Are we living in a computer simulation?
I'm moderately confident that I don't live in a computer simulation. My reasoning goes like this.
A priori, simulation scenarios are less probable than non-simulation scenarios.
My evidence is more likely in non-simulation scenarios than in simulation scenarios.
So: It is highly improbable, given my evidence, that I'm in a simulation scenario.
By a "simulation scenario", I mean a scenario in which a subject's experiences of themselves and their environment are generated by a computer program that simulates an ordinary (non-simulated) subject and their environment.
I assume that it is a priori possible for a computer program to generate experiences (and a "subject") by simulating an ordinary subject with experiences. I'm not 100% sure this is true. (If not, premise 1 can be strengthened: simulation scenarios have probability 0.) But it seems plausible, especially if we're liberal about what qualifies as a computer program and as a simulation.
(Strictly speaking, a simulation scenario isn't just a scenario in which somebody lives in a computer simulation. It's a scenario in which I do.)
Now, why do I think that simulation scenarios are a priori less probable than non-simulation scenarios (assuming they are possible)? In short, because they are skeptical scenarios, and skeptical scenarios have low a priori probability.
Consider first a more standard type of skeptical scenario.
There are worlds that match our world in all the ways we have observed, but whose unobserved parts are utterly different. For example, there are worlds that are just like this world until today, but in which everything turns into plum jam tomorrow. On any way of counting possibilities, there are at least as many such "counterinductive" worlds as "inductive" worlds. But we shouldn't take them seriously. The plum jam extinction scenario deserves negligible credence, even though it is compatible with all our evidence. In a Bayesian framework, this implies that counterinductive scenarios must have much lower a priori probability than inductive scenarios.
Another type of skeptical scenario involves hallucinations and illusions. There are scenarios in which it visually seems to me as if I am looking at a dagger even though there's no real dagger there. For a more extreme version, there's a scenario in which I have all my actual experiences while I am living the external life of Napoleon Bonaparte: it seems to me as if I am quietly typing into my laptop at a cafe, but in fact I am standing on a battlefield in 1800s France, directing my troops. Such scenarios, too, deserve negligible credence, even though they are compatible with my evidence.
Back to simulation scenarios.
Many simulation scenarios are skeptical scenarios of the second type. My current experiences suggest to me that I have a physical body, that I move around in a physical space with tables and trees etc. If I'm in a simulation scenario, arguably none of that is true: my sensory experiences are radically deceptive. But rationality requires giving negligible credence to scenarios in which my experiences are deceptive. Hence simulation scenarios have low a priori probability.
This is a little too quick. David Chalmers has argued that our mundane positive beliefs about ourselves and our environment need not be false in simulation scenarios. (See, for example, Chalmers 2018, and relevant parts of Chalmers 2012 and Chalmers 2022b.) Roughly, the idea is that all it takes for 'there are trees' to be true is that there are things that play a certain structural/causal role, linking the things to one another and to our experiences. I'm not convinced by these arguments, but I'm not fully convinced that the conclusion is false either.
So perhaps there are simulation scenarios in which my sensory experiences are not radically deceptive. These scenarios then don't belong in the second group of skeptical scenarios. But they still belong into the first. Such scenarios involve two layers of reality: a simulated layer (with genuine tables and trees, or so we assume) and an outer layer in which the simulation is running. We only have access to the simulated layer. How is this different from other counterinductive hypotheses that posit intricate events in unobserved parts of the world? There is a possible world in which this cafe is populated by angels made of ectoplasm that don't interact with ordinary matter. I can't rule out such a world, but I don't take it seriously. Layers of reality should not be multiplied without necessity.
So much for my first premise. In sum, simulation scenarios are scenarios in which our empirical methods don't work. Such scenarios deserve negligible a priori credence.
(Isn't this begging the question against someone who holds that we are living in a simulation? Of course it is. We shouldn't try to engage with a skeptic on their own ground.)
I also claim that my evidence is somewhat more likely in non-simulation scenarios than in simulation scenarios. This is my second premise. It's based on the thought that people in simulation scenarios generally don't have the kind of rich and coherent experiences that I have. "Most" computer programs that generate a simulation scenario would generate much poorer, more fragmented and chaotic experiences.
Objection: The people running these simulations would have an interest in generating rich and coherent experiences.
Response: Why should we assume this? Who said that there are people running these simulations anyway? Perhaps the computers that run the simulations are created by random fluctuations of matter in the outer universe. And even if there are people running the simulations, why are we entitled to any a priori views about their aims and interests?
This concludes my argument.
I need to say something about "the simulation argument", which supposedly proves that we are likely to live in a simulation. There are many versions of this argument. I'll focus on one problem that all of them seem to share.
Let's begin with a version that makes the problem especially obvious. The argument (intended to be a simplified version of the one in Bostrom 2003) goes like this.
Given what we know about physics, the brain, computer technology, etc., it is likely that we will create lots of simulated beings, including beings with experiences just like mine.
