The Simulation Hypothesis, articulated by philosopher Nick Bostrom, argues that at least one of three propositions must be true.

In his seminal 2003 paper 'Are You Living in a Computer Simulation?', Nick Bostrom of the University of Oxford laid out a probabilistic argument known as the 'Simulation Trilemma'. He posits that at least one of the following must be true: (1) The fraction of human-level civilizations that reach a posthuman (technologically mature) stage is very close to zero; (2) The fraction of posthuman civilizations that are interested in running ancestor-simulations is very close to zero; or (3) The fraction of all observer-moments in an ancestor-simulation that belong to beings who are simulated is very close to one. Bostrom argues that if the first two propositions are false, then we are almost certainly living in a simulation. His argument rests on the premise that a sufficiently advanced civilization would have immense computing power, capable of running a vast number of highly detailed ancestor simulations. If even a small fraction of posthuman civilizations chose to do so, the number of simulated realities would vastly outnumber the single 'base reality.' Therefore, by simple probability, it's far more likely for any given observer (like us) to be in one of the many simulated realities than in the one base reality. This isn't a direct proof, but a statistical argument for the likelihood.