Economic models fall into two broad genres. Macroeconomic models, the distant descendants of Phillips’s machine, belong mostly in central banks. They capture the economy’s ups and downs, providing a compass for the folks with their hands on the monetary tiller. The second species, known as computable general equilibrium (CGE) models, largely ignore the vagaries of the business cycle. They concentrate instead on the underlying structure of production, shedding light on the long-term repercussions of such things as the Doha trade round, a big tax reform or climate change.