Tradition or emergent social arrangements (however you want to call it) are an important part of law. But all of this does not answer the issue of the currency.
Emergent social arrangements, like state, government and taxes for instance...
By the way, I beleive there was a time when something called the
Tally Stick was fiat, and used to pay taxes. Eventually the bankers managed to overturn the Tally Stick system and replace it with gold fiat instead. The bankers like gold and always have done, because it's easier to control than notches on sticks I guess, warehousing the stuff is after all how bankers started out...