As distinct from revealed law, it is "nothing else than the rational creature's participation in the eternal law" (Summa Theologica, 1a 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 which 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" (Romans 2:14-15).