There are different types of stealing, since stealing is the taking of things belonging to another aginst the owner’s will. This is done either by pure deceit, by violence, or by fraud. If it is done by pure deceit and in secret, then it is called stealing. If it is done by violence, either the violence is done in the open and is called plundering, or it is hidden and it is called a robbery. If the taking of things belonging to another is done by fraud, this can involve an agreement in one of three ways. It can be done with an agreement that is either fraudulent, sinful, or sacrilegious. The first way happens in business and can be done in one of three ways: in weight, or in number, or in measure. Merchants rarely escape committing this sin. If it is done by a sinful agreement, it is called usury, in which that which is sold is public, namely time. If it is done by a sacrilegious agreement, in which things belonging to God are sold, it is called simony.
Some people believe that usury is evil because it is prohibited, but clearly it is prohibited because it is evil. In a monetary loan, that which is mine becomes yours. But if you acquire something from that loan by your industry, and I claim anything of that, then I sell time, which is public and cannot be sold lawfully.
–St Bonaventure, Collations on the Ten Commandments. trans. Paul J. Spaeth (St. Bonaventure, New York: The Franciscan Institute, 1995), 91.