The Philosophy of Law of Thomas Aquinas

Social Philosophy and Policy Foundation, Bowling Green, OH, USA


All translations are by the author unless otherwise indicated.

12.1 Life and Work

The great thirteenth-century philosopher and theologian Thomas Aquinas (ca. 1226–1274) played a pivotal role in the history and development of Western jurisprudence. During his productive but short life, Aquinas wrote extensively on moral matters, and as a corollary, on topics in political and legal philosophy. His exposition in Summa Theologiae on matters of law is often referred to as the classical canon of natural law theory.

Aquinas was born in 12261 of Italian noble parents of the family of Aquino. His birthplace was Roccasecca, which was not far from Naples in south central Italy. At an early age, while a beginning student at the University of Naples, he aspired to join the then newly formed mendicant friars known as the Order of Preachers or, more popularly, the Dominicans after their founder, Dominic de Guzman. Aquinas embarked upon this religious life voyage against his parents’ wishes—for they had visions of their son becoming the reigning abbot of the wealthy monastery of Monte Casino near their ancestral home. Nonetheless, young Aquinas persevered in his decision to join the Dominicans. Sent first to Paris and then to Cologne in order to study under the Dominican friar Albert the Great, Aquinas soon showed great intellectual promise.

The Dominicans saw themselves above all else as preachers and teachers. Moving away from the pastoral conditions of the countryside, which traditionally had served as the setting for most Western religious orders, the Dominicans established priories in the large European cities, usually associating themselves with major universities. Under the tutelage of Albert the Great, Aquinas’ intellectual star shone brightly. In fact, Albert accurately predicted that Aquinas would reach stellar intellectual achievements—the “bellow” of the “dumb ox” (as his fellow students at Cologne called him) would be heard around the world! He taught at the University of Paris on two distinct occasions; this institution was the leading center of academic and intellectual work in the thirteenth century. Albert the Great called it “a city of philosophers.” Aquinas was also assigned to Rome and to Naples at various times during his life.

Aquinas’ principal contribution to Western thought was his attempt to reconcile Aristotelian science and philosophy with the tenets of Western Christianity. He was a prolific writer; Kenny (1993, 10–1) claims that when one considers only the works generally acknowledged to be authentic, Aquinas’ omnia opera total over eight and one half million words. Including the texts of questionable authenticity increases the number of words to eleven and one half million. Aquinas wrote several commentaries on Aristotle’s texts, including his Commentary on the Nicomachean Ethics (In X Libros Ethicorum), which is important in the development of his own moral theory. Yet his greatest achievement was the composition, organization, and writing of his monumental Summa Theologiae.2 While it is correct to say that Aquinas never wrote a specific, “stand alone” treatise devoted to law, one finds what is often referred to as his “Treatise on Law,” which comprises Questions 90–97 of the Prima Secundae of the Summa Theologiae. He wrote the Summa Theologiae during the last few years of his scholarly life. It remained incomplete at the time of his death. Aquinas died at the premature age of forty-eight, probably due to a stroke. Tugwell (1988, 261) claims that today we would probably call Aquinas a “workaholic.”

This section concludes with a brief overview of Aquinas’ legal writings and their historical antecedents. McNabb (1929) traces many of the influences on Aquinas’ work in legal matters, and he also discusses the influence that Aquinas’ writings in turn had in the development of Western jurisprudence.3 McNabb (ibid., 1048) writes that Aquinas rethought the ethical system of Aristotle, and even argues boldly that the Nicomachean Ethics “is so enriched in form and matter by Aquinas that it might be well disputed who is the real founder of Ethics as a Science.” Aquinas’ texts indicate that he undertook reflective studies on law throughout his scholarly life. His early Commentary on the Sentences of Peter Lombard demonstrates that the “Law of the Decalogue” was foremost in his mind. Even at this early date in his intellectual career, Aquinas refers often to Aristotle’s Nicomachean Ethics and Metaphysics. Both of these treatises influenced Aquinas’ own construction of natural law moral and legal theory. His Summa Contra Gentiles, which was written five years after the commentary on Lombard, bears witness to the continued development of his legal insights. Nonetheless, his mature consideration on the nature and scope of law, De Lege, which is often translated as “The Treatise on Law,” is found in the Prima Secundae of Summa Theologiae, where Aquinas discusses human action. Aquinas’ references resemble a listing of the “Great Books” of ancient and medieval philosophy—the works of Plato, Aristotle, Cicero, Ulpian, John Chrysostom, Hilary, Jerome, the Pseudo-Dionysius, Augustine, Boethius, Isidore, and Moses Maimonides, among others. Aristotle the pagan philosopher, Augustine the Christian philosopher, and Moses Maimonides the Jewish philosopher are quoted most often. McNabb (ibid., 1055) notes that Maimonides served as a special influence on Aquinas’ understanding of the nature and scope of law: “[H]ad Moses Maimonides not written his famous book, Guide of the Perplexed, there would never have been written a still more famous book, St. Thomas’s treatise on Law.”

