N OT FOR the first time this century, the global economy is rebounding from crisis. The new normal will differ from the old one. The pandemic shifted resources around, destroyed firms, and subtly adjusted habits. The economy has evolved, in other words. Strangely, most economic models do not treat the economy as an evolving thing, undergoing constant change. They instead describe it in terms of its equilibrium: a stable state in which prices balance supply and demand, or the path the economy follows back to stability when a shock disturbs its rest. Though such strategies have sometimes proved...