Here is the idea at the center of it. Money has come in two families for about as long as we have had money. There is value that travels by possession: the coin, the note, the bill in your pocket that asks nothing of anyone. And there is value that travels by permission: the tally, the ledger, the account entry that moves only when a chain of institutions agrees to move it. Neither family is wicked. A healthy monetary world has always kept both, because each covers the other's failures. The story of our moment is that the first family is being retired, and the second family is being wired to a switch.

There is a coin in your pocket right now, and there is an identical coin in someone else's pocket across town, and those two coins are worth different amounts. Same metal, same stamp, same date. The difference between them comes from how each coin arrived. One of them was spent early, while prices still belonged to yesterday. The other arrived late, after the prices had already risen to swallow it. That gap, between the coin in the nearest hand and the coin in the other pocket, is the subject of the book I am holding today.

In the winter of 1393, the King of France gave his court an order that no one knew how to obey. He asked them not to touch him: not to brush against him in a corridor, not to take his arm on the stairs, not to embrace him on his name day. Charles the Sixth had become certain that his body was made of glass, and that one clumsy hand or one careless shoulder would shatter him on the floor of his own palace. He had the front of his clothing reinforced with rods, so that if he fell, the pieces of him might hold together long enough to be gathered up. He was a king at the center of a crowded court, and he spent his days in terror that the people closest to him would break him by accident. It is tempting to file that away as a medieval oddity, a story about one sick man six hundred years gone.