Aquinas also discusses references to law in the Bible. His treatment of the “old law” and the “new law” is in Summa Theologiae, QQ. 98 to 114, where the Prima Secundae ends. For example, Aquinas considers the Torah in the following manner:

We must, therefore, distinguish three kinds of precepts in the old law: a) moral precepts, which are dictated by the natural law; b) ceremonial precepts, which are determinations of the divine worship; and c) judicial precepts, which are determinations of the justice to be maintained among human persons. (STh IaIIae.99.a.4)

In addition to the analysis of law in the Summa Theologiae, Aquinas wrote several political documents, one of which is his unfinished Commentary on Aristotle’s Politics. He also authored selected notes on political leadership to the King of Cyprus, On Kingship (De Regimine Principum),4 and a brief letter on political theory to the Duchess of Brabant.5

12.2 The Treatise on Law

Aquinas’ discussion on law is a component of his substantive treatise on hu-man action, which is the topic of the Prima Secundae. His philosophy of law must accordingly be viewed as part of a more comprehensive philosophy of ac-tion. His analysis of law covers eight questions in the Summa Theologiae, and includes significant discussion of many topics germane to the study of Western jurisprudence:

  • Question 90: Considerations on Law

  • Question 91: The Different Kinds of Law

  • Question 92: The Effects of Law

  • Question 93: The Eternal Law

  • Question 94: The Natural Law

  • Question 95: Considering Human Law in and of Itself

  • Question 96: Concerning the Power of Human Law

  • Question 97: Concerning the Possibility of Changing the Law

Aquinas defines law in the following way: “Law of its very nature is an ordi-nance of reason for the common good, which is made by the person who has care of the community, and this rule is promulgated” (STh IaIIae.90.a.4). A suitable reading, as Finnis (1998, 226) suggests, for “common good” is the “public good.” In his Summa Contra Gentiles, Aquinas writes that “in human affairs, there is a common good that is, in fact, the good of the state [civitas or polis]” (SCG III.80, no. 14). In many respects, what Aquinas meant by civitas is similar structurally to the Greek concept of polis.

Aquinas uses his definition of law for the four categories of law he discuss-es: eternal law, natural law, positive or human law, and divine law. It is neces-sary to pay careful attention to the distinctions between these four kinds of law. In particular, one must not conflate eternal law with divine law, a practice that happens frequently in discussions of Aquinas on law. Moreover, one must not equate natural law with divine law in Aquinas, even though these two cat-egories of law are coextensive in several medieval treatises on law. The first three divisions of law—eternal, natural, and human or positive—are all inter-related yet distinct; all three are the result of a fairly rigorous philosophical analysis. Divine law, to the contrary, is in a class by itself and is entirely a mat-ter of theological investigation.

12.3 Eternal Law

One issue that contemporary students of natural law must confront is the role that eternal law plays in Aquinas’ general theory of natural law. Aquinas writes that the natural law in some way participates in the eternal law: “Hence, it is obvious that the natural law is nothing other than the participation of the eternal law in the rational creature or human being” (STh IaIIae.91.a.2). Many commentators assume that, according to Aquinas in Summa Theologiae, the existence of God is a necessary condition for understanding natural law. D’Entreves, for example, writes:

Now it seems to me that in our divided world the first and most serious stumbling block to the Thomist conception of natural law lies precisely in its premise […] of a divine order of the world, which St. Thomas recalls at the very beginning of his theory of law, and from which he infers, with unimpeachable logic, the most detailed and specific consequences: supposito quod mundus divina providentia regatur, ut in primo habitum est [it is assumed that the world is ruled by divine providence, as we demonstrated in the first part of Summa Theologiae]. Once that premise is granted, the whole majestic edifice of laws can be established on it: eternal law, the natural law, human law, and divine law. All are ultimately based on and justified through the existence of a supreme benevolent being. (D’Entreves 1970, 153–4)

In addition, O’Connor (1967, 60) writes that “the nature of law depends upon establishing the existence of a provident God who planned and guides the universe. St. Thomas, of course, believed that he had done this”; and A. Ryan (1985, 180) argues that “a secular natural law theory is simply incoherent.” The argument set forth in this chapter claims, to the contrary, that the eternal law is reducible to a Platonic archetype in the divine mind, which renders the objections articulated by D’Entreves, O’Connor, and A. Ryan, among others, moot.

