The natural law tradition has a long and venerable history. Hints of its beginning can be found in the writings of the ancient Greek philosophers Plato and Aristotle, and natural law was the dominant view of the Greek stoics. It was widely adopted by Roman jurists and orators such as Cicero. It was something like the official doctrine of the medieval scholastics. It had such powerful advocates as Hugo Grotius, Samuel Pufendorf, Francisco Suarez, and John Locke in the 17th century. And it has many thoughtful contemporary champions as well.
The proponents of natural law span such vast tracts of time and are drawn from so many different countries that there is, as might be expected, considerable variation within the natural law tradition. No one thinker can be taken to be the definitive representative of the group. Ye t we can cull from these many sources an outline of the theory that will suffice for our purposes.
Natural law, as traditionally understood, has sometimes also been called higher law, and for good reason. Traditional natural law theorists typically believed that there is a source of law beyond human creation. The dictates of natural law have their source in divine command, human nature, or the order of nature itself. Depending on which version of natural law is adopted, there will be variation concerning how human beings come to know the natural law: through divine revelation, by studying human nature, or by studying the order of the natural world. Despite these differences of detail, however, all natural law theorists believe there is a source of law that is independent of human creation, and that it can be discovered by human beings through the exercise of their reason. As rational creatures, human beings have access to the natural law. Moreover, since the source of natural law (divine command, human nature, or the order of the universe) is unchanging and universal, the natural law is also unchanging and universal. That is, the natural law is the same for all people, in all places and times.
This unchanging and universal higher law is contrasted with human-made law, which of course is often changing and governs only a particular group of people in a particular place and time. When we speak of the laws of the United States or Canada, for instance, we are speaking of human-made law that governs the behavior of a particular group of people at a particular time. We might indicate that the law has changed in a specific way, as when we note that in 1791 the Constitution of the United States was changed to include ten amendments that have subsequently become known as the Bill of Rights. Other amendments followed. And we can contrast the constitutional law of the United States with that of another legal jurisdiction, such as Canada. In 1982 Canada adopted a new law called the Canadian Charter of Rights and Freedoms, which plays a similar role to the American Bill of Rights. (Both are included as appendices at the end of this book.)
The two types of law, human and natural, are related in two important ways, however. First, natural law provides an independent standard that all human laws must meet if they are to be genuine laws. Second, the most general account of law that we can give, as philosophers of law, must capture both kinds of law. This chapter explains each, beginning with the relation between natural and human law.
The central task of much philosophy of law, or jurisprudence, has been thought to be that of providing a definition, or general explication, of law. Because natural law theorists had to provide a definition that covered both natural and human law, they were naturally led to provide a normative definition of law. This was so because the natural law was understood to be fundamentally normative. Natural law does not merely order the world, but orders it well. Likewise, human law, which directs the behavior of persons, does not seek merely to direct behavior, but to direct it aright. What is distinctive to the natural law position is the insistence that the direction provided by law must be toward ends that are rationally defensible or objectively good; law must direct behavior toward the common good. It is this requirement—that all genuine law aims at what is truly good, not just for the ruler but also for the ruled—that sets natural law theory apart from others. In short, natural law theorists believe that law, as law, must aim at morally right ends.
The insistence that law has as its end the common good makes natural law theory teleological. Teleology is the view that some things (perhaps all) have an end or function proper to them, and that they cannot be fully understood without reference to that end or function. So, for example, we cannot fully understand the essence of a knife without reference to its end, which is to cut things. Now, one need not be a teleologist about everything in order to be a natural law theorist. But one must be a teleologist about law itself. That is, one must believe that law has an end, which is truly good, and that all genuine law aims at that end; there is some end that law must serve, qua law. Stated in terms of functions, the natural law theorist is committed to the view that law has some function, which is objectively good, and that we cannot fully understand law independently of that function. There have been many functions proposed as essential to law: the preservation of order, to assist us in achieving the good appropriate to our nature, to make us virtuous, and so on. It is important to keep in mind, however, that it is not only natural law theorists who may attribute an essential function to law; one might think that the purpose of law is just to ensure obedience to the political officials of one’s society and yet not be a natural law theorist. One will only be offering a natural law position if the end or function claimed to be essential to law must be objectively morally good. The important point to all of this is that if one does offer such a position, then one makes moral conditions validity conditions of law.
To say that law must have a certain moral content in order to be valid is to say that we cannot identify something as genuine law by its history, form, or structure alone; we must look also to its content to determine whether it is law. For most natural law theorists, this requirement takes the form of providing necessary conditions of law: for something to be a valid law, it is necessary, though not sufficient, that it be aimed at the common good and that it be just. That is, it is enough to make a law invalid that it aims at something contrary to the common good or is unjust. Thus, the view that “an unjust law is no law at all” has come to be one of the defining tenets of the natural law position.
The claim that a directive which is immoral or unjust cannot be a genuine law has been a source of considerable controversy, both as to its precise meaning and its truth. Both its meaning and its plausibility can be understood, however, if one takes notice of a different feature of the natural lawyer’s position. Thinkers in the natural law tradition believe that genuine laws impose a moral obligation of obedience upon those to whom they apply. We have a moral obligation to obey the law because of its independent moral rightness. This moral obligation to obey the law must be distinguished from any prudential reason we have to obey the law, such as the desire to avoid punishment. While the latter may hold even with respect to “unjust laws,” the former holds only because the law, as genuine law, enjoins what we morally ought to do. Thus the claim that “unjust laws” are not genuine laws is based on the assumption that we are morally obligated to obey genuine laws. If a “law” is seriously immoral or unjust, not only do we not have a general moral obligation to obey it, but to obey it might even be morally wrong. Thus such “laws” must not be genuine laws after all.
We take our example of traditional natural law theory from St. Thomas Aquinas, whose writings on law provide our first reading. Modern natural law theorists offer quite different accounts than does Aquinas; some of these are presented later in the book.
St. Thomas Aquinas (c. 1225–1275) was a Dominican monk who wrote extensively on both theological and secular topics, including law, morality, and politics. Of particular interest for us are his writings on law, which come from his greatest work, Summa Theologica. The following excerpt from Summa Theologica is quite challenging for students first embarking on the study of the philosophy of law. The effort is worthwhile, I believe, because Aquinas identifies and discusses in the following passages many questions that have dominated jurisprudence from his time to ours. The difficulty of the readings is due in part to the nature of the subject, which will be new to most readers, and in part to the differences in expression that Aquinas used compared to those that we would use today. Aquinas organizes his thought in terms of a number of central Questions that he will address. Many of these are further divided into Articles or subquestions.
To assist the reader in understanding Aquinas, introductory comments precede each Question. These comments provide an overview of the topic to be addressed in the Question and an indication of how each Question relates to the central themes identified in the Introduction as the organizing themes of this book. They also identify the central themes that have come to characterize the traditional natural law theory. I have taken the liberty of changing the order of many Questions and Articles so as to present a more linear statement of Aquinas’s thought.