Given that there are N beings in the world with experiences just like mine, it is equally likely that I am any one of them.
So: It is likely that I am simulated.
There's something odd about this line of reasoning. The central empirical premise (premise 1) says that there will probably be simulated beings in the future. From this, the argument seems to infer that I am probably one of these beings. But I know that I'm not in the future!
Let Physics be the (relatively) uncontroversial empirical assumptions invoked in the argument's first premise: that the laws of physics allow for the existence of computers to simulate a human brain, that we don't have such computers now but might have them in the future, and so on. To bring out the problem, let's pretend that Physics makes it highly probable that there will be simulated beings in the future and highly improbable that there are simulated beings now. (You might deny that we have such information, but let's pretend!) The first premise is then true. But the empirical information that it appeals to also entails that the conclusion is false. So there must be something wrong with this kind of argument, although it's not obvious where the mistake lies.
A similar problem arises for the following argument that we are Boltzmann brains.
Given what we know from physics, it is likely that there are many Boltzmann brains with experiences just like mine.
Given that there are N beings in the world with experiences just like mine, it is equally likely that I am any one of them.
So: It is likely that I am a Boltzmann brain.
The first premise mentions "what we know from physics". But let's list a few things we know from physics! We know, for example, that we live on a planet orbiting a star at a distance of about 150 million km. This is not true if we are Boltzmann brains. As before, the empirical information that is supposed to make the Boltzmann hypothesis probable belongs to a larger corpus of information that actually contradicts the Boltzmann hypothesis.
Intuitively, these arguments are self-undermining: if we accept the conclusion, we are no longer justified in accepting the first premise. But this doesn't explain where the arguments go wrong.
Well, the conclusion undermines the first premise because it implies that we can't trust the empirical methods on which that premise rests.
As before, let Physics comprise what we take ourselves to know about the physical world. Let's assume that Physics makes it highly likely that there are many Boltzmann brains with experiences just like mine. Let Boltzmann be the hypothesis that I am a Boltzmann brain.
Since Physics entails that we live on a planet, we have P(Physics | Boltzmann) = 0.
By the self-locating indifference principle in premise 2 of the Boltzmann brain argument, P(Boltzmann | Physics) is high.
It follows by elementary probability theory that P(Physics) must be low.
So we have two options. We must reject either the self-locating indifference principle or the trustworthiness of our empirical methods (i.e., take P(Physics) to be low).
I think it's clear which of these options we should choose. We should reject self-locating indifference.
This should be unsurprising from what I said above. I argued that skeptical scenarios deserve negligible a priori credence. Since skeptical scenarios can coexist with non-skeptical scenarios in the same world, we should not be indifferent between all scenarios in a world that we can't rule out. Boltzmann scenarios deserve negligible a priori credence.
The simulation argument fails for the same reason. Let's look at Chalmers's version (from Chalmers 2022b, ch.5 and Chalmers 2022a), which doesn't have the flaw of suggesting that we somehow live in the future.
The conclusion of Chalmers's official argument states that 'if there are no sim blockers, we are probably sims'. But he also indicates that he regards the antecedent as somewhat probable. So we can rephrase the argument as follows.
It is somewhat probable that most beings with experiences like mine are sims.
If most beings with experiences like mine are sims, I am probably a sim.
So: it is somewhat probable that I am a sim.
Premise 1 is supposed to be supported by empirical information about computer technology, the number of neurons in the human brain, etc. This assumes that our empirical methods are reliable. But the conclusion implies that our empirical methods are probably unreliable. Like the previous two arguments, this argument is self-undermining. And as in the previous two cases, the flaw lies in the second premise.
(I have no strong views about the first premise. This is an empirical matter.)
Again, this line of thought is a little too quick, if we consider Chalmers's view that our ordinary, positive thoughts about tables and trees are true in some simulation scenarios. Suppose for the sake of the argument that this is correct. In the relevant simulation scenarios, our empirical methods for arriving at positive conclusions may then be fairly reliable. It's only our methods for arriving at negative conclusions that are unreliable. (A "negative" conclusion is a conclusion about the non-existence of extra things or extra structure or extra layers of reality.) If the empirical information adduced in the first premise is entirely positive, the argument isn't self-undermining.
Of course, I would still say that premise 2 is false: rationality requires giving low credence to scenarios in which our empirical methods for arriving at negative conclusions are unreliable.
Curiously, Chalmers might argue that the simulation scenarios in question are ones in which our methods for arriving at negative conclusions are reliable as well. The argument would go as follows.
Our positive information suggests that there are more simulated beings with our present experience than non-simulated beings.
Our empirical methods include highly restricted indifference principles. In particular, they license indifference between sim and non-sim scenarios in which we have our present experiences.
So, our empirical methods imply that it is highly likely that we are simulated, and therefore that there is an extra layer of reality.
I guess this shows that there may be nothing inherently unstable or incoherent about believing in the simulation hypothesis on empirical grounds. But it doesn't persuade me that the hypothesis is plausible.