In 1963, Martin Luther King Jr. wrote his famous “Letter from Birmingham Jail.” In it, the protestant clergyman would cite two of the most influential saints of the Roman Catholic Church, Augustine and Aquinas, to justify civil disobedience in the face of unjust segregation laws:

I would agree with St. Augustine that “an unjust law is no law at all.” Now, what is the difference between the two? 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.

As much as people today speak of moral relativism and legal positivism, the truth of the matter is that we can’t escape the natural law. Anytime we’re debating what the law should be, we’re appealing—at least implicitly—to some conception of justice, some conception of mon good, in order to justify why we think the law ought to be whatever our proposal entails. And the same is true whenever we’re deliberating about what we should do: The natural law governs our personal actions just as much as it does mon life as munities. As Samuel Gregg, research director of the Acton Institute, explains in his new book, The Essential Natural Law, “natural law is primarily ethics insofar as it is concerned with practical reasoning about how individuals munities do good and avoid evil when making choices and acting.”

Reason tells us that there are things one cannot not know, among them: to do good and avoid evil. It is from this starting point that one can begin speaking of natural and civil rights.

The Essential Natural Law

By Samuel Gregg

(Fraser Institute, 2021)

Theories of the natural law are one thing—and theorists will debate them until the ing. But the natural law is first and foremost a reality before it is theorized. There is a truth about human nature and the goods that perfect it, just as there is a truth about the moral norms that should govern our actions in pursuit of those goods. And the natural law tradition, as Gregg clearly lays out, holds that reason can know these truths, and that at some level we all make appeal to these basic truths even if we fail to follow reason all the way through: “Natural law maintains that for us to be rational in the fullest sense is to choose and act in accordance with what our reason tells us is the truth about the right course of action.”

Gregg opens this short book by tracing the tradition of natural law theorizing back to its classical roots in Plato, Aristotle, and Cicero. Far from being merely a translation of Christian theology into secular language, Gregg argues that critical reflection on human nature and its perfection gets started in a systematic way in the ancient Greek and Roman thinkers who sought a standard of justice beyond mere convention, grounding justice in nature. From there Gregg turns to the Christian thinkers who develop this tradition of philosophy and incorporate it into Christian theology, particularly the medieval thinker Thomas Aquinas, whose foundational theory of natural law Gregg presents in some detail. Continuing his historical sweep, Gregg explores later medieval Catholic thinkers, such as Francisco de Vitoria and Francisco Suárez, particularly concerned with what the natural law entailed for the exploration and settlement of the so-called New World, international relations, and trade, along with Protestant natural law thinkers such as Hugo Grotius, Samuel von Pufendorf, and Emmerich de Vattel.

Of particular importance in Gregg’s presentation of natural law theory is that the goods that perfect human nature are the foundational starting points. Our grasps of certain ends that are good in themselves, not mere means to other ends, is what allows thinking about action to get off the ground. From there we can discern various moral norms that should guide our action, and then various conclusions about particulars such as the virtues that shape our character, the actions that should never be done because they always mitting immorality (so-called moral absolutes), and rights understood as the entailments of justice. This last point is critically important: Natural rights for the natural law tradition are conclusionsof a chain of moral reasoning, not starting points (as they are for certain social contract thinkers). Gregg explains: “Natural rights derived their moral, legal, and political force from giving effect to requirements of natural law. Absent that foundation, natural rights would be understood simply as assertions of will and thus having little to do with reason.” That is, it is only from a sound conception of human nature and human flourishing, of the demands of justice and mon good, that we can then reason to conclusions about natural rights—and, I would add, any justified political and civil rights.

From here, Gregg moves on to discuss what the natural law tradition means for political authority and the distinctively mon good. In a chapter titled “Limited Government and the Rule of Law,” Gregg explains that it is precisely a concern for human flourishing that both justifies and limits government, and that demands that people be governed by law. Here Gregg attends both to those things that government must do in order for people to flourish and the ways in which government could overreach and subvert that flourishing, with the principle of subsidiarity proving crucial. In the next chapter Gregg turns to the natural law foundations and limits to the ownership of private property, emphasizing the foundations of property rights in service to mon good, but not saying quite enough about property duties. And in the final substantive chapter, Gregg explores the historic roots of the jus gentium—the law of nations—and its implications for international trade. In both these chapters on economic relations, Gregg examines the role that various late medieval and early modern Catholic and Protestant natural law thinkers played in the development of theorizing about markets, prices, trade, merce in general—showing how many of Adam Smith’s particular conclusions were already arrived at and with greater clarity and rational justification by these earlier thinkers.

The book concludes with Gregg’s discussion of the centrality of natural law for societies that want to maintain and protect ordered liberty, arguing that “it may well be natural law’s insistence that there are universal moral and philosophical truths knowable through right reason that represents one of its most important contributions to the maintenance of free societies.” Against skepticism about our ability to know the human good, or relativism and “neutrality” about the state’s promotion of the good, Gregg argues that it is precisely a sound—truthful—conception of human nature and human goods that will be the best bulwark for authentic freedom. Indeed, he closes the book with this clarion call: “Understanding natural law and the principles that it embodies surely has enormous potential to serve as a powerful ballast for the free society and to remind us of why liberty is important and why the protection of freedom merits eternal vigilance.”

Gregg’s book is an outstanding introduction—concise and accessible—to the broad natural law tradition. The choice to focus on economic and international relations leaves other topics less explored, and the Thomistic theory advanced is clearly influenced by Germain Grisez and John Finnis’s re-presentation of Aquinas’s works—which may rub some Thomists the wrong way. For the lay reader looking for a reliable guide, however, The Essential Natural Law is a fine place to start.