[Paper] On the 'Simulation Argument' and Selective Scepticism

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.

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.

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 mutually exclusive, my credence in SIM should be >= x.

I accept that this is a valid argument. The problem with it, in my view, is that (A ∨ B) is not a complete description of what my evidence says.

Let V represent the proposition that my evidence regarding f-sim is veridical (i.e., the true value of f-sim is indeed what it appears to be). If A is true, then V is also true. So a more complete description of what my evidence says is (A ∧ V) ∨ (B ∧ ~V).

Now we need to ask: is it true that, conditional on (A ∧ V), my credence in SIM should be >= x?

BIP doesn’t entail that it should be, since BIP takes no account of the relevance of V. And V is surely relevant, since (on the face of it, at least) V is far more likely to be true if I am not simulated (i.e., Cr (V | ~SIM) >> Cr (V | SIM)).

Indeed, if one were to learn that (A ∧ V) is true, one might well rationally assign credence =< x to SIM. However, it’s not important to my argument that one’s credence should be =< x: all that matters is that there is no compelling reason to think that it should be >= x.

In short, then, your argument shows that, conditional on a certain description of what my evidence indicates, my credence in SIM should be >= x. But that description is the not the most complete description available—and we must always use the most complete description available, because we often find in epistemology that incomplete descriptions of the evidence lead to incorrect inferences.

Nevertheless, I think your response does expose an error in the paper. I should have formulated BIP* like this, explicitly introducing V:

BIP*: Cr [SIM | ((f-sim = x) ∧ V) ∨ ((f-sim ≠ x) ∧ ~V)] >= x

When BIP* is formulated like this, it is not entailed by BIP. Yet this is the modified principle Bostrom actually needs, if he wants to recover his original conclusion while rejecting Good Evidence. So I think the overall argument still stands, once the error you point out is corrected.

If your case is that BIP is insufficient to establish the conclusions Bostrom wants to establish, I'm pretty sure it does in fact suffice. If you accept both of these:

BIP: Cr[SIM|f-sim ≥ x] ≥ x (where f-sim is over all observers in our evidential situation)
Cr[f-sim ≥ x |V ] Cr[V|¬SIM] ≥ y_x

then we derive Cr[SIM] ≥ 1- (1-x)/y_x. x is some estimate of what f-sim might be in our world if we are not in a simulation and our current evidence is veridical, and y_x is our estimate of how likely a large f-sim is given the same assumptions; it's likely to be around f_I f_p.

5Benya13y

Well -- look, you can't possibly fix your argument by reformulating BIP*, because your paper gives a correct mathematical proof that its version of BIP*, plus the "quadripartite disjunction", are enough to imply the simulation argument! :-) (For people who haven't read the paper: the quadripartite disjunction is H1 = almost no human civilizations survive to posthuman OR H2 = almost no posthuman civilizations run ancestor simulations OR H3 = almost all humans live in a simulation OR SIM = I live in a simulation. To get the right intuition, note that this is logically equivalent to "If I live in the real world, i.e. if not SIM, then H1 or H2 or H3".) More formally, the argument in your paper shows that BIP* plus Cr(quadripartite disjunction) ~ 1 implies Cr(SIM) >~ 1 - Cr(H1 or H2). I think that (a) there's a confusion about what the symbol Cr(.) means, and (b) what you're really trying to do is to deny Bostrom's original BIP. Your credence symbol must be meant to already condition on all your information; recall that the question your paper is examining is whether we should accept that Cr(SIM) ~ 1, which is only an interesting question if this is supposed to take into account our current information. A conditional credence, like Cr(SIM | A), must thus mean: If I knew all I know now, plus the one additional fact that A is true, what would my credence be that I live in a simulation? [ETA: I.e., the stuff we're conditioning on is not supposed to represent our state of knowledge, it is hypothetical propositions we're taking into account in addition to our knowledge! The reason I'm interested in the BIP* from the paper is not that I consider (A or B) a good representation of our state of knowledge (in which case Cr(SIM | A or B) would simply be equal to Cr(SIM)); rather, the reason is that the argument in your paper shows that together with the quadripartite disjunction it is sufficient to give the simulation argument.] So Bostrom's BIP, which reads Cr(SIM | f_sim =

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[Paper] On the 'Simulation Argument' and Selective Scepticism

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[Paper] On the 'Simulation Argument' and Selective Scepticism

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