Thomas Aquinas is acclaimed as the father of Roman Catholic theology. A student of Aristotle’s philosophy, he applied logic and moral discernment to the complex realities of medieval life. Here we excerpt from his master work, Summa Theologica, parts of his treatises on “Cheating” and “The Sin of Usury.”

Whether, in Trading, It Is Lawful to Sell a Thing at a Higher Price Than What Was Paid for It?

The greedy tradesman blasphemes over his losses; he lies and perjures himself over the price of his wares. But these are vices of the man, not of the craft, which can be exercised without these vices. Therefore trading is not in itself unlawful.

I answer that. A tradesman is one whose business consists in the exchange of things. According to the Philosopher, exchange of things is twofold; one, natural as it were, and necessary, whereby one commodity is exchanged for another, or money taken in exchange for a commodity, in order to satisfy the needs of life. Suchlike trading, properly speaking, does not belong to the tradesmen, but rather to housekeepers or civil servants who have to provide the household or the state with the necessaries of life. The other kind of exchange is either that of money for money, or of any commodity for money, not on account of the necessities of life, but for profit, and this kind of exchange, properly speaking, regards tradesmen, according to the Philosopher. The former kind of exchange is commendable because it supplies a natural need: but the latter is justly deserving of blame, because, considered in itself, it satisfies the greed for gain, which knows no limit and tends to infinity. Hence trading, considered in itself, has a certain debasement attaching thereto, in so far as, by its very nature, it ...

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