A central bank can pull the string (raise interest rates) to rein in an economy that is galloping ahead unsustainably. But it cannot push the string up: if economic growth stalls, as when confidence is seriously damaged, lowering interest rates may not be enough to stimulate demand.
约翰·梅纳德·凯恩斯（John Maynard Keynes）曾经说过，货币政策就像一根绳索。中央银行可以收紧绳索（提高利率），控制飞速发展但却不可持续的经济。但它却无法使绳索垂直立起：如果经济增长像信心严重受损时那样陷入停滞，那么单靠降低利率也许并不足以刺激需求。如果出现上述情况，那么即便央行竭尽全力，经济衰退也依然会发生。