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. So, in order for the trade value of money to be relatively fixed, not only must money be made of a material that is relatively rare, money can't have any real value, that is, it can't have use value. This is an odd way of doing business. And the only reason he'll do it is because he can take those thousands of pieces of useless paper and hand them to someone else who will then give him something back that is useful and valuable.
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 and then use that money to purchase the sweater. In a money economy, then, money can be substituted for any object whatsoever. This has been a long digression to explain indulgences, but there is in fact a clear relationship. The medieval Catholic church was the source of almost all social welfare and charity, and all this charity needed to be paid for. Beginning in the twelfth century, various hospitals and other organizations affiliated with the church began to send people out begging for money.