The Natural Moral Law was defined by the Ancient Roman and Stoic philosopher Cicero, who wrote in 52 BC, “For there is a true law: right reason. It is in conformity with nature, is diffused among all men, and is immutable and eternal; … (it is the) supreme law which existed through the ages, before the mention of any written law or established state … Nor may any other law override it, nor may it be repealed as a whole or in part, nor have we power through Senate or people to free ourselves from it … Nor is it one thing at Rome and another at Athens, one thing today and another tomorrow, but one eternal and unalterable law, that binds all nations forever.”
Saint Paul said in his epistle to the Romans (2:14), “For when the Gentiles who do not have the law by nature observe the prescriptions of the law, they are a law for themselves even though they do not have the law. They show that the demands of the law are written in their hearts.”
The Natural Moral Law is the universal set of basic ethical norms every man and woman knows by virtue of reason. No rational person can be ignorant of it, nor can they claim ignorance. It applies to everyone, always and everywhere. There are no exceptions.
This was the basis for the Nuremburg Trials after World War II and for all trials of those charged with crimes against humanity. The Nazis, who participated in the slaughter of six million Jews, could not excuse their guilt; whether they were generals, privates, judges, or laborers did not matter. Any human being is expected to know it is gravely immoral to intentionally kill one innocent person. Helping murder six million innocent people is a crime against humanity.
Regardless of religious, political, ethnic, academic, or economic background, every human person knows the basic fundamentals of the Natural Law: murder, theft, lying, adultery, and the like are intrinsically immoral and evil. People still commit these sins, but no one can claim ignorance of knowing they are wrong.
Catholic morality is governed by both the Natural Moral Law (known by reason) and the Divine Positive Law (revealed by God in the Ten Commandments). The principles of the Natural Law shaped and formed the Catholic teachings on the immorality and sinfulness of acts such as abortion and euthanasia, as well as the evils of social ills such as racism, anti-Semitism, and the exploitation and abuse of women and children.