The problem with quantitative easing (QE), quipped then-US Federal Reserve Chair Ben Bernanke in 2014 about the Fed’s bond-buying program, “is it works in practice but it doesn’t work in theory”. One could say the same about macroeconomic policy in general, in the sense that there is no robust theory behind it. Governments routinely “stimulate” the economy to “fight” unemployment, but with a theory that denies there is any unemployment to fight.

Mathematical refinement aside, economics has returned to what it was a century ago: the study of the allocation of given resources, plus the quantity th...