Understanding Natural Law from a Catholic Perspective

My teens always looked forward to the homilies of a particular priest, because he made the truths of Catholicism exciting and substantial. He inspired us to think about our Faith, and one way he did that was through a simple piece of philosophy. He said, “If we want to know what is real and true, we must always ask, ‘What is the nature of a thing?’” Another way to put it would be, “What is this thing and what is it made for?”

These questions unlock the force of the natural law, guiding us to understand more deeply the truth, goodness, and beauty that is our Catholic faith.

The first time I heard the term “natural law” was when my friend Nikolas Nikas of the Bioethics Defense Fund gave a presentation at my parish. By that time I had already spent many years teaching my children, my RCIA students, and others about God’s universal moral law and how it can be known by the use of reason alone—but I had never heard it called by the name of natural law. Nik demonstrated how America’s Constitution and our system of laws were based on natural-law principles that transcend any merely human laws based on popular opinion or human will (what is called positive law).

Abraham Lincoln, for example, cited moral truths that could be known through reason when he worked to deliver our nation from the evil of slavery. In a debate with Stephen Douglas, Lincoln said that slavery wasn’t just a political issue but one that represented “the eternal struggle between these two principles—right and wrong.”

A century later, Martin Luther King, Jr. was told that racial segregation was the “law of the land” and must be obeyed. MLK replied that unjust laws may be disobeyed. As he sat in jail for disobeying those laws, he wrote:

How does one determine whether a law is just or unjust? A just law is a man-made code that squares with the moral law or the law of God. An unjust law is a code that is out of harmony with the moral law. To put it in the terms of St. Thomas Aquinas: an unjust law is a human law that is not rooted in eternal law and natural law.

Natural law is another term for the universal moral law of God. Aquinas said that the natural law “is nothing other than the light of understanding infused in us by God, whereby we understand what must be done and what must be avoided.”

This law is universal because everyone—including people who have never received divine revelation—can know it. St. Paul proclaimed that even people who have never encountered something as basic as the Ten Commandments can still apprehend God’s important moral truths, because “what the law requires is written on their hearts” (Rom. 2:15).

Popes have used natural law to appeal to the world’s consciences. In recent memory, Pope Pius XII’s first encyclical was written to oppose humanity’s “drift toward chaos,” which he blamed on “the disregard, so common nowadays, and the forgetfulness of the natural law itself” (Summi Pontificatus 28). And Paul VI, writing in Humanae Vitae on the nature of human sexuality and the evils of contraception, stressed that “this teaching is in harmony with human reason.”

Some Catholics today argue against the use of natural law to instruct, calling it outdated or ineffective and suggesting pedagogical techniques that they consider more in line with modern sensibilities. But no less a thinker than Robert Cardinal Sarah has affirmed that “the Church’s social teaching argues on the basis of reason and natural law, namely on the basis of what is in accord with the nature of every human being.”

It is because of the natural law that Christians, Jews, Muslims, Buddhists, Hindus, and even atheists and agnostics can hear their consciences tell them what they may do, what they must do, and what they must never do.

Natural law explains people’s deep-seated understanding that crimes like murder and theft are wrong, whereas charitable acts like feeding the hungry and clothing the naked are right. It helps us instinctively relate such actions to the way human beings have been made. When we ask, “What is human life for?” or “What is the nature of a human being?” we want to know our ultimate goal in life and how to reach that goal. Natural law shows us both, or as professor Charles Rice says, it’s “a set of manufacturer’s directions written into our nature so that we can discover through reason how we ought to act.”

Just as we would not harm our car’s mechanical nature by putting molasses in the tank—which is made for gas—we should not harm our human nature by acting immorally; that is, in ways that contradict its design.

But before discussing natural law any further we need to address some common misunderstandings about it.

Natural law is not the same as the “laws of nature.” Scientific laws explain how matter and energy behave in the physical world. These laws cannot be “disobeyed” because they describe what usually happens rather than what should happen. For example, the law of gravity (science) describes how a falling bomb accelerates toward the earth. Natural law (morality) tells us whether it is right or wrong to drop the bomb on a certain target.

Natural law is not “what happens with other mammals in nature.” When Christians say that certain human behaviors go against nature, they mean that those acts contradict how human beings ought to act, not merely what happens in the natural world. Humans are animals, but we are rational animals, and so we should not make the behavior of lower animals our standard of morality. Lots of behaviors are “natural” for other animals, like stealing, forced sex, or infanticide, but that doesn’t make those behaviors natural for rational human beings.

Natural law is not “what feels natural to me.” A person may have a strong inclination to eat a whole box of cookies or a persistent desire to cheat on a spouse, but the consequences of those decisions show that we shouldn’t always follow our “natural feelings.” This doesn’t mean we should never follow our instincts and feelings; it just means we should use our minds to tell us which of them we should follow.

C.S. Lewis used the analogy of a piano to explain the relationship between our animal instincts that tempt us to do evil and our rational minds that know the moral law.

He said that instincts are like the keys on the piano, and morality (the natural law) is like the sheet music that tells us when to play the keys at the right time. For example, the fight instinct is good for a young boy if a stranger is trying to pull his sister into a car and abduct her; it’s bad if he is frustrated with his sister during play and uses fists to settle the matter. Likewise, the flight instinct is good when running away from a tsunami; it’s cowardly when running away from an infant drowning in a play pool.

Unlike the lower animals, we human beings are created with an intellect and a will (made in God’s image), which means that we are the only ones with the ability to reason morally—to choose between right and wrong. According to St. Thomas Aquinas’s classic definition, natural law is man’s participation in God’s eternal law, an intersection between human reason and divine wisdom.

Want to help your children navigate today’s toughest moral issues? Made This Way by Leila Miller and Trent Horn gives parents practical, age-appropriate tools for teaching Catholic morality through reason and natural law.