“It should not be: I will sell my wares as dear as I can or please, but thus: I will sell my wares as dear as I should, or as is right and proper. … but because thy selling is a work that thou performist to thy neighbor it should be restrained within such law and conscience that thou mayest practice it without harm or injury to thy neighbor, but heed rather that thou do him no injury which is thy gain.

“In distributive justice the mean is not taken according to equality of thing to thing, but according to the proportion of things to persons, so that in proportion as one person exceeds another, so also the thing that is given to the one person exceeds the thing that is given to the other. But in exchanges we must equalize thing to thing, so that whatever excess one party gets, over and above what is his own, of what belongs to another, so much exactly he should restore to the party to whom it belongs.”

“Community of goods is set down as a point of natural law, not as though it were a dictate of natural law that all things should be possessed in common, and that there should be no private property: but because the marking off of separate possessions is not done according to natural law, but rather according to human convention, which belongs to positive law. Hence private property is not against natural law, but is an institution supplementary to natural law invented by human reason.”

"The Church didn’t decide that usury was okay, however. Rather, it became much more precise in defining usury. Usury isn’t charging interest on a loan to offset the risk of the loan and the cost of forgoing other uses for the money. It’s charging someone for something that has no value. The word is also used to refer to the sin of exploiting someone when they’re in dire straights.”