This is sort of a follow up to Brian Leiter’s post on philosophy blogs in Newsweek. And equally belated.
Last week the Sydney Morning Herald ran an article by Paul Davies about the possibility that our universe is a simulation. What was interesting, to me at least, was that the article cited Nick Bostrom’s argument to this effect in The Philosophical Quarterly. (An online version of Bostrom’s paper is here.) It isn’t every day you see a philosophy journal cited in the morning newspaper. Sadly Davies didn’t cite, or even talk about, my refutation of Bostrom’s argument also in The Philosophical Quarterly. So I thought I may as well take the chance to revisit that debate and say what I thought was most important about it. Anyone who wants to write to the SMH making either of the points below is much more than welcome!
(Blog history note: I first found out about Bostrom’s paper through a chain of links starting with Instapundit, which probably makes my paper the first philosophy paper to be the result of a blog entry.)
The core of Bostrom’s argument is fairly simple. Given some plausible assumptions it is likely that there will be many more simulated conscious beings than material conscious beings over the course of the history of the world. (Bostrom makes all his conclusions conditional on these assumptions, but it makes life easier to just assume they are true, and I won’t question any of the assumptions in any case.) Now if we accept that we’re just as likely to be any one of the simulees as one of the material entities, it follows that we’re more likely to be a simulation than a material entity.
There are two problems with this argument, both due in different ways to the fact that we actually have quite a bit of evidence about just what our place in the universe is.
First, I know right now that I’m not feeling cold. So it’s not just as likely that I’m one of the people as any other, because I’m not one of the ones feeling cold. A much more plausible principle is that for any of the conscious beings whose conscious experiences are indiscriminable from mine I’m just as likely to be any one as any other. (I think even that’s not true, but let’s grant it for now.) The problem is that the assumptions don’t give us the conclusion that there are likely to more simulees than materials with conscious experiences indiscriminable from mine. Indeed, when we think about the vast variety of conscious experiences it starts to seem unlikely that there is any other being, simulation or material, with the same experiences as I have. So for all the argument has told us, the proportion of beings with conscious experiences indiscriminable from mine who are material is 1. And then applying the more plausible principle, we get that the probability that I’m material is 1.
To get the original argument to work, Bostrom has to come up with stronger principles than the plausible one I listed. But these principles all seem either ad hoc or inconsistent for reasons related to Goodman’s ‘grue’ paradox. I won’t go into the detail, because what really seems important is that the makeup of classes of beings that I know I’m not in just doesn’t seem that relevant to what probability I should assign to my being material or simulated.
The other point is that part of Bostrom’s argument relies on a principle of indiscriminability that is importantly false. I said above that Bostrom-style arguments can’t affect the probability I assign to being cold, because I can just tell that I’m not cold. Most modern theorists of perception would say the same thing about the probability I should assign to being a material being. I can just look at see that I’m material, whatever other philosophers would tell us.
Why wouldn’t we believe this? One intuitive reason turns out to be quite bad, for reasons Tim Williamson has recently stressed. It’s intuitive to think that if we were simulees rather than material beings we couldn’t tell the difference, which I guess is true. And it’s intuitive to think that this implies if we are material rather than simulees that we couldn’t tell the difference. But this doesn’t follow, because ‘is indiscriminable from’ is not symmetric.
We can make this intuitive by considering real-life cases. After 2 martinis there’s all sorts of things I can safely do, e.g. walk down steep steps, that I can’t safely do after 4 martinis. But when I’ve had 4 martinis I can’t tell the difference between by abilities and what they were when I’d only had 2 martinis. My ability to tell what’s happening is deteriorating as what is happening changes. That in no way means that after the first 2 martinis I can’t tell that I can safely walk down the steps.
The point generalises. Just because I can’t tell A from B when in situation A doesn’t mean I can’t tell A from B when in situation B if my capacities are greater in B than in A. And that’s just what happens in the simulation/material case. True, I can’t tell if I’m a simulee that I’m the product of a simulation rather than material. But if I’m material I have epistemic capacities that I don’t have if I’m a simulation. E.g. I can easily form reliable beliefs by perception. So nothing follows about what I can and can’t detect as a material being by thinking about what happens if I’m the product of simulation. The catchphrase version of this is that what happens in bad scenarios is not that relevant to what I can and can’t know by perception if I’m actually in a good scenario.
The upshot of all this is that we don’t need to worry about philosophical arguments talking us out of our naive belief that we are material beings any more than we could possibly worry about philosophical arguments talking us out of our naive beliefs that we are feeling warm (if we are). Compare: even if we knew that 99.999% of conscious beings feel cold, if we feel warm we just won’t be moved to change our beliefs by any philosophical arguments based on that statistic. I’m inclined to think that questions about whether we are material or a simulation are similar.