Let's say that we used toilets for money. If you could buy three chemistry textbooks for one toilet, this would perhaps seem reasonable to you if you have had nothing to eat or drink in a day or two. If, on the other hand, you had just finished drinking a case of soda pop this morning, you'd probably be less willing to trade your toilet away at such a low price. This is an odd way of doing business. Now follow me on this next argument.
Money works, then, by substituting for real things.
Rather than trade your CD player for the sweater, you can sell the CD player and take fifty dollars instead as a substitute for the CD player and then use that money to purchase the sweater. This was a long digression to explain indulgences, but bear with me as I put the two together. The medieval Catholic church was the source of almost all social welfare and charity and all this social welfare and charity needed to be paid for. In the late thirteenth century, the church came up with the idea of indulgences. In the spiritual life of sinners, indulgences function exactly the same way money functions in their economic life.