Nick Bostrom - Are You Living In a Computer Simulation?
Philosophical Quarterly, 2003, Vol. 53, No. 211, pp. 243-255

This paper argues that at least one of the following propositions is true: (1) the human species is very likely to go extinct before reaching a “posthuman” stage; (2) any posthuman civilization is extremely unlikely to run a significant number of simulations of their evolutionary history (or variations thereof); (3) we are almost certainly living in a computer simulation. It follows that the belief that there is a significant chance that we will one day become posthumans who run ancestor-simulations is false, unless we are currently living in a simulation. A number of other consequences of this result are also discussed.

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+ http://www.simulation-argument.com/
On this website you can peruse the debate that followed the paper presenting the Simulation argument. The original paper is here, as are popular synopses, scholarly papers commenting on the first paper, and a couple of replies to these comments.