[From Susan Dimock, “St. Thomas Aquinas: Law for the Common Good.” Copyright © Susan Dimock, 2001; all rights reserved. The excerpts from St. Thomas Aquinas are drawn from Summa Theologica 1–2, First Part of the Second Part. The original translation from Latin to English is by the Fathers of the English Dominican Province 1903; I have altered the translation where needed for clarity.]

I. The Essence of Law (from Question 90, Of the Essence of Law)

Aquinas begins his discussion of law with a consideration of the nature, or essence, of law in general. In this way, he sets the tone and task of future philosophy of law. What makes a particular rule, or directive, a law? What is it that all laws have in common and that gives them the force of law? This is the search for the nature of law as law. In the course of his discussion of this matter, Aquinas offers the following definition of law: law “is nothing else than an ordinance of reason for the common good, made by him who has care of the community, and promulgated.” He begins by offering an explication of each of the four component parts of this definition.

I.1 Reason (from Article 1: Is law something pertaining to reason?)

Law is a rule and measure of acts whereby man is induced to act or is restrained from acting; for “lex” [law] is derived from “ligare” [to bind], because it binds one to act. Now the rule and measure of human acts is the reason, which is the first principle of human acts … ; since it belongs to the reason to direct to the end, which is the first principle in all matters of action, according to the Philosopher. [Ed. “The Philosopher” is Aristotle.] Now that which is the principle in any genus is the rule and measure of that genus: for instance, unity in the genus of numbers, and the first movement in the genus of movements. Consequently it follows that law is something pertaining to reason.
Since law is a kind of rule and measure, it may be in something in two ways. First, as in that which measures and rules; and since this is proper to reason, it follows that, in this way, law is in the reason alone. Secondly, as in that which is measured and ruled. In this way, law is in all those things that are inclined to something by reason of some law, so that any inclination arising from a law may be called a law, not essentially but by participation as it were. And thus the inclination of the members to concupiscence is called “the law of the members.” [Ed. Concupiscence is a too-strong inclination to sensuality. It is an inclination that both precedes reason and threatens to lead persons into actions that are contrary to reason.]
Just as, in external action, we may consider the work and the work done, for instance the work of building and the house built, so in the acts of reason, we may consider the act itself of reason, i.e., to understand and to reason, and something produced by this act. With regard to the speculative reason, this is first of all the definition; secondly, the proposition; thirdly, the syllogism or argument. And since also the practical reason makes use of a syllogism in respect of the work to be done, … as the Philosopher teaches, hence we find in the practical reason something that holds the same position in regard to operations as, in the speculative intellect, the proposition holds in regard to conclusions. Suchlike universal propositions of the practical intellect that are directed to actions have the nature of law. And these propositions are sometimes under our actual consideration, while sometimes they are retained in the reason by means of a habit.
Reason has its power of moving from the will … for it is due to the fact that one wills the end, that the reason issues its commands as regards things ordained to the end. But in order that the volition of what is commanded may have the nature of law, it needs to be in accord with some rule of reason. And in this se...