Objection 1. It would seem that the reason of any man is competent to make laws. For the Apostle says (Romans 2:14) that "when the Gentiles, who have not the law, do by nature those things that are of the law,... they are a law to themselves." Now he says this of all in general. Therefore anyone can make a law for himself.
Objection 2. Further, as the Philosopher says (Ethic. ii. 1), "the intention of the lawgiver is to lead men to virtue." But every man can lead another to virtue. Therefore the reason of any man is competent to make laws.
Objection 3. Further, just as the sovereign of a state governs the state, so every father of a family governs his household. But the sovereign of a state can make laws for the state. Therefore every father of a family can make laws for his household.
On the contrary, Isidore says (Etym. v. 10): "A law is an ordinance of the people, whereby something is sanctioned by the Elders together with the Commonalty."
I answer that, A law, properly speaking, regards first and foremost the order to the common good. Now to order anything to the common good, belongs either to the whole people, or to someone who is the viceregent of the whole people. And therefore the making of a law belongs either to the whole people or to a public personage who has care of the whole people: since in all other matters the directing of anything to the end concerns him to whom the end belongs.
Reply to Objection 1. As stated above (A. 1 ad 1), a law is in a person not only as in one that rules, but also by participation as in one that is ruled. In the latter way each one is a law to himself, in so far as he shares the direction that he receives from one who rules him. Hence the same text goes on: "Who shows the work of the law written in their hearts."
Reply to Objection 2. A private person cannot lead another to virtue efficaciously: for he can only advise, and if his advice be not taken, it has no coercive power, such as the law should have, in order to prove an efficacious inducement to virtue, as the Philosopher says (Ethic. x. 9). But this coercive power is vested in the whole people or in some public personage, to whom it belongs to inflict penalties, as we shall state further on (Q. 92, A. 2 ad 3; II-II, Q. 64, A. 3). Wherefore the framing of laws belongs to him alone.
Reply to Objection 3. As one man is a part of the household, so a household is a part of the state: and the state is a perfect community, according to Polit. i. 1. And therefore, as the good of one man is not the last end, but is ordained to the common good; so too the good of one household is ordained to the good of a single state, which is a perfect community. Consequently he that governs a family, can indeed make certain commands or ordinances, but not such as to have properly the force of law.
Source: Summa Theologica (New Advent)