Or to be more precise, all currency is consensus currency.
Units of exchange (dollar bills, great big rocks at the bottom of the ocean) can have value merely because everyone in a community agrees that they have value. That value need not be declared, defined, or enforced by by some “fiat” authority with powers of (ultimately physical) coercion — though it often or usually is.
That’s one big realization I came to from Graeber’s Debt: The First Five Thousand Years. (Though he doesn’t state it so succinctly, and I’m not sure he’d agree with it.)
Think of gold coins. If their exchange/consensus value is (enough) less than the (commodity) value of their metal content, people will melt them down and sell the metal. Arbitrage happens. Their consensus exchange value must be higher than the exchange value of their constituent metal, or they’re simply not currency any more; they’re chunks of commodity.
That differential between currencies’ consensus value and their commodity value is their very sine qua non: the thing that makes them what they are, without which they would not be currencies.
So why gold coins? Because the next city-state over, or the one 500 miles away, might not have the same consensus about those coins’ value as in your city-state. But there is a much wider consensus as to the value of gold. As far as they’re concerned, your ruler’s gold coins have the value of their gold content, and that’s all. But at least they have that value, because the gold-value consensus is widespread.
So if you’re a trader looking to buy 1,000 yak skins from the remote Azbakalians, you can carry a bunch of gold coins there and trade them instead of carrying 1,000 amphoras of lima-bean oil. Yeah, you sacrifice the extra consensus value you’d have if you instead used those gold coins to buy things locally, but the cost of transporting all that oil is higher than that loss.
So is a physical one-ounce lump of gold a unit of “currency”? I’d say no. Because while the consensus value of gold might be different and might change at different times and places, there is no particular time and place where it has two different exchange values: its local consensus value and its foreign-trade commodity value. It only has its commodity value.
How does this work for cigarettes in a POW camp? They’re obviously used as some type of currency, as units of exchange. I’d suggest that because some people smoke and some don’t — some people value them highly for the utility their consumption can deliver, while others have no use for them — their average commodity value is lower than their average exchange/consensus value. (Also: new cigarettes are constantly being delivered and shared out in some way by the Red Cross or whoever, and they’re constantly being consumed. This is never true of coins or paper bills, which can’t be consumed.) This raises issues of distribution, power, wants, needs, and satisfactions, which I’d love to hear discussion of by my gentle readers.
Cross-posted at Asymptosis.