Catholics, following St. Augustine, differentiate between coveting a neighbor’s wife and between coveting a neighbor’s goods. Protestants follow Judaism and Origen in combining both types of covetousness into the tenth commandment, “Thou shalt not covet.” Now the species of a sin is defined by its object (Summa 2a.72.1) just as an action takes its species from its end (Summa 2a.72.3.r2). What does it mean for an action to “take its species from its end”? It means that one act differs specifically from another in respect to the end (goal) of the action. In the same way, one sin is specifically different from another if it has a different end. Consider this example. The species of an act whereby a doctor desires to heal a man in surgery and accidentally severs a vital artery is distinct from the species of an act whereby a man desires to kill a man and does so by severing a vital artery. These two acts differ specifically because they had different ends although the same thing happened in both acts.

Like all sins, the species of covetousness takes its act from its object, but not all objects are the same. Desiring a man’s material possessions is a distinct sin from desiring his wife because the object differs in kind. Consent to the former leads to theft, but consent to the latter leads to the greater, and specifically distinct, sin of adultery. Therefore, Sts. Augustine and Aquinas are correct in apprehending that there are two specifically distinct sins of covetousness forbidden in the Decalogue.

St. Thomas Aquinas raises an interesting objection as to whether there should be prohibitions against covetousness:

Further, murder is a more grievous sin than adultery or theft. But there is no precept forbidding the desire of murder. Therefore neither was it fitting to have precepts forbidding the desire of theft and of adultery. (Summa 2b.122.6.o4)

To which he replies:

Murder in itself is an object not of concupiscence but of horror, since it has not in itself the aspect of good. On the other hand, adultery has the aspect of a certain kind of good, i.e. of something pleasurable, and theft has an aspect of good, i.e. of something useful: and good of its very nature has the aspect of something concupiscible. Hence the concupiscence of theft and adultery had to be forbidden by special precepts, but not the concupiscence of murder. (Summa 2b.122.6.r4)

All men desire good, and every deliberate act is an act towards acquiring some good (real or perceived). Man’s pursuit of a particular good is moved by the concupiscible appetite. The object of concupiscence is a pleasurable good. Protestants generally believe that concupiscence is simply our proclivity to sin. But St. Thomas argues that there are different types of concupiscence. Some concupiscences are natural (called irrational) and some are unnatural (called rational). The natural concupiscences are called irrational because they are common to animals and men, but the unnatural concupiscences are called rational because they are proper only to rational beings. (Summa 2a.30)

Natural concupiscence leads a man (or an animal) to seek things because they are suitable to one’s nature (like food and drink). These things, because they are suitable to nature, are pleasurable. Thus, natural concupiscence does not lead to sin. The unnatural concupiscence is what leads a man to seek a thing because he apprehends it “as good and suitable, and consequently takes pleasure in it.” (Ibid., article 3)

It is an unnatural concupiscence which leads a man to covet either another man’s material objects or another man’s wife. He perceives (by coveting) that he will have pleasure in theft or in adultery. But neither of these are suitable to his nature (as are food or drink for example). So they are not driven by natural concupiscence. Therefore the sins of theft and adultery arise from unnatural concupiscence. In the case of covetousness, a man seeks the pleasure of a good rather than a good suitable to nature which is pleasurable for that reason.

St. Thomas explains that, while murder is a greater evil than adultery or theft, murder itself is not an object of concupiscence. No one can desire the “pleasurable good” of murder because murder is not a pleasurable good. Now adultery is not a good and neither is theft. But in both cases, there is “an aspect of a certain kind of good.” The aspects he refers to are sex and acquisition of material goods respectively. Both of these are aspects of a certain kind of good but killing a man does not have such an aspect of good.

The significance of the prohibitions against covetousness is the condemnation of unnatural concupiscence. This principle is foundational for many other precepts and doctrines; some of which have been abandoned by most non-Catholic traditions. It is a divine precept that underlies the prohibition of covetousness: you shall not seek mutable goods merely for the sake of pleasure. Rather, “Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy strength, and with all thy mind;” (Lk 10:27) and “seek ye first the kingdom of God, and His righteousness; and all these things shall be added unto you.” (Mt 6:33)