Quote from Martinghoul:
But it isn't it also true that you hold gold because you're implicitly assuming someone will pay you for it? I.e. you own gold, because, in your mind, it represents the world's promise to pay. So, again, it's hard to see the difference between a fiat commodity, like gold, and fiat money.
Let me be the devil's advocate for a moment. Why is it that you think that people will always be willing to sell you something you need in exchange for gold? Why can't it be that, in the future, people value some other random decorative commodity, such as seashells (yes, I know I am sounding like a broken record) more than gold?
For your first point - the supply of gold is hard to increase, whereas fiat money can have its supply increased infinitely at the whim of a government, or significantly each year by the commercial banking system. That's a major difference, given the impact that supply has on price.
As for the second point - this one is obvious. Apart from fiat currency, gold has by far the best collection of properties beneficial for a monetary substitute - it's high value per oz so relatively easy to store and transport high amounts of money, it is very durable (high melting point, low reactivity), it is relatively easy to measure its purity, it is fungible/homogenous (unlike sea shells), it is attractive and has reserve demand for jewelry, and it is time-tested as a monetary substitute. If fiat currency falls from favour, gold is the only serious monetary substitute out there at the moment.