[daxfohl]

I think there's a problem with this ((d) in particular) because you don't need a stable supply of gold per se, but a stable supply of all things fulfilling the other four properties, plus (d) independently.

So if you start finding smold or plold or frold (or creating bitcoin or smitcoin or whatever) that each has those five properties, currency-wise they're in essence all the same thing.

If they're all the same thing, just call them all "gold", (d) for this aggregate no longer holds. However there's no property that distinguishes one from the other as a currency, so you can't just "pick one" and move on. So essentially currency no longer exists.

[svs]

The value of gold depends on the supply of it. The value of silver depends on the supply of it. The value of gold does not depend on the supply of silver. The two are separate currencies.

So for a currency to be stable, its supply has to be stable. The supply of other things need not be.