# Does Bostrom's simulation argument make sense?

Bostrom has famously argued that we live in a simulation. One of his key assumptions is that other civilizations exist that can simulate us. Why do we make this assumption?

Why assume that other civilizations exist? We obviously would not be able to see them in our universe. If we did, that would imply that we do, in fact, live in "base reality." If that would be the case (that we live in base reality, yet it is simulated by others in this base reality), it would imply an infinite chain of causality, which seems dubious at best.

Why, then, assume that other universes exist besides ours? We simply do not know if 1) other universes exist at all, and 2) life can be supported in that universe, much less intelligent life. It doesn't seem that probability can be applied here, either. How can one possibly, rationally, assign probability to this?

As a bit of an aside, it seems that the argument also assumes reductive physicalism. Is this not contested heavily?

• Where does he make this assumption? That looks more like part of the conclusion. Can you cite the part of the paper that makes this assumption? – Eliran Aug 1 '19 at 21:12
• @Eliran If you're talking about the assumption that other civilizations exist that can simulate us, it is a requirement of the argument. If that's not the case, then we ourselves must simulate us and that's... circular at best. If it's the assumption of reductive physicalism, he assumes that consciousness can arise from the hardware of a computer, which I think implies that it can reduced to it. Is that not correct? – Josh Aug 1 '19 at 21:25
• What is infinite change causality? Also see philosophy.stackexchange.com/a/56834/5759 – alanf Aug 2 '19 at 8:31
• The assumption is just that whatever the base reality, many civilizations arise within it. Nowhere does Bostrom assume that many civilizations must exist in our reality even if our reality is a simulation. – Hypnosifl Aug 2 '19 at 12:51
• @alanf Sorry, that was a typo. I meant infinite chain of causality. – Josh Aug 2 '19 at 17:43

You have almost answered your first question in your reply to Eliran: we make that assumption for the sake of argument, and some of us are interested in the argument because it seems, at least at first sight, to provide an interesting perspective on questions about what is real and about our true nature (others might be interested because they want to definitively put a stop to such nonsense!)

You may not think it at all plausible that such a situation could arise, but there are others who do. I cannot speak for anyone else, but I would start from the position that it is not completely ruled out that we could, in principle, create such a simulation, and then the question of whether I am in one follows naturally.

Assuming that such a thing is possible, it also seems at least possible that a conscious entity in it would be incapable, in principle, of discerning that to be so, but its inability to discern the greater truth of its situation would not be a prima facie argument against the possibility of such simulations.

Putting aside the question of whether the simulation argument does necessarily require reductive physicalism, if it did, then the fact that the latter is contested would hardly be grounds for dismissing the former: just about every nontrivial position in philosophy is contested. Whether reductive physicalism is heavily contested is rather a matter of opinion. As it happens (at least, according to the physicist Sabine Hossenfelder), the simulation hypothesis is not popular among physicists.

• Thank you. It seems that Bostrom's argument is essentially probabilistic (I believe he assigns 1/3 probability to us living in a simulation). How can one possibly assign probability in a case like this? How can you meaningfully assign a probability to other universes existing which support life? We simply do not know. Modern physics would have to tell us nothing, I think (if it's all simulated anyway). As far as I can tell, the only way would be a subjective, Bayesian sort of assignment. But, this obviously does not hold much weight. Is this the case here? – Josh Aug 2 '19 at 17:46
• @Josh If you could estimate the probability P that a civilization would reach a level of technology where it could run such a simulation and chooses to do so (and calculate the expected number it will run), and you assume that every simulation has the same probability of reaching that point and running its own sims, then you can calculate the expected number of total simulated civilizations. Our probability of being a simulation is then >= 1/(1+that number). But we can't estimate those probabilities, so if he actually assigned a probability of 1/3, he probably just made a wild guess. – Ray Aug 2 '19 at 19:53
• @Josh If each civilization creates $n$ simulations capable of creating their own simulations with probability $P(n)$, then the expected number of simulated civilizations is the closed form of $f() = 1 + \sum_{i=0}^∞ i * f() * P(i)$. For some distributions P(n), this will be infinite and require infinite processing power (or infinite time), at which point Bostrom's argument really falls apart. – Ray Aug 2 '19 at 19:59
• @josh Maybe my notion of relative probabilities came from some commentary on Bostrom's thesis. I guess what you recall would explain the 1/3, but I do not know that there is any justification for Bostrom to assume the unknown probabilities are equal... WRT laws: Conifold has identified a common fallacy in these discussions (i.e. assuming 'our' laws are 'their' laws.) In a very tongue-in-cheek sense, I wonder if the growing 'weirdness' of known fundamental physics is because the laws of our simulation are not entirely consistent! – sdenham Aug 3 '19 at 18:30
• @Josh Bostrom's argument assumes that the toplevel civilization is capable of running perfect-fidelity simulations of the history of its own universe, up to and including the point where the universe starts simulations of its own, and that it chooses to do so. If you grant these assumptions, a lower bound on the expected number of simulated universes can be expressed as a simple recurrence relationship. But if you don't accept those assumptions (and in my view, you shouldn't; they're utter nonsense given everything we know of physics), then the probability I stated doesn't apply at all. – Ray Aug 4 '19 at 3:36