Martin takes a very deep and entertaining look at how people have perceived money since its invention a couple of thousand years ago, how different societies have dealt with it, and how he thinks it should be defined (which matters a lot). The traditional conception of money as something of value (gold or silver, say) that is used as a medium of exchange is wrong, he says. The idea that money replaced barter is also wrong, in his view. Money is, instead, transferrable debt that is based on a universally accepted measure of value (a dollar, for example). A dollar bill, then, is a token showing that the holder has that amount of credit. And who will honor that token? Since the dollar is universally accepted within some society, then anybody in that society will accept it. So, if you have a dollar bill, then society as a whole owes you a debt of one dollar. And how about a gold coin? The same thing is true. The gold may have some current value as a metal (measured in dollars, of course), but that just makes a gold coin a token of credit with some built-in collateral. As for barter, Martin says that no evidence has ever been found of a true barter society.