Objection 1. It would seem that there is no natural law in us. Because man is governed sufficiently by the eternal law: for Augustine says (De Lib. Arb. i) that "the eternal law is that by which it is right that all things should be most orderly." But nature does not abound in superfluities as neither does she fail in necessaries. Therefore no law is natural to man.
Objection 2. Further, by the law man is directed, in his acts, to the end, as stated above (Question 90, Article 2). But the directing of human acts to their end is not a function of nature, as is the case in irrational creatures, which act for an end solely by their natural appetite; whereas man acts for an end by his reason and will. Therefore no law is natural to man.
Objection 3. Further, the more a man is free, the less is he under the law. But man is freer than all the animals, on account of his free-will, with which he is endowed above all other animals. Since therefore other animals are not subject to a natural law, neither is man subject to a natural law.
On the contrary, A gloss on Romans 2:14: "When the Gentiles, who have not the law, do by nature those things that are of the law," comments as follows: "Although they have no written law, yet they have the natural law, whereby each one knows, and is conscious of, what is good and what is evil."
I answer that, As stated above (90, 1, ad 1), law, being a rule and measure, can be in a person in two ways: in one way, as in him that rules and measures; in another way, as in that which is ruled and measured, since a thing is ruled and measured, in so far as it partakes of the rule or measure. Wherefore, since all things subject to Divine providence are ruled and measured by the eternal law, as was stated above (Article 1); it is evident that all things partake somewhat of the eternal law, in so far as, namely, from its being imprinted on them, they derive their respective inclinations to their proper acts and ends. Now among all others, the rational creature is subject to Divine providence in the most excellent way, in so far as it partakes of a share of providence, by being provident both for itself and for others. Wherefore it has a share of the Eternal Reason, whereby it has a natural inclination to its proper act and end: and this participation of the eternal law in the rational creature is called the natural law. Hence the Psalmist after saying (Psalm 4:6): "Offer up the sacrifice of justice," as though someone asked what the works of justice are, adds: "Many say, Who showeth us good things?" in answer to which question he says: "The light of Thy countenance, O Lord, is signed upon us": thus implying that the light of natural reason, whereby we discern what is good and what is evil, which is the function of the natural law, is nothing else than an imprint on us of the Divine light. It is therefore evident that the natural law is nothing else than the rational creature's participation of the eternal law.
Reply to Objection 1. This argument would hold, if the natural law were something different from the eternal law: whereas it is nothing but a participation thereof, as stated above.
Reply to Objection 2. Every act of reason and will in us is based on that which is according to nature, as stated above (Question 10, Article 1): for every act of reasoning is based on principles that are known naturally, and every act of appetite in respect of the means is derived from the natural appetite in respect of the last end. Accordingly the first direction of our acts to their end must needs be in virtue of the natural law.
Reply to Objection 3. Even irrational animals partake in their own way of the Eternal Reason, just as the rational creature does. But because the rational creature partakes thereof in an intellectual and rational manner, therefore the participation of the eternal law in the rational creature is properly called a law, since a law is something pertaining to reason, as stated above (Question 90, Article 1). Irrational creatures, however, do not partake thereof in a rational manner, wherefore there is no participation of the eternal law in them, except by way of similitude.
Source: Summa Theologica (New Advent)