In his letter to the Romans, St. Paul describes Gentiles who do not have the law yet who “do by nature what the law requires,” saying that they prove that “what the law requires is written on their hearts, while their conscience also bears witness and their conflicting thoughts accuse or perhaps excuse them” before God (Rom. 2:14-16).
This “law written on their hearts” is what theologians mean by natural law. St. Thomas Aquinas talks about it as man’s rational participation in the eternal law. What he means is that God’s universe is an orderly, rational one, and we are rational creatures. Thus, there’s a lot we’re able to rationally discern about what we should and shouldn’t do. Those who have never heard of the Ten Commandments already know that it’s wrong to murder—even if they do it anyway, or even if they rationalize a certain type of murder as not really murder.
At its most basic level, natural law says to “do good and avoid evil.” We start moral discussions from an advantage: we don’t have to show that we should do good or avoid evil; we can merely show what is good and evil, and let conscience do the rest.
Natural law is important for how we understand civil society. If a legitimate authority (e.g., a democratically elected majority) passes an immoral law, does that law have binding authority? St. Paul writes, “Let every person be subject to the governing authorities. For there is no authority except from God, and those that exist have been instituted by God” (Rom. 13:1). On the other hand, Paul was put to death by those same governing authorities.
This is no idle question. In 2014, the Supreme Court sided with Hobby Lobby in refusing to provide abortifacient contraception in its company health plan, citing religious objections. A year later, in Obergefell v. Hodges, that same Court discovered a never-before-seen constitutional right to “gay marriage.” Is it hypocritical for Catholics to seek legal cover in the Hobby Lobby decision while rejecting Obergefell as a violation of God’s law?
The same question arose during the civil rights movement, in which figures such as Dr. Martin Luther King, Jr. disobeyed Jim Crow laws while simultaneously calling for the enforcement of the Supreme Court’s decision in Brown v. Board of Education. King explained the intellectual coherence of these two positions, laying out a whole theory of natural law rooted in Catholic theology. He wrote: “One has not only a legal but a moral responsibility to obey just laws. Conversely, one has a moral responsibility to disobey unjust laws.”
What’s the difference? A just law is “a man-made code that squares with the moral law or the law of God,” while an unjust law “is out of harmony with the moral law.” King then cites Aquinas for the proposition that “an unjust law is a human law that is not rooted in eternal law and natural law.”
Natural law, in other words, gives us a secular framework in which to evaluate the morality of “positive law” (that is, legislation and court rulings created by human governing institutions). It’s how the courts at Nuremberg were able to say that what the Nazis did amounted to “crimes against humanity,” regardless of what German law permitted or even ordered. They had a duty higher than their duty to the state: a duty to obey natural law, which is ultimately a duty to obey God.
But understanding the role of natural law is also important for interpersonal apologetics. J. Budziszewski points out that we often assume that our job as evangelists is to “pump in” knowledge, when it’s more often the case that we can (and should) instead draw it out of others. This matters, because, as Blaise Pascal astutely noted, “People are generally better persuaded by the reasons which they have themselves discovered than by those which have come into the mind of others.”
If you can use the natural law to show your non-believing friend that he thinks murder is wrong, and that he thinks murder is the killing of an innocent human being, and that human life (scientifically speaking!) begins at conception, that’s a good deal more effective than haranguing him about the fact that, from your perspective, he’s wrong.
Never forget that, due to the law written on their hearts, nonbelievers still want to be moral. The American Humanist Association’s self-righteous slogan is “Good without a God,” after all. If the humanists mean “You can behave morally without knowing whether or not God exists,” we Catholics would agree, at least to an extent. But if they mean that there can be objective morality without a divine Lawgiver, the answer is no. And so, this desire for moral goodness ends up pointing directly toward God as the source and ground of objective morality.