In the last few months the world economy has been saved from a near-depression. That feat has been achieved by a range of extraordinary government stimulus measures: In the U.S. and in China, and to a lesser extent in Europe, Japan and other countries, governments have pumped liquidity, slashed policy rates, cut taxes, primed demand and ring-fenced and back-stopped the financial system. All of this has worked, but at a cost. Governments have been spending and borrowing like never before. The question now is: how do they stop?