Plato’s analogy of the Demiurge in his Timaeus provides an instructive paradigm for understanding the function of eternal law. A consistent analysis elucidates the concept of eternal law as the set of divine ideas in the divine mind. One idea in this set is the archetype for human nature. Following Plato’s suggestion offered in the Phaedrus, the archetypes in the divine mind “divide nature at its joints” (Plato, Phaedrus 265e). Aquinas uses this Platonic insight in rendering an interpretation of the following scriptural passage: “Let us make mankind in our image and likeness” (Gen. 1:26–8; New Catholic Edition). Aquinas argues that human nature is what it is, that is, the quidditas (“quiddity”) or set of essential properties determined by materia prima (“prime matter”) and forma substantialis (“substantial form”), because it is a reflection of the archetype of human nature in the divine mind. Working in a manner generally familiar to most medieval philosophers, Aquinas adopts insights from what he understood to be Platonic philosophy. In many ways, his was the received interpretation of Plato, which asserted that a subsistent world of the Forms existed in a transcendental realm. Aquinas situates these forms, which Plato articulated as freestanding, eternal, unchangeable essences, in the divine mind; most early medieval philosophers and theologians did likewise. Hence, a Platonic Form functions as a divine idea or archetype. This appropriation of Platonic Forms, which is rooted in Plotinus and Augustine, is accepted by most of the early medieval philosophers and theologians.

This Platonic schema serves as the philosophical basis for what Aquinas refers to as eternal law. The Renaissance philosopher Domingo de Soto (1494– 1560) of early modern Scholasticism6 at Salamanca in Spain, commenting on Aquinas’ account of law, explained the role of eternal law in some detail. De Soto explains that the eternal law is the ratio (“explanatory principle”) for understanding the order of the created world. Eternal law, as a formal cause, exists causally in the divine mind. A formal cause is that which provides the structure or organization for a natural object. For Aristotle and Aquinas, it refers more to a principle of explanation than to source of movement. De Soto writes:

God […] out of eternity conceived in his mind the order and dispensation and rule of the universe of things, in the likeness of which conception all laws are to be constituted: that ordainment and commandment therefore is called the eternal law in accord with its nature. (De Iust. et Iure I.3, Ad. 1 as quoted in Brett 1997, 142)

Since Aquinas writes that the natural law “participates” in the eternal law, many commentators (e.g., D’Entreves 1970) claim that the natural law depends on the eternal law. It would follow then that any understanding of the natural law requires the existence of God. This account entails a theological definist position for Aquinas (i.e., a metaethical position that defines the basic rightness or wrongness of an action by means of theological principles alone). Hence, in principle, it is in opposition to a natural law position, where the moral qualities of actions are determined by their connection with human nature rather than God. Such a position suggests the following puzzle: Must one understand the eternal law prior to coming to terms with the natural law?

There are two possible responses to this query, one metaphysical and the other epistemological. The metaphysical position articulated by Aquinas is that the archetype of human nature in the divine mind is the metaphysical principle after which all humans in the terrestrial realm are patterned. This is, to be sure, a rather rarefied ontological position, and the foundation for what the medievals often called “the truth in things,” also referred to as “ontological truth.” However, a human knower is able to determine the set of necessary properties that constitute an essence without understanding that this essence fundamentally is a copy of the archetype in the divine mind. In other words, one can understand the set of necessary properties that make up the content of human nature as just that—an essence of human nature—without realizing that this essence is patterned after the archetype in the divine mind. Now, how might this be explained? We must look to Aquinas’ epistemological position to answer this question.

The epistemological position depends upon insights gleaned from Aquinas’ philosophy of mind. Through the use of abstraction via the intellectusagens (“agent intellect”), a knower can determine the content of a human essence totally within the human sphere of awareness. One need not know that this essence depends on a divine archetype in order to flesh out the set of synthetic necessary properties that comprise the content of a human essence. Aquinas claims that human beings never have propter quid (“essential knowledge”) of God but only secundum quid (“incidental knowledge”). To assert or imply that a human being needs to know the eternal law, which is an archetype in the divine mind, in order to understand the natural law is inconsistent with Aquinas’ philosophy of mind, natural theology, and epistemology. The archetype of human nature is, to be sure, the foundation of human nature, which in turn is the foundation of natural law. Nonetheless, it is possible to understand the content of human nature without realizing its dependence on the divine archetype. This is an important point that Aquinas appropriated from Aristotle’s doctrine of abstraction and used in his own philosophy of mind. Hence, what Aquinas needs for his theory of natural law is a theory of natural kinds rather than the existence of God. (This issue will be discussed further in the next section.)

