In Catholic circles the concept of natural law is fairly familiar. I say “fairly” because I’m not sure that many Catholics could give an account of what natural law is. I’m even less sure that many Catholics could draw on natural-law thought when, and if, they enter debates in the public forum.

But Catholic hesitancy about natural law, and how to argue from natural law, is worrisome. Or worse. It might reflect a culpable intellectual ignorance. So I’d like to consider some pivotal points about natural law, its foundation, and how to introduce it into public debate.

A caveat for those looking for ready answers: Catholic intellectuals are likely to think that each of the points I introduce leads to further questions. And they are right. So be it; that’s how philosophy works.

Let’s begin with a definition. Aquinas teaches that because we are free and rational, we have a share in God’s providence. Thus, he writes, “this participation of the eternal law in the rational creature is called the natural law” (ST I-II, 91, 2). Very well. But how are we to live in accord with natural law?

Here a basic comparison comes into play. There are first principles in theoretical reason, that is, our reasoning about how things are. Chief among them are the principles of non-contradiction, of identity, and of sufficient reason. So also there are first principles in practical reason, that is our reasoning about how to act. These are the precepts of natural law. They are keyed to our deepest human inclinations. We seek, for example, life and self-preservation, knowledge, sexual communion, the care of our children, and social harmony. Such are the core goods of human flourishing. We ought never attack these goods, and we ought to come together to advance them.

Given the definition of natural law and the dynamic of how to live in accord with it, we can turn to the question of how to appeal to natural law in the public forum. That is, how are we to engage in political and social policy debates about, for starters, marriage and family, economic resources, military action, life issues, and religious liberty?

We can, broadly speaking, do so by showing how a given policy either advances or undermines a core human good. We can make the case that these core goods are incommensurable and non-fungible and so cannot be “cashed out” in utilitarian terms. Princeton University’s Robert George is a public intellectual who does just this with acuity and aplomb.

To be sure, as Prof. George would admit, such natural law arguments do not always prevail. Indeed, some weary veterans of the culture wars might say that they seldom do. Moreover, even a sound argument is not a thumbscrew.

Whatever their success or lack thereof, it’s noteworthy that natural law arguments in the public forum typically rely on a “thin version” of natural law. What does this mean? First, it means that wherever possible empirical arguments about the human goods take the place of arguments from philosophical anthropology, that is, arguments that focus on the very nature of the human person. Second, it means that God is “bracketed.” No reference is made to the Creator, to the author of human nature. To do so, it’s assumed, would introduce theology into a secular environment that has long banished it. Nonetheless, Aquinas grounds his account of natural law in the Creator of the whole of nature and the distinctive character of human nature. On his view, “this participation of the eternal law in the rational creature is called the natural law.” Here we have the “thick version” of natural law. (Ironically, probably most critics of “thin version” natural law arguments are already suspicious of what lies behind them.)

So, what to do in public forum debates? My view is that “thin version” arguments are themselves sound and often strategically in order. At the same time, we ought not to leave God waiting in the wings. We need to be ready and willing to ground our arguments in a Christian philosophical anthropology. We should speak out loud the reasons for the faith that we have, the faith that animates our intelligence.

Aquinas famously presents fruitful lines of argument for the existence of God. So, too, do some of our contemporaries. The Calvinist Alvin Plantinga does so with great rigor. And let me introduce an audacious and analytic newcomer, Patrick Flynn. His book The Best Argument for God (Sophia, 2023) is near the top of my reading list. After all, I’m a frequent flyer in the public forum, and I don’t like to pull my punches.