While the mania for cryptocurrencies may have peaked, new units continue to be announced, seemingly by the day. Prominent among the new arrivals are so-called stable coins. Bearing names like Tether, Basis and Sagacoin, their value is rigidly tied to the dollar, the euro or a basket of national currencies.

It's easy to see the appeal of these units. Viable monies provide a reliable means of payment, unit of account and store of value. But conventional cryptocurrencies, such as bitcoin, trade at wildly fluctuating prices, which means that their purchasing power — their command over goods and services — is highly unstable. Hence they are unattractive as units of account.

No grocer in his right mind would price the goods on his shelves in bitcoin. No worker would want a long-term employment contract that paid her a fixed number of those units.