Natural law is a concept. The notion is ancient and at the risk of bringing tears of lament; I’ll avoid the temptation to go on and on about Aristotle, Scholasticism and Aquinas. It is sufficient to note the natural law is universal in scope and duration and applicability. Natural law is the universal law of nature and Nature’s God. We are endowed with reason and with free will in order to determine good from evil so that we might conform our behavior to the former.
Our Declaration of Independence relies on Natural Law. The first paragraph refers to, “the Laws of Nature and of Nature’s God” and the second paragraph begins with, “We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.”
Why does this matter? It matters because the eternal and universal law of nature is not of man but of God. Positive (all non natural law) law is of man. Natural law is an expression of reason and morality. Positive law is an expression of the interests of those who made the law. Those interests can, and often have, included a great many things that are in direct violation of Natural Law. The historical example of communism in the twentieth century provides sufficient evidence of what happens when natural law is abandoned in favor of alternative theories of law (positive or other). The various communist regimes banished God and elevated man, or rather some few men. The law was to serve the greater good which meant starving Ukrainian peasants, killing anyone in Cambodia who spoke French, imprison millions for dissenting political views or for exercising their religion, etc. When they had finished the communists who discarded natural law killed somewhere in the ball park of one hundred million of their own people… all for the greater good of course. Natural law, being objective and eternal is not subject to the irrational abuses that spilled so much blood in such a short time.