As distinct from revealed law, natural law is “nothing else than the rational creature’s participation in the eternal law” (Summa Theologiae, la, 2ae, quest. 91, art. 2).
As coming from God, the natural law is what God has produced in the world of creation; as coming to human beings, it is what they know (or can know) of what God has created. It is therefore called natural law because everyone is subject to it from birth (natio), because it contains only those duties that are derivable from human nature itself, and because, absolutely speaking, its essentials can be grasped by the unaided light of human reason.
St. Paul recognizes the existence of a natural law when he describes the moral responsibility of those ancients who did not have the benefit of Mosaic revelation . “Pagans,” he says, “who never heard of the Law but are led by reason to do what the Law commands, may not actually ‘possess’ the Law, but they can be said ‘to be’ the Law. They can point to the substance of the Law engraved on their hearts— they can call a witness, that is, their own conscience — they have accusation and defense, that is, their own inner mental dialogue” (Rom 2:14-15).
The Declaration of Independence begins, “When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.”
Therefore, let us also see Father Hardon’s very similar definition of Law of Nature:
The moral law as universally binding on human nature, unchangeable in its essence, and knowable to all mankind. The law of nature is universally binding no matter how many people may violate it. Its universality is not to be confused with the universality of its observance. No matter how many or flagrant the transgressions, they do not change the value or extent of the law. The law of nature is adaptable but not essentially changeable . There have been modifications adopted at one time or another in the law. On closer study, however, they are seen to be in fact changes in circumstances or in matters to which the law is applied. What cannot change is the substance of the law itself. Also the law of nature can be perceived, however dimly, by every human being who has the full use of his or her reason. Certain social customs, clearly in opposition to this law, do not change the fact that it can be perceived by all. What such customs prove is that fallen human nature is prone to evil and needs divine assistance as revelation even for a correct and generally available knowledge beyond the primary duties of the moral order. Obscured by passion and exposed to the moral pollution of a secularist culture, human reason is weakened in its perception of what is right and wrong. Nevertheless its light is not completely extinguished, and besides, having access to revelation, the light of grace is also always available.