[notahacker]

"Fiat" money is backed by enormous sums of debt repayable only in that currency and a tax burden representing a large proportion of GDP payable only in that country, and its supply is kept in balance with demand on a day to day basis by central banks. (And even then, people are entirely justified in worrying that certain currencies will struggle to retain value because their state and central bank are very bad at doing the latter jobs) So it's basically backed by years' worth of guaranteed future demand to obtain that currency from people who will therefore be willing to provide goods and services in return.

Bitcoin is backed by optimism.

[Kyotoku]

Never have seen this so well explained. Alternatively, fiat can be printed. So there is a case for something that is limited, transportable etc. You can always sell your bitcoin to pay the taxes. Is just that the limited comodity would have the tendency to sell for more if the fiat is being printed