In part, the letter reads:
Paul Krugman cites a concern for mathematical elegance over truth and reliance on efficient market theory as reasons for economists underestimating the instability of markets. But similar arguments could be made about the Keynesianism that Krugman advocates. In its heyday, Keynesianism included elegant mathematical models that demonstrated markets are inherently unstable and had its own version of an efficiency theory, only it was government that was efficient; a wise and good government could fine-tune the economy through appropriate fiscal policy. . . .