The central tenet is the underlying intrinsic value. There are many fungible concepts that operate on a valuation that seems like it is being bought and sold solely on the basis that others think it is valuable too. However, usually there's a seed of intrinsics, something fundamental whose value will always be there.
For 'fiat currency', it's the fact that the state has a monopoly on violence and has decreed both [A] that they will accept the fiat currency for fulfillment of tax obligations, and [B] that they will not enforce a claim of debt if you have offered to pay it in fiat currency and the debtee didn't accept your offer.
For gold it's the fact that gold is considered intrinsically pretty by some, and has industrial uses.
The same can be said for iron, wood, or even air - but those are far less rare. Wood has value just as gold does - it's just that folks tend not to trade trailers full of wood as a fungible because it is unwieldy.
Stocks have intrinsic value too: It's a tiny voting share, and gives the right to enjoy a share of dividends.
Bitcoin has __nothing__. Whatsoever. It's real easy to look at the ridiculous price of stocks in companies that make it incredibly difficult to use your vote and which never pay out any dividends, or the sheer unfathomable levels of business done in terms of USD or EUR and how it seems to dwarf the intrinsic, and conclude that the intrinsic is just not important...
But is that a jump you can make?
As you said:
> I accept 100 euro bills for my labour because I believe I'll continue to be able to trade them for food and shelter.
You sure? Maybe it's 1% 'because I have absolute guarantees I can pay my taxes with this, and I have a guarantee that I can trade them for food and shelter because if I pay my bills with dollars and the recipient no longer wants them, they have no legal recourse to force me to make good in any other way'. Presumably you have certain outstanding debts that work like this (if not just simply your tax bill, which is inevitable, then your power bill, your rent or mortgage, etc), so that euro bill __already has value__ the moment you receive it. You can take your mental bookkeeping of 'oh yeah the month is halfway through so I absolutely do, unambiguously, owe my bank half a month's mortgage at the very least', and immediately reduce that amount by €100,-, given that you are holding a €100,- bill in your hands and the state has decreed that they will tell the bank to get fucked if they decide they no longer want to accept your euro for paying off that mortgage.
Well, in my country my taxes are deducted from my salary before I receive it, so the psychology of that is a bit different. But I think we agree, what I'm saying is fiat money is valuable because of the stability of the institutions that order our society/ies, and because there is an unspoken agreement that most of us want things to stay roughly as they are and believe that they will. If too many cracks appear there, people start acting very different.
As for Bitcoin, the original Bitcoin paper talks about replacing trust with code, and I just don't buy it. Sure, you can trust that Bitcoin aren't spent more than once and all that, the maths take care of that. But it fails to consider all the institutions that integrate the Bitcoin world with the rest of social/economical reality, and all that arises there like price volatility, transactions taking forever, exchanges getting hacked, and that Bitcoin-as-a-casino, excuse me, object of speculation, has a far bigger impact on its day-to-day than for e.g. the Euro. The paper doesn't consider that reality at all, but those things mean that 1000EUR worth of Bitcoin is a lot less useful than a stack of fifties.
For 'fiat currency', it's the fact that the state has a monopoly on violence and has decreed both [A] that they will accept the fiat currency for fulfillment of tax obligations, and [B] that they will not enforce a claim of debt if you have offered to pay it in fiat currency and the debtee didn't accept your offer.
For gold it's the fact that gold is considered intrinsically pretty by some, and has industrial uses.
The same can be said for iron, wood, or even air - but those are far less rare. Wood has value just as gold does - it's just that folks tend not to trade trailers full of wood as a fungible because it is unwieldy.
Stocks have intrinsic value too: It's a tiny voting share, and gives the right to enjoy a share of dividends.
Bitcoin has __nothing__. Whatsoever. It's real easy to look at the ridiculous price of stocks in companies that make it incredibly difficult to use your vote and which never pay out any dividends, or the sheer unfathomable levels of business done in terms of USD or EUR and how it seems to dwarf the intrinsic, and conclude that the intrinsic is just not important...
But is that a jump you can make?
As you said:
> I accept 100 euro bills for my labour because I believe I'll continue to be able to trade them for food and shelter.
You sure? Maybe it's 1% 'because I have absolute guarantees I can pay my taxes with this, and I have a guarantee that I can trade them for food and shelter because if I pay my bills with dollars and the recipient no longer wants them, they have no legal recourse to force me to make good in any other way'. Presumably you have certain outstanding debts that work like this (if not just simply your tax bill, which is inevitable, then your power bill, your rent or mortgage, etc), so that euro bill __already has value__ the moment you receive it. You can take your mental bookkeeping of 'oh yeah the month is halfway through so I absolutely do, unambiguously, owe my bank half a month's mortgage at the very least', and immediately reduce that amount by €100,-, given that you are holding a €100,- bill in your hands and the state has decreed that they will tell the bank to get fucked if they decide they no longer want to accept your euro for paying off that mortgage.