The result would be a series of intriguing but catastrophic complications. The economy wouldn’t stop, exactly, but finding ways to conduct everyday transactions would suddenly become all-consuming. For as long as banks stayed closed, Greeks would have to find ways to get by without using the financial system and accessing their own savings—using cash they had withdrawn before the closures or IOUs, for example. (This is the vicious cycle of a bank run—people are worried about closures, so they withdraw their funds; those withdrawals only make a closure even more likely.)

Meanwhile, the government would have to quickly switch course from frantically negotiating a way to remain in the eurozone to implementing a new currency—presumably a return to some variety of the drachma, used from 1832 to 2001. (CNBC recently guessed it would take around $430 million to mint new money; who knows where funding would come from.)

There’d be all sorts of complications to that. Johnson laid out the simplest: If I loan you €10 on Friday and Greece leaves the euro on Monday, what do I owe you on Tuesday? Is it €10? Or some equivalent in drachmas? If the latter, what is the exchange rate?