Bostrom sets up his logical premise based on three propostions as follows:

1. 1. The fraction of all human-level technological civilizations that survive to reach a posthuman stage is close to zero
2. 2. The fraction of posthuman civilizations that are interested in running simulations of their own evolution or variants thereof is close to zero
3. 3. The fraction of all observers with human-type experiences that live in simulations is close to unity

Bostrom argues that if 1 and 2 are false, then 3 must be true true because we know that, if the fraction of human-level civilizations reaching a posthuman stage is significantly greater than zero, and if the fraction of such civilizations interested in running ancestor-simulations is significantly greater than zero, then the computer technology available to these civilizations will allow them to run such a vast number of simulations that the number of simulated observers will vastly exceed the number of real, physical observers.

According to Bostrom, if our civilization reaches a ‘posthuman’ stage—in which our descendants’ capacities ‘‘so radically exceed those of present humans as to be no longer unambiguously human by our current standards’’ –we can logically assume that the computing power available to our descendants will enable them to run hugely many ‘ancestor- simulations’, each containing vast numbers of simulated beings with conscious experiences much like our own. This source effectively offers a philosophical consideration using appeals to logic in the form of probabilities.