The New York Times Science section recently ran this item on Nick Bostrom's Simulation Argument. It is an odd article, because the Science section usually touts recent or upcoming research. Bostrom's paper touting the simulation argument was in Phil Quarterly in 2003 and had been circulating on the web for a couple of years before that. Moreover, Bostrom has been promoting it as something important for most of this century. He has a website at with a simulation argument FAQ and the proviso, "I regret that I cannot usually respond to individual queries about the argument. However, I try to repond to reporters."*

At its heart, the argument is a twist on the standard brain-in-a-vat argument for scepticism. The usual argument points out that there is nothing in our immediate experience of the world to prove that the experiences are not fed to us by a system simulating such a world. Bostrom adds a twist by imagining who the mad scientists running the simulation might be.

Suppose that the human race lasts long enough to be able to run simulations which include people like us. If it does, then descendants of ours might run many similar simulations. There would thus be one actual historical 2007 and many simulated 2007s in which we might be living. Put a uniform probability distribution over them, and you get the conclusion that we are probably not in the actual 2007 but instead in one of the simulations.

As Bostrom notes, the argument really gives you a dilemma: Either future humans will not run so many simulations (because they die out, never develop the capability, or decide not to do it) or we are probably in a simulation.

OK, but what does this add to the evil demon worries that have been with us since the seventeenth-century? Instead of the mere possibility that I might be a brain in a vat, it is supposed to yield the high (conditional) probability that I am a brain in a vat. Yet the probability assessment requires thinking about how the world works, which I must do as informed by what I know about the world.

Either we have an answer to the traditional worry or we do not. If we do not, then the new argument is redundant. So suppose we do have an answer to the traditional worry. There are two kinds of answers we might think we have:

First, we might accept a reliabilist premise that our natural faculties are a reliable guide to the truth. If we unflinchingly accept that premise, then we believe already that we are not in a simulation.**

Second, we might trust our natural faculties without an explicit premise that they deliver the truth. Once we accept that standard of evidence, my seeing the world is enough of a ground for me to believe in it. The simulation argument requires that trust to get started and so comes along too late to undercut it. To paraphrase Thomas Reid, starting with trust won't get you a sceptical conclusion.

Suppose, contrary to all that, that the argument leaves me mired in scepticism. I can imagine a great many creatures who might do simulations of creatures like us. I can also imagine creatures that would do simulations of creatures like them. Computational constraints don't put the brakes on this speculation, because powerful gods might want to simulate worlds more constrained than their own; perhaps the computational constraints we know are just be features of our world as simulated. There is no sensible way to put a probability distribution over these possibilities. In the Times article, Bostrom is quoted as saying: "My gut feeling, and it's nothing more than that, is that there's a 20 percent chance we're living in a computer simulation." I have no gut feeling on the subject, because I can't make sense of 'chance' here at all.

Apart from the merits of the argument, the story in the Times is a bit disconcerting. It just encourages the all too popular conception of philosophers as purveyors of headtrips and wacky sophisms. But wouldn't I return the call if they wanted to do a story on some wacky sophism of mine? Perhaps I could feign interest.

* The argument has also gotten attention from philosophers; see, inter alia Brian Weatherson's blogging on the subject.

** David Chalmers has argued that simulation is not a sceptical possibility, but simply an alternate metaphysics. If we are in a simulation, then everything we know about tables, chairs, dogs, ducks, and the rest of the world is true; it's just that those things are (considered fundamentally) part of simulation just as we are.


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