Money matters because it creates or distorts incentives. Central banks have used cheap money before to boost growth, but never on the this scale, and usually only in wartime. So they don’t know how economies respond to this policy — or indeed how people respond. But common sense would say that whereas benefits are likely to show in the short-term, the costs only become evident in the long. We are now in transition, taking the first halting steps to a more normal policy, and so far only in the US. There lies the danger.