As we ceased to trade, we moved all of our transactions to the abstract world of money. And the thing about an abstract trade is that it happens over time, not all at once. So I trade you this tuition money today in exchange for degree in four years which might get me a better job in nine years. Not only is there risk involved, but who knows what the value of anything nine years from now is?

Because of the abstraction and time shift, we're constantly re-evaluating what money is worth. Five dollars to buy a snack box on an airplane is worth something very different than five dollars to buy a cup of coffee after a fancy meal, which is worth something different than five dollars in the grocery store. That's because we get to pretend that the five dollars in each situation is worth a different amount--because it's been shifted.