Objection 1. It would seem that the Old Law contains others besides the moral, judicial, and ceremonial precepts. Because the judicial precepts belong to the act of justice, which is between man and man; while the ceremonial precepts belong to the act of religion, whereby God is worshipped. Now besides these there are many other virtues, viz. temperance, fortitude, liberality, and several others, as stated above (Question 60, Article 5). Therefore besides the aforesaid precepts, the Old Law should comprise others.
Objection 2. Further, it is written (Deuteronomy 11:1): "Love the Lord thy God, and observe His precepts and ceremonies, His judgments and commandments." Now precepts concern moral matters, as stated above (Article 4). Therefore besides the moral, judicial and ceremonial precepts, the Law contains others which are called "commandments." [The "commandments" (mandata) spoken of here and in the body of this article are not to be confused with the Commandments (praecepta) in the ordinary acceptance of the word.]
Objection 3. Further, it is written (Deuteronomy 6:17): "Keep the precepts of the Lord thy God, and the testimonies and ceremonies which I have [Vulgate: 'He hath'] commanded thee." Therefore in addition to the above, the Law comprises "testimonies."
Objection 4. Further, it is written (Psalm 118:93): "Thy justifications (i.e. "Thy Law," according to a gloss) I will never forget." Therefore in the Old Law there are not only moral, ceremonial and judicial precepts, but also others, called "justifications."
On the contrary, It is written (Deuteronomy 6:1): "These are the precepts and ceremonies and judgments which the Lord your God commanded... you." And these words are placed at the beginning of the Law. Therefore all the precepts of the Law are included under them.
I answer that, Some things are included in the Law by way of precept; other things, as being ordained to the fulfilment of the precepts. Now the precepts refer to things which have to be done: and to their fulfilment man is induced by two considerations, viz. the authority of the lawgiver, and the benefit derived from the fulfilment, which benefit consists in the attainment of some good, useful, pleasurable or virtuous, or in the avoidance of some contrary evil. Hence it was necessary that in the Old Law certain things should be set forth to indicate the authority of God the lawgiver: e.g. Deuteronomy 6:4: "Hear, O Israel, the Lord our God is one Lord"; and Genesis 1:1: "In the beginning God created heaven and earth": and these are called "testimonies." Again it was necessary that in the Law certain rewards should be appointed for those who observe the Law, and punishments for those who transgress; as it may be seen in Deuteronomy 28: "If thou wilt hear the voice of the Lord thy God... He will make thee higher than all the nations," etc.: and these are called "justifications," according as God punishes or rewards certain ones justly.
The things that have to be done do not come under the precept except in so far as they have the character of a duty. Now a duty is twofold: one according to the rule of reason; the other according to the rule of a law which prescribes that duty: thus the Philosopher distinguishes a twofold just--moral and legal (Ethic. v, 7).
Moral duty is twofold: because reason dictates that something must be done, either as being so necessary that without it the order of virtue would be destroyed; or as being useful for the better maintaining of the order of virtue. And in this sense some of the moral precepts are expressed by way of absolute command or prohibition, as "Thou shalt not kill, Thou shalt not steal": and these are properly called "precepts." Other things are prescribed or forbidden, not as an absolute duty, but as something better to be done. These may be called "commandments"; because they are expressed by way of inducement and persuasion: an example whereof is seen in Exodus 22:26: "If thou take of thy neighbor a garment in pledge, thou shalt give it him again before sunset"; and in other like cases. Wherefore Jerome (Praefat. in Comment. super Marc.) says that "justice is in the precepts, charity in the commandments." Duty as fixed by the Law, belongs to the judicial precepts, as regards human affairs; to the "ceremonial" precepts, as regards Divine matters.
Nevertheless those ordinances also which refer to punishments and rewards may be called "testimonies," in so far as they testify to the Divine justice. Again all the precepts of the Law may be styled "justifications," as being executions of legal justice. Furthermore the commandments may be distinguished from the precepts, so that those things be called "precepts" which God Himself prescribed; and those things "commandments" which He enjoined [mandavit] through others, as the very word seems to denote.
From this it is clear that all the precepts of the Law are either moral, ceremonial, or judicial; and that other ordinances have not the character of a precept, but are directed to the observance of the precepts, as stated above.
Reply to Objection 1. Justice alone, of all the virtues, implies the notion of duty. Consequently moral matters are determinable by law in so far as they belong to justice: of which virtue religion is a part, as Tully says (De Invent. ii). Wherefore the legal just cannot be anything foreign to the ceremonial and judicial precepts.
The Replies to the other Objections are clear from what has been said.
Source: Summa Theologica (New Advent)