> In contemporary times, they have some value because everyone that lives in a particular country is legally required to pay taxes and those taxes can only be p

> In contemporary times, they have some value because everyone that lives in a particular country is legally required to pay taxes and those taxes can only be paid in the relevant government issued currency. You've got it half right. In most countries, private individuals and organizations are also required to accept the state currency for at least some purposes. In the USA, for example, creditors are required to accept US banknotes in payment of debts. This places US dollars firmly at the core of the US financial system. This doesn't create any legal obligation for US businesses to accept dollars, but it does create a very strong incentive for them to accept dollars in some form. That's the currency in which their finances will almost certainly need to be conducted if they want to have access to things like banking. That creates a situation where the US dollar isn't just valuable insofar as you can use it to pay your taxes. It is, de facto but also very nearly de jure, backed by virtually the entire United States economy. Bitcoin, on the other hand, really is backed by nothing but its own popularity. And the problem there is, it's really not that popular. It does have a lot of users worldwide, and those users can communicate over the Internet, but the same is true of Esperanto.