SINCE cash was invented in the seventh century BC, it has generally been the most convenient way to pay for everyday purchases. But as electronic payments get easier—most recently with the launch of a “contactless payment” system by Apple—economists are beginning to ask whether notes and coins have had their day. Kenneth Rogoff, of Harvard University, reckons they have. Scrapping physical currency, he argues, would help governments to collect more tax, fight crime and develop better monetary policy. On the surface, Mr Rogoff’s plan seems like a minor change. Notes and coins make up only a tiny part of the money in circulation: just 3% in Britain, for instance. (In America, the proportion is 10%, partly because foreigners hold lots of dollar notes.) The rest is simply records of balances in accounts, either at a bank (in the case of businesses and individuals) or at the central bank (in the case of banks). It tends to be moved around by electronic transfer, never taking physical for...