The glut of savings, then, has become a constraint on current demand. But since it is connected to weak investment, it also implies slow growth of prospective supply. This difficulty predates the crisis. But the crisis has made it even worse.

So what is to be done? One response to an excess of desired savings over investment would be even more negative real rates of interest. That is why some economists have argued for higher inflation. But that would be hard to achieve, even if it were politically acceptable. Another possibility, stressed by Andrew Smithers in The Road to Recovery, is to tackle obstacles to corporate investment head-on. His biggest villain is the "bonus culture", which encourages management to manipulate stock prices, via buybacks, rather than raise productive investment.

Yet another possibility, discussed by Mr Summers and supported by many economists (including myself), is to use today's glut of savings to finance a surge in public investment. That might be partly linked to a shift to lower-carbon growth. Another possibility is to facilitate capital flows to emerging and developing countries, where the best investment opportunities must lie. It makes no sense for so much of the world's savings to seek investment opportunities where they do not apparently exist and shy away from places where, one hopes, they do.