Natural law refers to moral and legal theories, and is independent of the laws of nature that science aims to describe.
According to moral theory, the standards governing human behavior are, in some sense, objectively derived from the nature of human beings and the nature of the world. While independent of the legal theory, the two theories intersect.
According to legal theory, the authority of legal standards partially derives from considerations about the moral merit of those standards. There are different kinds of legal theories differing with respect to the role morality plays in determining the authority of legal norms. Classical natural law theory, such as the theory of Thomas Aquinas, focuses on the overlap between moral and legal theories.
Classical natural law theory subscribes to the Overlap Thesis, which asserts the relation between law and morality. The notion of law cannot be fully articulated without some reference to moral notions. Moral relates to principles of right and wrong in behavior. There are different interpretations of the Overlap Thesis.
Aquinas distinguishes four kinds of law: (1) eternal law; (2) natural law; (3) human law; and (4) divine law. Eternal law is comprised of those laws that govern the nature of an eternal universe. Divine law concerns the standards a human being must meet to achieve eternal salvation. The precepts of divine law are disclosed only through divine revelation.
The natural law comprises those precepts of the eternal law that govern the behavior of beings possessing reason and free will. The first precept of the natural law, according to Aquinas, concerns to practice good and to avoid evil. What is good and evil is derived from the rational nature of human beings. Good and evil are thus both objective and universal.
Aquinas is also a legal theorist, since a human law promulgated by human beings is valid only if its content conforms to natural law. If a human law sins against natural law, then the human law would be an unjust law. To paraphrase Augustine’s famous remark, “an unjust law is really no law at all”.
Is Roe vs Wade a law at all? Legal norms promulgated by human beings are valid only if the laws are consistent with morality.