When Kublai Khan invaded China, in addition to spreading disease and starting a postal system, he introduced paper money. It would take the Chinese a few hundred years to overcome the Mongol hordes, build a wall to keep them out, and abolish currency in the early Ming Dynasty. But now, almost 500 years later, the rest of the world is catching up with those Chinese emperors.

There is an effort gaining momentum to limit, or even abolish, physical money. There are some very positive upsides, but there are also some very disturbing downsides too. It’s that way with most disruptive events in history – gunpowder, the steam engine, music videos – but this time the ledger of pluses and minuses is a little more difficult to balance.