Objection 1. It would seem that the precepts of the decalogue are unsuitably set forth. Because sin, as stated by Ambrose (De Paradiso viii), is "a transgression of the Divine law and a disobedience to the commandments of heaven." But sins are distinguished according as man sins against God, or his neighbor, or himself. Since, then, the decalogue does not include any precepts directing man in his relations to himself, but only such as direct him in his relations to God and himself, it seems that the precepts of the decalogue are insufficiently enumerated.
Objection 2. Further, just as the Sabbath-day observance pertained to the worship of God, so also did the observance of other solemnities, and the offering of sacrifices. But the decalogue contains a precept about the Sabbath-day observance. Therefore it should contain others also, pertaining to the other solemnities, and to the sacrificial rite.
Objection 3. Further, as sins against God include the sin of perjury, so also do they include blasphemy, or other ways of lying against the teaching of God. But there is a precept forbidding perjury, "Thou shalt not take the name of the Lord thy God in vain." Therefore there should be also a precept of the decalogue forbidding blasphemy and false doctrine.
Objection 4. Further, just as man has a natural affection for his parents, so has he also for his children. Moreover the commandment of charity extends to all our neighbors. Now the precepts of the decalogue are ordained unto charity, according to 1 Timothy 1:5: "The end of the commandment is charity." Therefore as there is a precept referring to parents, so should there have been some precepts referring to children and other neighbors.
Objection 5. Further, in every kind of sin, it is possible to sin in thought or in deed. But in some kinds of sin, namely in theft and adultery, the prohibition of sins of deed, when it is said, "Thou shalt not commit adultery, Thou shalt not steal," is distinct from the prohibition of the sin of thought, when it is said, "Thou shalt not covet thy neighbor's goods," and, "Thou shalt not covet thy neighbor's wife." Therefore the same should have been done in regard to the sins of homicide and false witness.
Objection 6. Further, just as sin happens through disorder of the concupiscible faculty, so does it arise through disorder of the irascible part. But some precepts forbid inordinate concupiscence, when it is said, "Thou shalt not covet." Therefore the decalogue should have included some precepts forbidding the disorders of the irascible faculty. Therefore it seems that the ten precepts of the decalogue are unfittingly enumerated.
On the contrary, It is written (Deuteronomy 4:13): "He shewed you His covenant, which He commanded you to do, and the ten words that He wrote in two tablets of stone."
I answer that, As stated above (Article 2), just as the precepts of human law direct man in his relations to the human community, so the precepts of the Divine law direct man in his relations to a community or commonwealth of men under God. Now in order that any man may dwell aright in a community, two things are required: the first is that he behave well to the head of the community; the other is that he behave well to those who are his fellows and partners in the community. It is therefore necessary that the Divine law should contain in the first place precepts ordering man in his relations to God; and in the second place, other precepts ordering man in his relations to other men who are his neighbors and live with him under God.
Now man owes three things to the head of the community: first, fidelity; secondly, reverence; thirdly, service. Fidelity to his master consists in his not giving sovereign honor to another: and this is the sense of the first commandment, in the words "Thou shalt not have strange gods." Reverence to his master requires that he should do nothing injurious to him: and this is conveyed by the second commandment, "Thou shalt not take the name of the Lord thy God in vain." Service is due to the master in return for the benefits which his subjects receive from him: and to this belongs the third commandment of the sanctification of the Sabbath in memory of the creation of all things.
To his neighbors a man behaves himself well both in particular and in general. In particular, as to those to whom he is indebted, by paying his debts: and in this sense is to be taken the commandment about honoring one's parents. In general, as to all men, by doing harm to none, either by deed, or by word, or by thought. By deed, harm is done to one's neighbor--sometimes in his person, i.e. as to his personal existence; and this is forbidden by the words, "Thou shalt not kill": sometimes in a person united to him, as to the propagation of offspring; and this is prohibited by the words, "Thou shalt not commit adultery": sometimes in his possessions, which are directed to both the aforesaid; and with this regard to this it is said, "Thou shalt not steal." Harm done by word is forbidden when it is said, "Thou shalt not bear false witness against thy neighbor": harm done by thought is forbidden in the words, "Thou shalt not covet."
The three precepts that direct man in his behavior towards God may also be differentiated in this same way. For the first refers to deeds; wherefore it is said, "Thou shalt not make... a graven thing": the second, to words; wherefore it is said, "Thou shalt not take the name of the Lord thy God in vain": the third, to thoughts; because the sanctification of the Sabbath, as the subject of a moral precept, requires repose of the heart in God. Or, according to Augustine (In Ps. 32: Conc. 1), by the first commandment we reverence the unity of the First Principle; by the second, the Divine truth; by the third, His goodness whereby we are sanctified, and wherein we rest as in our last end.
