The simulation argument: is at least one of the three disjuncts almost certainly true, and what follows for our credence about whether we are in a simulation?

Nick Bostrom, answered from the texts and cited to the page.

The position in brief

The simulation argument is a logical disjunction, not a claim about our world. At least one of three propositions is almost certainly true: (1) almost no civilizations reach a posthuman level of technological capability; (2) almost no posthuman civilizations run significant numbers of ancestor simulations; or (3) we are almost certainly living in a computer simulation.12

The argument trades on an indifference principle over observers in epistemically-equivalent situations, combined with realistic estimates of the computational resources a posthuman civilization would have available. The conclusion is not that we are in a simulation; it is that credence should be roughly distributed across the three disjuncts unless there are positive grounds to think one is improbable.

The argument is consistent with classical theism, with substrate-independent functionalism about mind, and with both finite and infinite cosmologies.3

Sources

  1. What is the simulation argument? The simulation argument purports to show 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…
    The Simulation Argument FAQ v2.0, p. 2
  2. Here fp is the fraction of civilizations at our current stage that eventually become technologically mature; fI is the fraction of technologically mature civilizations that apply some non-negligible portion of their computational power to running ancestor-simulations; and fsim is the fraction of all people with human-like experiences who live in simulations.
    Reply to Weatherson, p. 3
  3. A PATCH FOR THE SIMULATION ARGUMENT Nick Bostrom Future of Humanity Institute Faculty of Philosophy & Oxford Martin School University of Oxford Marcin Kulczycki Institute of Mathematics Faculty of Mathematics and Computer Science Jagiellonian University www.simulation-argument.com This article reports on a newly discovered bug in the original simulation argument.
    A Patch for the Simulation Argument, pp. 1–2

Where the argument is made

  • "Are You Living in a Computer Simulation?" (Philosophical Quarterly, 2003)
  • "The Simulation Argument: Reply to Weatherson" (Philosophical Quarterly, 2005)
  • "The Simulation Argument: Reply to Brueckner" (Analysis, 2009)
  • "A Patch for the Simulation Argument" with Marcin Kulczycki (Analysis, 2011)
  • "The Simulation Argument FAQ v2.0" (2025)

How the argument was received

The most-cited contribution. Published 2003 in Philosophical Quarterly; replied in print to Brian Weatherson (2005) and Anthony Brueckner (2009); the 2011 Patch (with Kulczycki) handles the indifference-over-reference-class technical objection. The argument has survived twenty-plus years of scrutiny. The trilemma's logical structure is owned firmly.