A lengthy discussion on the nature of debt from an anthropological perspective - http://goo.gl/gyeXO
(4500 words, David Graeber, source: Naked Capitalism)
Summary: The barter system is a founding myth of economics, but spot trade isn't usually how exchange happens. Exchange accrues obligation; and to quantify and transfer this obligation, debt emerged - in the context of potential violence (and plunder). Historically, money alternates between being commodity (object) and credit (social relation). We're in a credit period, but without the Mesopotamian "clean slates" which prevent debt traps.