In 1960 the physicist Eugene Wigner published an essay called "The Unreasonable Effectiveness of Mathematics in the Natural Sciences." The title was the argument. Mathematics, an abstract activity invented by humans for reasons that had nothing to do with empirical reality, turns out to describe physical phenomena with extraordinary precision. Centuries later, equations developed for pure curiosity become the foundations of quantum field theory or general relativity. There is no obvious reason this should be the case. Why does the universe care about complex numbers? Why is the geometry of curved spacetime exactly the geometry of Riemann's nineteenth-century abstraction? Why is symmetry, of all things, the deepest organising principle of the laws of nature? Wigner offered no answer. He concluded only that the unreasonable effectiveness was a gift, and that we should be grateful for it. There is one explanation that makes the gift comprehensible. If the universe was written, it was written in mathematics because mathematics is what universes get written in. The map is so good because it is the territory.