Jonathan Birch recently published an interesting critique of Bostrom's simulation argument. Here's the abstract:
Nick Bostrom’s ‘Simulation Argument’ purports to show that, unless we are confident that advanced ‘posthuman’ civilizations are either extremely rare or extremely rarely interested in running simulations of their own ancestors, we should assign significant credence to the hypothesis that we are simulated. I argue that Bostrom does not succeed in grounding this constraint on credence. I first show that the Simulation Argument requires a curious form of selective scepticism, for it presupposes that we possess good evidence for claims about the physical limits of computation and yet lack good evidence for claims about our own physical constitution. I then show that two ways of modifying the argument so as to remove the need for this presupposition fail to preserve the original conclusion. Finally, I argue that, while there are unusual circumstances in which Bostrom’s selective scepticism might be reasonable, we do not currently find ourselves in such circumstances. There is no good reason to uphold the selective scepticism the Simulation Argument presupposes. There is thus no good reason to believe its conclusion.
The paper is behind a paywall, but I have uploaded it to my shared Dropbox folder, here.
EDIT: I emailed the author and am glad to see that he's decided to participate in the discussion below.
It's interesting, but it's also, as far as I can tell, wrong.
Birch is willing to concede that if I know that almost all humans live in a simulation, and I know nothing else that would help me distinguish myself from an average human, then I should be almost certain that I'm living in a simulation; i.e., P(I live in a simulation | almost everybody lives in a simulation) ~ 1. More generally, he's willing to accept that P(I live in a simulation | a fraction x of all humans live in a simulation) = x; similar to how, if I know that 60% of all humans have a gene that has no observable effects, and I don't know anything about whether I specifically have that gene, I should assign 60% probability to the proposition that I have that gene.
However, Bostrom's argument rests on the idea that our physics experiments show that there is a lot of computational power in the universe that can in principle be used for simulations. Birch points out that if we live in a simulation, then our physics experiments don't necessarily give good information about the true computational power in the universe. My first intuition would be that the argument still goes through if we don't live in a simulation, so perhaps we can derive an almost-contradiction from that? [ETA: Hm, that wasn't a very good explanation; Eliezer's comment does better.] Birch considers such a variation and concludes that we would need a principle that P(I live in a simulation | if I don't live in a simulation, then a fraction x of all humans lives in a simulation) >= x, and he doesn't see a compelling reason to believe that. (The if-then is a logical implication.)
But this follows from the principle he's willing to accept. "If I don't live in a simulation, then a fraction x of all humans lives in a simulation" is logically equivalent to (A or B), where A = "A fraction x of all humans lives in a simulation" and B = "the fraction of all humans that live in a simulation is != x, but I, in particular, live in a simulation"; note that A and B are mutually exclusive. Birch is willing to accept that P(I live in a simulation | A) = x, and it's certainly true that P(I live in a simulation | B) = 1. Writing p := P(A | A or B), we get
P(SIM | A or B) = [P(SIM | A) p] + [P(SIM | B) (1-p)] = [x * p] + [1-p] >= x.
Thanks Benja. This is a good objection to the argument I make in the 'Rejecting Good Evidence' section of the paper, but I think I can avoid it by formulating BIP* more carefully.
Suppose I’m in a situation in which it currently appears to me as though f-sim = x. In effect, your suggestion is that, in this situation, my evidence can be characterized by the disjunction (A ∨ B). You then reason as follows:
(1) Conditional on A, my credence in SIM should be >= x.
(2) Conditional on B, my credence in SIM should be 1.
(3) So overall, given that A and B are mutua... (read more)