Simulation theory — in its most rigorous form, the Simulation Argument — is a philosophical thesis about the nature of physical reality, not a conspiracy theory or science fiction premise. Its central claim, most precisely stated by the philosopher Nick Bostrom in 2003, is that at least one of three propositions must be true: either virtually all civilisations at our level of development go extinct before reaching the technological capability to run realistic consciousness simulations; or virtually all technologically mature civilisations choose not to run such simulations; or we are almost certainly living in a simulation. The argument is purely probabilistic: if simulations are possible and civilisations run many of them, then simulated consciousnesses vastly outnumber biological ones, making it statistically more likely that any given consciousness is simulated than biological.
Nick Bostrom's 2003 paper "Are You Living in a Computer Simulation?" established the modern philosophical framework for simulation theory. The argument does not claim we are definitely in a simulation — it claims the question is more serious than it might appear, because the statistical case for simulation is surprisingly strong if certain assumptions about future technology hold.
The key assumptions are: that consciousness is substrate-independent (that a sufficiently complex computational process can produce genuine conscious experience regardless of whether it runs on carbon-based neurons or silicon processors); that a sufficiently advanced civilisation could in principle create simulations of sufficient fidelity to fool their simulated inhabitants; and that such civilisations would have both the capability and the motivation to run large numbers of such simulations. If these assumptions hold, the number of simulated minds in any given universe would quickly come to exceed the number of biological minds by orders of magnitude — making simulation the statistically more probable state for any given mind to find itself in.
The physics connection: simulation theory has attracted attention from physicists as well as philosophers, because several features of physical reality seem more natural in a computational framework than in a purely physical one. The fact that the universe is mathematically describable — that physical laws take precise mathematical forms — is more expected in a simulation (which is fundamentally mathematical) than in a brute physical reality. The quantisation of energy and matter (the fact that physical quantities come in discrete packets rather than continuous values) resembles the pixelation of a digital system. The speed of light as an absolute upper limit on information transfer resembles a computational clock speed. Max Tegmark's Mathematical Universe Hypothesis — that physical reality is fundamentally mathematical structure rather than physical "stuff" — provides a framework in which the simulation and base reality may be less distinct than they appear.
Simulation theory resonates with several ancient philosophical and religious frameworks that describe physical reality as a secondary or constructed phenomenon:
Plato's Allegory of the Cave (Republic, 514a-520a) — prisoners in a cave who mistake the shadows on the wall for reality, never perceiving the actual objects and sunlight that produce the shadows — is the Western tradition's most precise anticipation of the simulation argument. The shadows are to the actual objects as our perceived reality is to whatever substrate runs the simulation. Hindu Maya — the concept that the phenomenal world of everyday experience is an appearance overlaid on a deeper reality (Brahman) — carries a structural similarity to simulation theory, though the nature of the "deeper reality" differs fundamentally (pure consciousness in Vedanta, a computational substrate in simulation theory). Buddhist emptiness (sunyata) — the teaching that phenomena lack inherent self-existence and arise in dependence on causes and conditions — has been connected by some Buddhist philosophers to simulation theory as both proposing that apparent reality is not ultimately self-sustaining. Gnostic cosmology — in which the material world is a creation of an inferior craftsman (the Demiurge) rather than the ultimate divine — maps onto simulation theory's distinction between the simulated environment and its creators.