From Jonathan Lethem's Chronic City:
"Simulated worlds theory says that computing power is inevitably going to rise to a level where it's possible to create a simulation of an entire universe, in every detail, and populated with little simulated beings, something like Biller's avatars, who sincerely believe they're truly alive. If you were in one of these simulated universes you'd never know it. Every sensory detail would be as complete as the world around us, the world as we find it."
"Sure," said Perkus. "Everybody knows that." He tried to dismiss or encompass Oona's description before she could complete it. "It's common knowledge we could be living in a gigantic computer simulation unawares. I think science established that decades ago, for crying out loud. Your Junrow was--huh!--behind the curve on that one."
"Right, right," said Oona slyly. "But here's the point. If we agree that the odds are overwhelming that it's already happened, then we're just one of innumerable universes living in parallel, a series of experiments just to see how things will develop. You know, whether we'll end up destroying ourselves with nuclear weapons, or become a giant hippie commune, or whatever. There might be trillions of these simulations going on at once."
"Why couldn't we be the original?" I asked.
"We could be," said Oona. "But the odds aren't good. You wouldn't want to bet on it."
This is an old philosophical musing, akin to life as a dream and vice versa, but why is it at once so compelling and so idle? It shows both the potentially maddening limits of our knowledge and the total lack of practical implications for these limits. If it could somehow be shown that our universe is in fact a virtual simulation, this would not, and should not, change anything about what we do. This thought experiment also undermines our now millenia-old assumption that God is singular...