Reply to Objection 1. This objection may be answered in two ways. First, because the precepts of the decalogue can be reduced to the precepts of charity. Now there was need for man to receive a precept about loving God and his neighbor, because in this respect the natural law had become obscured on account of sin: but not about the duty of loving oneself, because in this respect the natural law retained its vigor: or again, because love of oneself is contained in the love of God and of one's neighbor: since true self-love consists in directing oneself to God. And for this reason the decalogue includes those precepts only which refer to our neighbor and to God.
Secondly, it may be answered that the precepts of the decalogue are those which the people received from God immediately; wherefore it is written (Deuteronomy 10:4): "He wrote in the tables, according as He had written before, the ten words, which the Lord spoke to you." Hence the precepts of the decalogue need to be such as the people can understand at once. Now a precept implies the notion of duty. But it is easy for a man, especially for a believer, to understand that, of necessity, he owes certain duties to God and to his neighbor. But that, in matters which regard himself and not another, man has, of necessity, certain duties to himself, is not so evident: for, at the first glance, it seems that everyone is free in matters that concern himself. And therefore the precepts which prohibit disorders of a man with regard to himself, reach the people through the instruction of men who are versed through the instruction of men who are versed in such matters; and, consequently, they are not contained in the decalogue.
Reply to Objection 2. All the solemnities of the Old Law were instituted in celebration of some Divine favor, either in memory of past favors, or in sign of some favor to come: in like manner all the sacrifices were offered up with the same purpose. Now of all the Divine favors to be commemorated the chief was that of the Creation, which was called to mind by the sanctification of the Sabbath; wherefore the reason for this precept is given in Exodus 20:11: "In six days the Lord made heaven and earth," etc. And of all future blessings, the chief and final was the repose of the mind in God, either, in the present life, by grace, or, in the future life, by glory; which repose was also foreshadowed in the Sabbath-day observance: wherefore it is written (Isaiah 58:13): "If thou turn away thy foot from the Sabbath, from doing thy own will in My holy day, and call the Sabbath delightful, and the holy of the Lord glorious." Because these favors first and chiefly are borne in mind by men, especially by the faithful. But other solemnities were celebrated on account of certain particular favors temporal and transitory, such as the celebration of the Passover in memory of the past favor of the delivery from Egypt, and as a sign of the future Passion of Christ, which though temporal and transitory, brought us to the repose of the spiritual Sabbath. Consequently, the Sabbath alone, and none of the other solemnities and sacrifices, is mentioned in the precepts of the decalogue.
Reply to Objection 3. As the Apostle says (Hebrews 6:16), "men swear by one greater than themselves; and an oath for confirmation is the end of all their controversy." Hence, since oaths are common to all, inordinate swearing is the matter of a special prohibition by a precept of the decalogue. According to one interpretation, however, the words, "Thou shalt not take the name of the Lord thy God in vain," are a prohibition of false doctrine, for one gloss expounds them thus: "Thou shalt not say that Christ is a creature."
Reply to Objection 4. That a man should not do harm to anyone is an immediate dictate of his natural reason: and therefore the precepts that forbid the doing of harm are binding on all men. But it is not an immediate dictate of natural reason that a man should do one thing in return for another, unless he happen to be indebted to someone. Now a son's debt to his father is so evident that one cannot get away from it by denying it: since the father is the principle of generation and being, and also of upbringing and teaching. Wherefore the decalogue does not prescribe deeds of kindness or service to be done to anyone except to one's parents. On the other hand parents do not seem to be indebted to their children for any favors received, but rather the reverse is the case. Again, a child is a part of his father; and "parents love their children as being a part of themselves," as the Philosopher states (Ethic. viii, 12). Hence, just as the decalogue contains no ordinance as to man's behavior towards himself, so, for the same reason, it includes no precept about loving one's children.
Reply to Objection 5. The pleasure of adultery and the usefulness of wealth, in so far as they have the character of pleasurable or useful good, are of themselves, objects of appetite: and for this reason they needed to be forbidden not only in the deed but also in the desire. But murder and falsehood are, of themselves, objects of repulsion (since it is natural for man to love his neighbor and the truth): and are desired only for the sake of something else. Consequently with regard to sins of murder and false witness, it was necessary to proscribe, not sins of thought, but only sins of deed.
Reply to Objection 6. As stated above (Question 25, Article 1), all the passions of the irascible faculty arise from the passions of the concupiscible part. Hence, as the precepts of the decalogue are, as it were, the first elements of the Law, there was no need for mention of the irascible passions, but only of the concupiscible passions.
Source: Summa Theologica (New Advent)