The idea that reality as we experience it may be a technologically based simulation is a staple of science fiction, showing up in films such as The Matrix, Total Recall and Vanilla Sky. It also has been of some interest to philosophers, who raise scenarios involving brains in vats and so forth as a way of contemplating the nature of knowledge.

In the past couple of years, though, the idea has gained some credence as a plausible or likely description of how things actually are. Philosopher Nick Bostrom of Oxford University has presented a simulation argument, which holds that there is a significant chance we do live in a simulation. His reasoning runs as follows: A technologically advanced civilization (such as humanity may become) would have enormous computing power, which could be used to create numerous simulations (complete with consciousness) of the civilization's own ancestors. Thus, there are three possibilities (which, in the absence of further information, Bostrom sees as equally likely):

One, such advanced civilizations almost never come into existence -- which suggests humanity is likely to go extinct before developing the requisite advanced technologies. Two, advanced civilizations have almost zero interest in running simulations of their ancestors. Or three, the overwhelming majority of people with experiences like ours are simulated -- in which case we almost certainly do live in a simulation.

Drawing upon Bostrom's ideas, Robin Hanson, an economist at George Mason University, wrote a paper on "How to Live in a Simulation." Hanson's guidelines are not so much an ethical theory as a way to minimize the possibility the simulation's controllers will get bored and pull the plug. Among other things, he suggests that people should "care less about others," "live more for today," and "keep the famous people around you happy and interested in you."

But are there really any substantial grounds for thinking we live in a simulation? A look at the assumptions and ambiguities built into the simulation argument suggests otherwise.

There is, to begin with, the question of whether consciousness can be created in a computer simulation. The slowness and difficulty of progress in artificial intelligence gives some reason for skepticism on this point. Nonetheless, as Bostrom points out, his argument requires no assumption about timescale; computer consciousness might be possible 100,000 years from now, if not in 50 or 100 years. This seems reasonable enough, although the possibility that computer consciousness may be impossible cannot be ruled out. If it is impossible, the simulation argument fails.

Let's assume, though, that our distant descendants can create such conscious simulations. The next question is what they would do with this power. Bostrom assumes that at least some of them would create large numbers of simulations of their ancestors. He notes that even if such things are illegal or frowned upon, all it would require is one or a few wealthy individuals with access to vast computer power and an interest in ancestor-simulations. Such simulations could be done using only a tiny fraction of the enormous computing resources that the advanced civilization is presumed to have.

But there's a problem here. Consider what computer simulations are used for today, particularly the vast assortment of games involving dragons, aliens and so on. Ancestor-simulation is low on the list. Even if our descendants have greater interest in ancestor-simulation than humans do today, there remains the likelihood that a far larger set of conscious simulations will be created that are not about realistic depictions of ancestors.

Thus, while simulated people may outnumber real people, people in nonrealistic simulated worlds should outnumber those in realistic simulated worlds. Notwithstanding accounts of alien abductions and the like, we appear to live in a realistic world -- which should puzzle us, if the simulation argument is true. We do not appear to be typical observers, yet the idea that we are typical is what led us to think we're simulated (since it is assumed that only a small fraction of people are real).

The simulation argument, in this respect, is similar to the doomsday argument (also popularized by Bostrom), which claims it is improbable we would find ourselves alive today if there will be numerous humans alive in the future. In the doomsday argument, it is not clear if only humans belong in our reference class or also aliens, sentient animals and posthuman robots. In the simulation argument, it is not clear what kind of simulated beings we should count ourselves among.

Then again, if the simulation argument is true, we have little reason to think that the real universe (where the programmers live) is anything like our own universe. It is possible that the programmers are giant octopuses, or that their universe has totally different laws of physics. This would solve the problem of the unexpected realism of our simulation. But it also undercuts our ability to form any assumptions whatsoever based on our own experience. The simulation argument requires such assumptions (for example, the assumption that our descendants will exist and want to simulate their ancestors' lives).

Finally, it is worth asking whether, if the simulation argument were true, the programmers would allow their creations to engage in conscious reflection about their true situation. One would think that speculations about simulated universes would be edited out, and that articles such as the one you're reading would never be allowed. The fact that you are reading this proves that ? GLITCH.

Note to film buffs: If you watch the 2001 movie Vanilla Sky, look for the scene early on where actor Jason Leigh walks down a building's steps and meets Tom Cruise in a waiting car. The scene was filmed on the stoop of the real-life building where I live.