The paper promise-to-pay that became money itself — a printed instrument whose value rests not on the worth of its material but on the trust that it can be redeemed or accepted by others. From the jiaozi of Song-dynasty China to the goldsmith receipts of seventeenth-century London, from notes redeemable for metal to pure fiat currency backed by nothing but the state, the banknote is one of the most successful confidence devices in economic history: a piece of paper that an entire society agrees to treat as wealth.
A coin is worth something because of what it is made of, or so people once believed. Gold is gold; you can weigh it, bite it, melt it down. A banknote is different. It is worth something only because of what is printed on it and who promises to honor that promise. The paper itself is nearly worthless. When Marco Polo returned from the court of Kublai Khan in the late thirteenth century, the detail Europeans found hardest to believe was not the size of the empire or the wealth of its cities. It was that the Great Khan made money out of mulberry bark, stamped it with his seal, and ordered his subjects to accept it on pain of death — and that they did. A piece of paper, Polo reported, bought silk and pearls and horses as readily as coined silver. To medieval European readers this sounded like alchemy, or a fraud. It was neither. It was the discovery that money does not need to be valuable. It only needs to be trusted. Every banknote ever printed is a wager on that idea, and the wager has been lost as often as it has been won.