Objection 1. It would seem that it is wrong to distinguish other moral precepts of the law besides the decalogue. Because, as Our Lord declared (Matthew 22:40), "on these two commandments" of charity "dependeth the whole law and the prophets." But these two commandments are explained by the ten commandments of the decalogue. Therefore there is no need for other moral precepts.
Objection 2. Further, the moral precepts are distinct from the judicial and ceremonial precepts, as stated above (99, A3,4). But the determinations of the general moral precepts belong to the judicial and ceremonial precepts: and the general moral precepts are contained in the decalogue, or are even presupposed to the decalogue, as stated above (Article 3). Therefore it was unsuitable to lay down other moral precepts besides the decalogue.
Objection 3. Further, the moral precepts are about the acts of all the virtues, as stated above (Article 2). Therefore, as the Law contains, besides the decalogue, moral precepts pertaining to religion, liberality, mercy, and chastity; so there should have been added some precepts pertaining to the other virtues, for instance, fortitude, sobriety, and so forth. And yet such is not the case. It is therefore unbecoming to distinguish other moral precepts in the Law besides those of the decalogue.
On the contrary, It is written (Psalm 18:8): "The law of the Lord is unspotted, converting souls." But man is preserved from the stain of sin, and his soul is converted to God by other moral precepts besides those of the decalogue. Therefore it was right for the Law to include other moral precepts.
I answer that, As is evident from what has been stated (99, A3,4), the judicial and ceremonial precepts derive their force from their institution alone: since before they were instituted, it seemed of no consequence whether things were done in this or that way. But the moral precepts derive their efficacy from the very dictate of natural reason, even if they were never included in the Law. Now of these there are three grades: for some are most certain, and so evident as to need no promulgation; such as the commandments of the love of God and our neighbor, and others like these, as stated above (Article 3), which are, as it were, the ends of the commandments; wherefore no man can have an erroneous judgment about them. Some precepts are more detailed, the reason of which even an uneducated man can easily grasp; and yet they need to be promulgated, because human judgment, in a few instances, happens to be led astray concerning them: these are the precepts of the decalogue. Again, there are some precepts the reason of which is not so evident to everyone, but only the wise; these are moral precepts added to the decalogue, and given to the people by God through Moses and Aaron.
But since the things that are evident are the principles whereby we know those that are not evident, these other moral precepts added to the decalogue are reducible to the precepts of the decalogue, as so many corollaries. Thus the first commandment of the decalogue forbids the worship of strange gods: and to this are added other precepts forbidding things relating to worship of idols: thus it is written (Deuteronomy 18:10-11): "Neither let there be found among you anyone that shall expiate his son or daughter, making them to pass through the fire:... neither let there by any wizard nor charmer, nor anyone that consulteth pythonic spirits, or fortune-tellers, or that seeketh the truth from the dead." The second commandment forbids perjury. To this is added the prohibition of blasphemy (Leviticus 24:15, seqq) and the prohibition of false doctrine (Deuteronomy 13). To the third commandment are added all the ceremonial precepts. To the fourth commandment prescribing the honor due to parents, is added the precept about honoring the aged, according to Leviticus 19:32: "Rise up before the hoary head, and honor the person of the aged man"; and likewise all the precepts prescribing the reverence to be observed towards our betters, or kindliness towards our equals or inferiors. To the fifth commandment, which forbids murder, is added the prohibition of hatred and of any kind of violence inflicted on our neighbor, according to Leviticus 19:16: "Thou shalt not stand against the blood of thy neighbor": likewise the prohibition against hating one's brother (Leviticus 19:17): "Thou shalt not hate thy brother in thy heart." To the sixth commandment which forbids adultery, is added the prohibition about whoredom, according to Deuteronomy 23:17: "There shall be no whore among the daughters of Israel, nor whoremonger among the sons of Israel"; and the prohibition against unnatural sins, according to Leviticus 18:22-23: "Thou shalt not lie with mankind... thou shalt not copulate with any beast." To the seventh commandment which prohibits theft, is added the precept forbidding usury, according to Deuteronomy 23:19: "Thou shalt not lend to thy brother money to usury"; and the prohibition against fraud, according to Deuteronomy 25:13: "Thou shalt not have divers weights in thy bag"; and universally all prohibitions relating to peculations and larceny. To the eighth commandment, forbidding false testimony, is added the prohibition against false judgment, according to Exodus 23:2: "Neither shalt thou yield in judgment, to the opinion of the most part, to stray from the truth"; and the prohibition against lying (Exodus 23:7): "Thou shalt fly lying," and the prohibition against detraction, according to Leviticus 19:16: "Thou shalt not be a detractor, nor a whisperer among the people." To the other two commandments no further precepts are added, because thereby are forbidden all kinds of evil desires.
Reply to Objection 1. The precepts of the decalogue are ordained to the love of God and our neighbor as pertaining evidently to our duty towards them; but the other precepts are so ordained as pertaining thereto less evidently.
Reply to Objection 2. It is in virtue of their institution that the ceremonial and judicial precepts "are determinations of the precepts of the decalogue," not by reason of a natural instinct, as in the case of the superadded moral precepts.
Reply to Objection 3. The precepts of a law are ordained for the common good, as stated above (Question 90, Article 2). And since those virtues which direct our conduct towards others pertain directly to the common good, as also does the virtue of chastity, in so far as the generative act conduces to the common good of the species; hence precepts bearing directly on these virtues are given, both in the decalogue and in addition thereto. As to the act of fortitude there are the order to be given by the commanders in the war, which is undertaken for the common good: as is clear from Deuteronomy 20:3, where the priest is commanded (to speak thus): "Be not afraid, do not give back." In like manner the prohibition of acts of gluttony is left to paternal admonition, since it is contrary to the good of the household; hence it is said (Deuteronomy 21:20) in the person of parents: "He slighteth hearing our admonitions, he giveth himself to revelling, and to debauchery and banquetings."
Source: Summa Theologica (New Advent)