"John Maynard Keynes first raised the question of what can be done to stabilize the economy when it has fallen into a liquidity trap-when interest rates have fallen to a level below which they cannot be driven by further monetary expansion-and whether monetary policy can be effective at all under such circumstances. Long treated as a mere theoretical curiosity, Keynes's question now appears to be one of urgent practical importance, but one with which theorists have become unfamiliar."