At the heart of this system is the ancient principle of fungibility: the idea that one unit of a particular product is perfectly interchangeable with another.

This principle depends upon an unspoken agreement between market participants that information about a product's history is not only hidden but is actually lost. A product having this quality is, more or less, the very definition of a commodity.

Fungibility is even more important to money. Our system of money requires that each dollar be completely interchangeable with any other dollar. For it to function perfectly, users can have no knowledge of the history of each of those dollars.

I like to define money as a communication system that uses a commodity (the currency) to convey information about transfers of value. If the commodity's fungibility is challenged, the power to communicate that information is diminished.

You could say that assuring the fungibility of a currency is a matter of free speech. Just as importantly, the breach of freedom means that the system of exchange itself breaks down.