Currencies have always been a promise of some sort, and the value of the currency depends ultimately on the value of that promise.

Now the REALLY interesting thing about this observation is that we can all make our own promises too.

Hear me out: The promise of gold and silver coinage was that the coins were pure and of a certain weight, and that the metal would continue to be in demand and supply would be limited - even though this promise was broken occasionally, e.g. by Mansa Musa or the Conquistadores.