I've got £90,000 in my pyjamas

Just what is this money stuff, anyway? I can see its effects—I can thumb a banknote, flip a coin—but what is it, actually? What do these abstract numbers stand for? What is the thing that’s being represented? Wouldn’t it be reassuring if it was more like a physical thing, and less like an idea? Wouldn’t the global financial system be less vertiginous, less bizarre, if your money actually stood for something?

The answer to that question is no...

-- from John Lanchester's review of Lords of Finance by Liaquat Ahamed.