It is written (Deuteronomy 6:1): "These are the precepts and ceremonies and judgments which the Lord your God commanded . . . you" where "precepts" stands for "moral precepts" antonomastically. And these words are placed at the beginning of the Law. Therefore all the precepts of the Law are included under them.
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.