In this part of the lecture I have argued that a number of works in development economics written during the 1950s contained, more or less explicitly… - Paul Krugman

The usual and basic Keynesian answer to recessions is a monetary expansion. But Keynes worried that even this might sometimes not be enough, particularly if a recession had been allowed to get out of hand and become a true depression. Once the economy is deeply depressed, households and especially firms may be unwilling to increase spending no matter how much cash they have, they may simply add any monetary expansion to their board. Such a situation, in which monetary policy has become ineffective, has come to be known as a ""; Keynes believed that the British and American economies had entered such a trap by the mid-1930s, and some economists believed that the United States was on the edge of such a trap in 1992.
The Keynesian answer to a liquidity trap is for the government to do what the private sector will not: spend. When monetary expansion is ineffective, fiscal expansion—such as public works programs financed by borrowing—must take its place. Such a fiscal expansion can break the vicious circle of low spending and low incomes, "priming the pump: and getting the economy moving again. But remember that this is not by any means an all-purpose policy recommendation; it is essentially a strategy of desperation, a dangerous drug to be prescribed only when the usual over-the-counter remedy of monetary policy has failed.

Keynesian theory initially prevailed because it did a far better job than classical orthodoxy of making sense of the world around us, and Friedman’s critique of Keynes became so influential largely because he correctly identified Keynesianism’s weak points. And just to be clear: although this essay argues that Friedman was wrong on some issues, and sometimes seemed less than honest with his readers, I regard him as a great economist and a great man.

It has been obvious for quite a while that Sanders — not just his supporters, not even just his surrogates, but the candidate himself — has a problem both in facing reality and in admitting mistakes.