Economists get a rough ride. For many, their consistent failure to forecast the future renders them irrelevant. In our bewilderingly complex world, their necessarily simplified models seem to offer little practical help. Often over-promising and under-delivering, the dismal scientists are inescapably soft.  But paradoxically, the contribution of the greatest of them has been revolutionary. The broad prosperity increasingly enjoyed around the globe, and the insights to sustain it, are the priceless legacy of his great mind. Remarkably, at both Versailles in 1919 and then Bretton Woods in 1944, John Maynard Keynes was the central figure labouring to rebuild the war-ravaged…