Objection 1. It seems that the other six precepts of the decalogue are unfittingly expressed. For it is not sufficient for salvation that one refrain from injuring one's neighbor; but it is required that one pay one's debts, according to Romans 13:7, "Render... to all men their dues." Now the last six precepts merely forbid one to injure one's neighbor. Therefore these precepts are unfittingly expressed.
Objection 2. Further, these precepts forbid murder, adultery, stealing and bearing false witness. But many other injuries can be inflicted on one's neighbor, as appears from those which have been specified above (Q72, seq.). Therefore it seems that the aforesaid precepts are unfittingly expressed.
Objection 3. Further, concupiscence may be taken in two ways. First as denoting an act of the will, as in Wisdom 6:21, "The desire [concupiscentia] of wisdom bringeth to the everlasting kingdom": secondly, as denoting an act of the sensuality, as in James 4:1, "From whence are wars and contentions among you? Are they not... from your concupiscences which war in your members?" Now the concupiscence of the sensuality is not forbidden by a precept of the decalogue, otherwise first movements would be mortal sins, as they would be against a precept of the decalogue. Nor is the concupiscence of the will forbidden, since it is included in every sin. Therefore it is unfitting for the precepts of the decalogue to include some that forbid concupiscence.
Objection 4. 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.
On the contrary, stands the authority of Scripture.
I answer that, Just as by the parts of justice a man pays that which is due to certain definite persons, to whom he is bound for some special reason, so too by justice properly so called he pays that which is due to all in general. Hence, after the three precepts pertaining to religion, whereby man pays what is due God, and after the fourth precept pertaining to piety, whereby he pays what is due to his parents--which duty includes the paying of all that is due for any special reason--it was necessary in due sequence to give certain precepts pertaining to justice properly so called, which pays to all indifferently what is due to them.
Reply to Objection 1. Man is bound towards all persons in general to inflict injury on no one: hence the negative precepts, which forbid the doing of those injuries that can be inflicted on one's neighbor, had to be given a place, as general precepts, among the precepts of the decalogue. On the other hand, the duties we owe to our neighbor are paid in different ways to different people: hence it did not behoove to include affirmative precepts about those duties among the precepts of the decalogue.
Reply to Objection 2. All other injuries that are inflicted on our neighbor are reducible to those that are forbidden by these precepts, as taking precedence of others in point of generality and importance. For all injuries that are inflicted on the person of our neighbor are understood to be forbidden under the head of murder as being the principal of all. Those that are inflicted on a person connected with one's neighbor, especially by way of lust, are understood to be forbidden together with adultery: those that come under the head of damage done to property are understood to be forbidden together with theft: and those that are comprised under speech, such as detractions, insults, and so forth, are understood to be forbidden together with the bearing of false witness, which is more directly opposed to justice.
Reply to Objection 3. The precepts forbidding concupiscence do not include the prohibition of first movements of concupiscence, that do not go farther than the bounds of sensuality. The direct object of their prohibition is the consent of the will, which is directed to deed or pleasure.
Reply to Objection 4. 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.
Source: Summa Theologica (New Advent)