There's a very simple objection to the simulation hypothesis that I came up with, and I've read others come up with this same idea. So I assume it's something philosophers should already be aware of.

There are 2 possibilities:

  1. We're in a simulation. In which case we cannot infer anything about the world or worlds above our own (be it also a simulated world or non-simulated real world). We can only make inferences about simulations that will be constructed within our own world.

  2. We're not a in a simulation. In which case there is no higher level world to make inferences about. We can only make inferences about simulations we may construct within our world.

Either way we can only make inferences about simulations constructed 'within' our world. We can't make any inferences about whether there's a world above our own. So all of Bostrom's arguments tell us nothing about the probability that our world is itself a simulation or not. All of Bostrom's references to worlds/civilizations have to refer to world/civilizations within our own. So when he calculates probabilities he's restricted to worlds under our own.

So Bostrom's paper is a bunch of elaborate reasoning about how worlds constructed within our world are probably simulations. It's really a bunch of useless reasoning, because worlds under our own are by definition simulations.

What is the response from philosophers to this objection?

Here's a link to his paper:


  • Are you familiar with Bostrom's "self-sampling assumption", a form of anthropic reasoning? The simulation argument is mainly just saying that given that assumption, we should take it as very unlikely that both of the following claims are true: 1) many civilizations in our universe reach some kind of transhuman breakout point that results in them performing huge numbers of ancestor simulations, so that simulated beings in such ancestor sims vastly outnumber real beings at a comparable level of technological development, and 2) we are not simulated beings of this type.
    – Hypnosifl
    Commented Apr 18, 2021 at 14:28
  • @Hypnosifl, no I'm not familiar with that. Does he explicitly mention it? If he's making that assumption, isn't it just begging the question? Commented Apr 18, 2021 at 14:33
  • The self-sampling assumption doesn't refer to the statements 1) and 2) I mentioned, rather it's an idea used in deriving the conclusion that 1) and 2) are unlikely to both be true. I tried to summarize the self-sampling assumption and some of its implications in this answer, and if you want to get into more details you can look at Bostrom's book Anthropic Bias which is posted on his site.
    – Hypnosifl
    Commented Apr 18, 2021 at 14:37
  • @Hypnosifl, ok. Thanks. I see what you mean. So the question is the validity of the assumption. Commented Apr 18, 2021 at 14:44
  • 1
    Bostrom says at 1:15:21 of this podcast interview that the bland indifference principle is basically a weaker version of the self-sampling assumption, they are both part of his project of "trying to figure out what are the legitimate ways of reasoning about these indexical facts when observer selection effects are in play, in other words, developing a theory of anthropics."
    – Hypnosifl
    Commented Apr 18, 2021 at 16:20

1 Answer 1


In his original paper on the simulation argument Bostrom stated his main claim in terms of a collection of three different propositions:

This paper argues that at least one of the following propositions is true: (1) the human species is very likely to go extinct before reaching a “posthuman” stage; (2) any posthuman civilization is extremely unlikely to run a significant number of simulations of their evolutionary history (or variations thereof); (3) we are almost certainly living in a computer simulation.

Using De Morgan’s laws, a claim of the form (A or B or C) is logically equivalent to a claim of the form ~(~A and ~B and ~C), and I find it conceptually clearer to think of Bostrom’s claim in these terms. If we take the negations of his three statements (and paraphrase and elaborate them a little), what he is arguing is that he thinks it’s very unlikely all three of the following statements are simultaneously true (i.e. at least one is likely false):

#1 A significant fraction of species that have reached our level of technological development avoid extinction and go on to reach a post-biological stage where they command vast computational power (enough so that they could perform vast number of ancestor simulations as described in #2)

#2 A sufficiently large fraction of post-biological civilizations with such computing power devote enough of that computing power to ancestor simulations such that non-simulated biological intelligences end up being significantly outnumbered by simulations of biological intelligences inhabiting simulations of the past before the post-biological era.

#3 We are non-simulated.

Now, there are all sorts of reasons #1 and #2 might be false, since they involve questions of futurology, technological feasibility, and sociological questions about what posthuman civilizations would tend to want to do with their computing power. But certainly a lot of transhumanists are hoping that #1 is true, since if this is a common fate for intelligence in the universe (or multiverse), then there’s a decent chance that will be our future as well. And if #1 is true, #2 seems plausible as well. So, the heart of Bostrom’s argument is that, based on a particular type of anthropic selection principle which he discussed at length in his book Anthropic Bias (available on his website here), #3 is very unlikely given #1 and #2.

Before going into his anthropic argument, I want to address two related misconceptions that I’ve seen people bring up, the second of which is most relevant to your question:

A. This is not a general argument for skepticism about our ability to know anything about external reality, like Descartes’ Demon or the brain in the vat. Instead, Bostrom’s argument is directed specifically at transhumanists and others who would find the assumption I labeled #1 plausible, pointing out that it naturally leads to seeing #2 as plausible as well, and that if we grant the likelihood of both we are forced to consider #3 unlikely. See Bostrom’s comments here in the section titled “Not the Old Brain-in-a-Vat Argument” where he says “The Simulation argument, by contrast, adopts as its starting point that things are the way they seem to be and that science gives us reliable information about the world. Part of this information concerns the technological capabilities that an advanced civilization would be able to develop.” (He makes similar comments at 5:40 in the podcast here.) Also note that Bostrom himself seems to be a transhumanist or at least keenly interested in the subject, see his writings about it here and here and here for example, along with his book Superintelligence: Paths, Dangers, Strategies.

