The most basic form of exchange is barter. This is represented by the equation C—C, when one commodity is directly exchanged for another. Because everyone must have their goods on hand, or they must be willing to barter with promises and obligations, barter is generally limited to local communities.
Money can make trading easier. A commodity can be sold for money, which is then used to purchase another commodity. This is represented by C—M—C. A person can sell (C—M) and then buy (M—C) in a totally different place, as long as the money is accepted. This allows for much larger than local markets, which can exist on a national or even global scale.
Here the crucial difference between use-value and exchange-value, as described by Aristotle, becomes essential. All trading involves exchange-value in the act of trading, but the end goal is use-value. The purpose of bartering a shoe for a chair is to sit in the chair or wear the shoe. “Aristotle saw the form C–M–C as natural, and as being part of the art of managing a family or a city. Since the family or the city need certain external things to live, and to live well, there is a natural art of wealth getting, which is concerned with satisfying those needs” (“Use Values and Corn Laws…”, Edmund Waldstein, O’Cist.).
However, money introduces a unique danger. The Blessed Apostle Paul warned that “the love of money is the root of all evils” (1 Timothy 6:10). C—M—C, for the rich, can easily be flipped around. If somebody goes to the market with money, instead of anything to sell, they can purchase a good and then sell it again for a profit. This is represented by M—C—M´. “Aristotle goes on to argue, there is a second kind of wealth-getting that is not natural, because it is not ordered to acquiring necessary instruments (use-values), but rather to getting as much money (i.e. as much exchange-value) as possible… This form of wealth-getting has no natural limit, since it is not ordered to getting certain needed goods, but just to increasing the quantity of money. Thus, there is no reason why M´ should not be again invested to yield M´´…” (Waldstein). Usury (M—M´) is the purest form of this unnatural profit-making.
Saint Thomas Aquinas agrees: “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 does not imply a virtuous or necessary end [telos].” (Summa Theologiae II-II. Q77. A4. co.)
M—C—M is a dangerous source of fueling endless greed, which has always needed to be restrained in premodern society. In modernity, it has been fully unleashed for the first time in history, creating the system we know as capitalism.