In 2016 Elon Musk gave a now-famous interview at the Recode conference. Asked about simulation theory, he made a brief and concrete argument. Forty years ago, he said, the most advanced video game was Pong — two rectangles and a square. Today we have photorealistic, multi-player virtual environments running on consumer hardware. Project that trajectory forward another forty years, then four hundred, then four thousand. At any reasonable rate of improvement, the line between high-fidelity simulation and reality becomes mathematically indistinguishable. And once it is possible to run such simulations, civilisations that survive long enough will run a great many of them. The conclusion follows. If the simulations are convincing and there are very many of them, the probability that you happen to be in the original is small. He put the odds at billions to one. Musk is not a philosopher. He builds things at scale. His interest in this question is not abstract. He is asking what kind of universe one builds for, given the answer.