"But what became clear in the policy debate after the 2008 crisis was that many economists — including many macroeconomists — don’t know the simplest multiplier analysis. They literally know nothing about models in which aggregate demand can be determined by more than the quantity of money. I’m not saying that they have looked into such models and rejected them; they are unaware that it's even possible to tell a logically consistent Keynesian story. We’ve entered a Dark Age of macroeconomics, in which much of the profession has lost its former knowledge, just as barbarian Europe had lost the knowledge of the Greeks and Romans.
"As long as monetary policy could bear the burden of macroeconomic stabilization, this didn’t seem to matter too much: even as equilibrium business cycle theory became increasingly dominant in graduate study, central banks, like medieval monasteries, kept the old learning alive. But once we were hit with such a severe banking and balance sheet crisis that monetary policy hit the zero lower bound, it was crucial that the economics profession be able to weigh in knowledgeably and coherently on other possible actions. And it turned out that it couldn’t."
--Paul Krugman, from his Presidential Address to the Eastern Economic Association, "The Profession and the Crisis," in Eastern Economic Journal (2011) 37, 307–312. doi:10.1057/eej.2011.8.