The abilities to know truth and choose the good are gifts of our reason. They are natural to us and make us fully human.

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Perhaps the most confusing aspect of natural law is the phrase itself: “natural law.” For many people, the word “natural” implies human biology or the physical environment. For others, it means “instinct.” Likewise, when some people hear the word “law,” it implies “constraint” or obedience to legislation, regulations, and codes decreed by institutions with the authority to do so.

There is obviously some validity to using these words in such ways. Yet such uses are not a good starting point for understanding what natural law is.

The origins of the expression “natural law” are to be found in debates between the Greek philosopher Plato and those thinkers known as the Sophists. In broad terms the Sophists believed that politics was not about questions of right, wrong, justice, or injustice. They maintained that social arrangements reflected whoever was the strongest. Hence, it was “natural” for the strong to rule the weak. Such was the “law” of human “nature.”

Plato disagreed with the Sophists. For him, politics and justice could not be reduced to the rule of the strong. Nevertheless, Plato recognized the rhetorical power of the term “natural.” He thus decided to use it for his own purposes. In Plato’s thought, “natural” became a way of saying “human,” and one distinctive feature of humans is that we have reason. This is what makes humans different from animals. They act according to instinct alone. We do not.

What did Plato mean by “reason?” First and foremost, he meant the mind’s ability to know truth, and how to choose and act rightly as individuals munities in light of truth. Reason was thus more than our mind’s ability to know how to weigh and calculate quantifiable objects, or our capacity prehend the workings of the material world in which we exist. Reason certainly included those capacities; it found expression in fields such as mathematics or natural sciences like physics. But reason, from Plato’s standpoint, was above all practical in the sense of helping us know ethical and philosophical truth and then how to choose and act rightly.

What is the “law” dimension of natural law? The law part concerns that which is right for human beings. Here “right” does not primarily mean “efficient” or “useful.” Insofar as efficiency means the optimal use of scarce resources and avoidance of waste, or utility means the usefulness or value that consumers experience from the use of a product, natural law regards efficiency and utility as valuable and, as we will see, potential factors to consider when making moral judgments.

But when the phrase “right” is used in natural law, the focus is upon what reason identifies as good and just. Much of this was neatly explained by Thomas Aquinas. To his mind, natural law consists of the basic principles of practical reason for humans. The most fundamental of these principles is that good is to be done and evil is to be avoided. Here good means reasonable while evil means unreasonable. A second key principle of practical reasoning is that knowledge is a good to be pursued while falsehood and ignorance are to be e. A third principle is that you may never do evil even if you anticipate that good e of it.

This third principle merits more explanation as it is one that many have found perplexing. Surely, the argument goes, there are instances in which one must choose means (e.g., bombing German cities in World War II) that we would not otherwise choose in order to realize a greater good (e.g., hasten the defeat of Nazi Germany).

In one sense, the idea that we may never do evil that good e of it is a logical derivative of the first principle of doing good and avoiding evil. That means avoiding the free choice of evil in every aspect of any action, whether it is the object or goal of the act (defeating Nazi Germany), or the means through which that goal is achieved (the waging of war). Once your act involves a conscious choice of an evil (consciously targeting civilian populations and batants while waging war), it follows that the act itself is evil, no matter how much good might be realized. In other words, there are some acts that cannot be rationally defended by reference to any end.

Right reason and truth

How then do we know these principles? Natural law holds that people possess a basic knowledge of these principles through their possession of reason (ST I-II a.94, a.4). In this sense, the principles of natural law are “natural” to human beings (ST I-II q.94, a2) not because of human biology but because they are universally knowable by human reason (ST I-II q. 94, a.4; a.94, a.6) and universally binding because of their basis in human reason (ST I-II q. 94, a.4). Reason thus permits us to know the truth about good and evil, even though the directedness of such knowledge can be undermined or obscured by the pull of powerful emotions, and the meaning of this information for human choice and action can be hard to determine (ST I-II q.94, a.6).

What is the content of this truth about good and evil? In basic terms, it is the truth about human flourishing. Such flourishing occurs when we can freely choose particular things that are good in themselves (such as knowledge or beauty) and therefore fulfilling (ST I-II q.94, a.2) for humans qua humans, intelligible to human reason as reasonable for humans to pursue, and which other species (like animals and plants) cannot know and cannot therefore choose because they lack reason. Our knowledge of such es about through our intrinsic orientation toward the various goods that reason bids us to pursue. These goods in turn provide reasons for humans as rational beings to make this implicit awareness explicit and propositional through reflection on human choice and action.

The study of natural law consequently involves identifying and applying the principles of rational thought to how we know and choose the good, right, and just when we make free choices. 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. Aquinas defined truth as adaequatio intellectus et rei [conformity between the intellect and reality] (ST I q.21, a.2c). What Aquinas meant by “reality” is the truth about something as it is in itself: that, for instance, the content of the most basic principle of justice is to give others what they are owed, and not something else; or that the content of the virtue of courage is not the same as being reckless or being a coward.

(This is an excerpt from chapter 1 of The Essential Natural Law by Samuel Gregg, published by the Fraser Institute, 2021.)