Fungibility is simply that it doesn’t matter where any given unit of currency came from; one unit is worth as much as any other unit. For example, it doesn’t matter, upon presentation, the euro you hold was stolen two transactions ago. No one can say “I will take that euro but not that euro.”
This was settled as a matter of law in the 16th century. The reason we have fungibility is because otherwise the currency wouldn’t work; if every time you received a euro, you had to open a database and check the provenance, currency wouldn’t work. Not just because you would have to track a database that was perfect and always updated, but because no euro would be worth one euro.
Every euro would be weighted by how liquid it is, whether you can use it and it will be accepted. Fungibility is understanding by law, practice, and technology that every unit is treated as indistinguishable. Bitcoin is somewhat fungible. You can trace the provenance of every coin; by custom we don’t do that but we are beginning to see some companies do it, which is a problem.
If accounts start being frozen because of where bitcoins came from three transactions ago, that’s a problem. Fungibility is tied to privacy and anonymity. There are systems being made where you can’t tell the amount being transacted, who the sender and receiver are.
Banning technology, especially open-source technology, because criminals use it only usually affects the people who aren’t criminals: the innocent and the idiots. People should be able to protect their money from predators, from corporations, from tyrannical governments.
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