That's an interesting answer. About halfway through my own post even I started to doubt what I was saying when I got to comparing the qualities of post-apocalyptic contracts to bank notes. Plus, I was sort of hoping that the whole "cash" thing would kinda slide through. *smirk*

Anyway --- Cash or Money, whether physical or "in the bank", is worth something because we all agree to exchange goods or services for it. A promissory note is worth something because one person has agreed to provide periodic money transfers to another entity.

One of the qualities of money is that every unit should be fungible, that is, I only care that I get a twenty if I go to the ATM and ask for 20 bucks. I'm not looking for a specific bill because any twenty dollar bill is as good as the next (aside from oddball numismatically interesting bills).

In contrast, a promissory note does not have a set value -- the amount of the loan is a fixed value set when the debt and the money were made. However, the value of the note itself can fluctuate greatly depending on how reliable the debtor is or how desirable the collateral is -- in other words, a $100k loan secured with an acre of Santa Barbara beach front as collateral, is far more valuable than a $100k loan on 79 Winnebago with a blown engine. Because the value of each $100k note might vary, every $100k note is not equivalent. If every unit of money must be equivalent, then it follows that debts are not money because the value of debt is variable.