special case. I think this is probably untenable, because your reasoning really isn’t that good. If you had to make 7 billion independent arguments as complicated as this one, I’d be surprised if you didn’t mess up one of them on a technicality.
Another response to this situation is to throw up your hands and discount the possibility P as an instance of Pascal’s mugging. Maybe we don’t understand why we shouldn’t act on the basis of such small possibilities of large upsides, but it’s intuitively obvious it would be wrong.
If we take the perspective of evidential or timeless decision theory, however, this problem vanishes. In these theories, we use a different decision rule: take the action which you would be happiest to learn that someone in your situation had taken. To decide what to do in situation S, compute E[ U | “in situation S I would pick action A” ] and E[U | “in situation S I would pick action B” ], and choose whichever action leads to the higher utility.
In this framework, we should no longer assign a non-negligible probability to being confused about anthropic questions, because such questions are never asked–the relevant properties are baked directly into the decision rule. Suppose that there are a billion people, P is true for exactly one of them, and I receive some evidence that is a million times more likely if P is true. Then I’m given the option to take some gambit, which increases U by 100 if P holds, and decreases U by 1 otherwise. Now if I am 99% sure that my basic picture about reality is correct I can reason:
In 99% of (impossible) possible worlds, there are 1000 observers with the evidence I have. P is true for one of them and not true for 999 of them. So if I choose to take the gambit, I will lose 999 utility and gain 100, which is a net loss.
In the remaining 1% of possible worlds, maybe it’s just me, and maybe property P is true. And in those worlds I would gain 100 utility. This is only 1 utility in expectation, which doesn’t offset the -900 from the other worlds.
Of course, I’ve swept a few important things under the rug; most importantly I’ve assumed that U is non-indexical. (It works fine if U = “total # of happy years of life” or U = “total # of happy years of life for people with my experiences so far” or so on. But if U = “# of happy years of life I have” then it is going to come down to anthropic questions in the definition of “I”.)
The original Pascal’s mugging
Incidentally, Pascal’s mugging is structurally identical to the argument we just discussed. Nick Bostrom describes an unarmed mugger who approaches M. Pascal:
Mugger: Let us say that the 10 livres that you have in your wallet are worth to you the equivalent of one happy day. Let’s call this quantity of good 1 Util. So I ask you to give up 1 Util. In return, I could promise to perform the magic tomorrow that will give you an extra 10 quadrillion happy days, i.e. 10 quadrillion Utils. Since you say there is a 1 in 10 quadrillion probability that I will fulfill my promise, this would be a fair deal. The expected Utility for you would be zero. But I feel generous this evening, and I will make you a better deal: If you hand me your wallet, I will perform magic that will give you an extra 1,000 quadrillion happy days of life.
Pascal: I admit I see no flaw in your mathematics.
Whatever clever argument we might suggest Pascal could use to decide that the mugger’s offer is unattractive, the mugger could always ask: “But surely, M. Pascal, there is some chance that you are mistaken?” This seems to be something of a reductio against unbounded utility. Robin Hanson is reported to have observed that, in any world large enough to contain 10 quadrillion (or whatever number) of valued objects, there are (reasonably likely to be) a comparable number of observers; most of them who believe that they have the power to create or destroy so much value must be deluded. But more importantly, each of them who is so deluded could create a constant amount of value themselves. So my large EV from the possibility I’m not deluded is balanced by my large EV from controlling more folks’ actions if I am deluded. And now the situation is transparently the same as with our proposition P. We don’t need to assign any probabilities near 1 to avoid the trouble.
(Of course, you are still going to run into divergent sums if you accept the kind of arguments Pascal does in the example, which seems to be a fundamental problem with unbounded utilities. But Pascal’s mugging is already a problem if you take some mind-boggling upper-bound on the size of the universe, and at least this works then.)