There are two uses of a thing, one proper, the other improper [says Aristotle]; a shoe, for instance, may be worn, which is its proper use, or exchanged, which is its improper use. It follows that there is something degraded about a shoemaker, who must exchange his shoes in order to live. Retail trade, we are told, is not a natural part of the art of getting wealth. The natural way to get wealth is by skillful management of house and land... Wealth derived from trade is justly hated, because it is unnatural. "The most hated sort, and with the greatest reason, is usury, which makes a gain out of money itself, and not from the natural object of it. For money was intended to be used in exchange, but not to increase at interest... Of all modes of getting wealth this is the most unnatural."
Medieval philosophers were churchmen, and the property of the Church was mainly in land; they therefore saw no reason to revise Aristotle's opinion. Their objection to usury was reinforced by Anti-Semitism, for most fluid capital was Jewish. Ecclesiastics and barons had their quarrels, sometimes very bitter; but they could combine against the wicked Jew who had tided them over a bad harvest by means of a loan, and considered that he deserved some reward for his thrift.
Bertrand Russell, The History of Western Philosophy