B. If we understand Bostrom as just claiming that #1 and #2 and #3 can’t all be true, then in no way does it refute his argument to point out that if we are in a simulation, it might not be an ancestor simulation and in fact the “real world” might be nothing like the apparent world we experience (different laws of nature, for example). This is certainly possible, but by postulating that we are simulated you aren’t disputing what he says about the three propositions. The point is to show that given the premise that the laws of nature are the ones we observe, we are led to some conclusions that are apt to seem strange for those that believe in the technological feasibility of this sort of transhuman future—either that we are living in a simulation, or that there is some kind of “Great Filter” (a possibility he talks about here) that makes it vanishingly unlikely that any civilization like ours will actually reach that transhuman level, or that there are some quasi-deterministic historical laws that lead the vast majority of transhuman civilizations to avoid detailed historical simulations even when they have the capability to run them.

Also note that when presented in my modified form with the statements #1-#3 that are negations of Bostrom's original 3 statements, we can see that the argument that #1 - #3 can't all be true amounts to a probabilistic version of a proof by contradiction (I think this is one of the main conceptual advantages of presenting the argument in the modified form). In a proof by contradiction one starts from some set of premises and proves they can't all be true by showing they lead to a contradiction; similarly if we start from premises #1 - #3 we can show that #1 and #2 together lead to the conclusion that #3 is extremely unlikely. #3 said that we are not in a simulation, so negating it implies we likely are in a simulation, and that may undermine whatever reasons we had for thinking #1 and #2 were plausible in the first place (since #1 and #2 are statements about the real world, and if we decide that we're in a simulation we can no longer claim to know with any confidence what the real world is like). But it's a general feature of proofs by contradiction that they undermine their own premises, so this is not in itself a reason for doubting the argument.

I think these two points address your original objection. As for Bostrom’s anthropic argument for saying #3 is unlikely given #1 and #2, it’s basically a variation on the “self-sampling principle” he details in his book Anthropic Bias linked above (I believe Bostrom invented this term, and he is more systematic in fleshing out the rules of its use, but he didn’t invent the basic idea which was already present in some of the work of Brandon Carter, who coined the term “anthropic principle”). I discussed the self-sampling assumption and its relation to the simulation argument (as well as the related doomsday argument) in a previous answer, so I won’t repeat that here. But one extra issue brought up in the comments is that Bostrom doesn’t actually invoke the self-sampling assumption in his simulation argument paper, instead he uses what he calls the “principle of bland indifference”. This is actually just a weakened version of the self-sampling assumption, as he says on p. 9 of the paper:

The Doomsday argument rests on a much stronger and more controversial premiss, namely that one should reason as if one were a random sample from the set of all people who will ever have lived (past, present, and future) even though we know that we are living in the early twenty‐first century rather than at some point in the distant past or the future. The bland indifference principle, by contrast, applies only to cases where we have no information about which group of people we belong to.

And as I noted in a comment, Bostrom says at 1:15:21 of this podcast interview that the bland indifference principle is "weaker than the self-sampling assumption" but that they are both part of his project of "trying to figure out what are the legitimate ways of reasoning about these indexical facts when observer selection effects are in play, in other words, developing a theory of anthropics."

  • Thanks for the thorough response. I'm still a bit troubled. Suppose we grant that we are in a simulation but we don't know what kind of simulation it is. Is it reasonable to believe that it is an "ancestor" simulation of the kind Bostrom describes? Commented May 10, 2021 at 2:48
  • If we are in a simulation, why is it reasonable to think that the knowledge we have obtained about the simulated world (which is all we know) applies outside the simulated world? Commented May 10, 2021 at 2:49
  • There might be some argument for why ancestor simulations would be more common than simulations with alternate laws of nature, but the question isn't relevant to assessing Bostrom's claim, which is equivalent to the claim that the statements I labeled #1 - #3 are very unlikely to all be correct. If you propose we are in a non ancestor simulation, in that case statement #3 would be incorrect, so under this proposal Bostrom was right! He never says that if we are in a simulation, it must be an ancestor simulation, he brings up ancestor sims to argue that if #1 and #2 are true, #3 is unlikely.
    – Hypnosifl
    Commented May 10, 2021 at 3:01
  • But my point is that if we are in a simulation, #1 and #2 do not even make sense (they are just gibberish). The terms "species", "civilization" etc... I don't see how they apply unless we can somehow reason that information from within the simulation applies outside. Commented May 10, 2021 at 3:05
  • They make sense in that it's logically possible they apply to the real world, and if they don't you can say they are false rather than meaningless (you can add "the real world obeys laws consistent with our observations, and..." to the beginning of #1 and #2 if it'd make it clearer).
    – Hypnosifl
    Commented May 10, 2021 at 3:10

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