“[T]he 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.” – Summa Theologiæ [Part II, FP, Q. 91]
Natural law is sometimes dismissed as just another religious theory, totally dependent on belief in God. But actually, while Catholics do believe that God wrote it on our hearts (Deuteronomy 6:4-7, Romans 2:12-16), it is a self-evident truth with which even the most rabid atheist should come to terms.
What is natural law?
“The natural law expresses the original moral sense which enables man to discern by reason the good and the evil, the truth and the lie” (CCC #1954).
“The natural law, present in the heart of each man and established by reason, is universal in its precepts and its authority extends to all men. It expresses the dignity of the person and determines the basis for his fundamental rights and duties” (CCC #1956).
It is an “immutable” and “permanent” element in society (CCC #1958), and civil law should reflect it (CCC #1959).
How can it be known?
Through the revealed wisdom of the Church and examinations of ourselves, we can become conscious of this law. “For everyone who asks receives, and he who seeks finds, and to him who knocks it will be opened” (Matthew 7:8, Luke 11:10). If you open yourself to evidence, you will discover truth.
Is it supported by secular science?
Yes. The concept of cognitive dissonance shows that lying makes us feel conflicted. Some diseases are directly tied to immoral sexual activity. The process of procreation and statistics on child-rearing point to the importance of heterosexual, sacramental marriage. The list truly goes on and on.