12.4 The Natural Law

Recently, modern historians have advanced the theory that a revived sense of the study of nature occurred in the twelfth century in several cathedral schools, especially Chartres. This renewed interest in the natural world accompanied the introduction of a systematic order into the matters of learning inherited from an earlier time (Southern 1995, 4–5; Haskins 1927, 303–40 passim). Consequently, as the universities began to blossom in the early-thirteenth century, full sets of lectures on the philosophy of nature became part of the curriculum.

Given this renewed interest in the study of nature, philosophical and theological discussions ensued within the context of a better understanding of the natural world. These new studies demonstrated a rationale for the intelligibility of the natural world and advocated the intrinsic goodness of the realm of nature. As Porter (1999, chaps. 1 and 2) argues, nature and revelation, under the direction of reason, worked in tandem during the formative stages of the development of natural law theory.7 Working with Albert the Great, Aquinas became immersed in these new studies, arguing that the development of a natural law theory in the Aristotelian mode followed coherently from the emerging studies in the philosophy of nature. Aquinas, furthermore, offered an interpretation of nature using the Aristotelian categories of matter and form. “Matter” refers to the material substratum or underlying “stuff” that is organized by the “form.” “Form” is the principle of organization much like the “blueprint” for a building. Any individual natural thing in the world is made up of matter and form; neither exists separately by itself. In order to explain the causal structures of nature, Aquinas argues for the existence of natural kinds, which is a metaphysical theory of essence that claims that there is a set of properties that renders an individual a member of a class or natural kind. A substantial form is a set of dispositional properties that determines the content of a natural kind. A dispositional property is a “capacity” that something has to become more developed or brought to fruition. In De Anima, Aristotle writes that the soul, which is nothing more than the substantial form of a living organism, is “the first act of a body with the potency of life” (de An. 415b5–10). Using the categories of contemporary analytic philosophy, we might suggest that this set of dispositional properties is a synthetic necessary set. It is necessary because it determines the essence—in all possible worlds, one might argue—and it is synthetic because it has a referent beyond the use of language.8 This synthetic necessary set is de re, or about the nature of things, and not de dicto, or only about the use of language. Aquinas’ realist ontology is apparent in his discussion of the philosophy of nature.

The concept of human nature is a necessary condition for Aquinas’ account of natural law to cohere consistently. What Aquinas needs, in turn, to account for his theory of human nature is a metaphysics of natural kinds. As noted above, human nature as elucidated by Aquinas is best analyzed as a natural kind. It follows that what Aquinas needs in order to explicate his account of natural law is the concept of a natural kind. The natural kind of human nature is defined as a certain set of dispositional properties.

In the latter part of the Prima Secundae of the Summa Theologiae, Aquinas delineates his exposition of the three generic categories of dispositional properties that determine what human nature is:

Insofar as good has the intelligibility of end and evil has the intelligibility of contrary to end, it follows that reason grasps naturally as goods (accordingly, as things to be pursued by work, and their opposites as evils and thus things to be avoided) all the objects which follow from the natural inclinations central to the concept of human nature. First, there is in human beings an inclination based upon the aspect of human nature which is shared with all living things; this is that everything according to its own nature tends to preserve its own being. In accord with this inclination or natural tendency, those things (actions, events, processes) by which human life is preserved and by which threats to human life are met fall under the natural law. Second, there are in human beings inclinations toward more restricted goods which are based upon the fact that human nature has common properties with other animals. In accord with this inclination, those things are said to be in agreement with the natural law (which nature teaches all animals) among which are the sexual union of male and female, the care of children, and so forth. Third, there is in human beings an inclination to those goods based upon the rational properties of human nature. These goods are uniquely related to human beings. For example, human beings have a natural inclination to know the true propositions both about God and those necessities required for living in a human society. In accord with this inclination arise elements of the natural law. For example, human beings should avoid ignorance and should not offend those persons among whom they must live in social units, and so on. (STh IaIIae.94.a.2)


Human nature as a generic set of dispositional properties might be rendered in the following manner: (1) The set of living dispositions (which humans share with plants), (2) the set of sensitive dispositions (which humans share with animals), and (3) the set of rational dispositions (which makes humans unique in the material world). Aquinas’ analysis of human nature is dependent on philosophical claims found in Aristotle’s Nicomachean Ethics, Metaphysics, and De Anima.

A living disposition is the capacity, active potency, or drive that all living beings possess that enables them to continue in existence. Had humans evolved or been created differently, there might be a different set of dispositions that comprise their set of essential properties. This living disposition is similar structurally to what Hart (1961, 190) calls the natural necessity of “survival.” In a similar fashion, one of the rational dispositions is the capacity that humans exhibit to know, which is best described as an innate curiosity. Aquinas was familiar with the opening passage in Aristotle’s Metaphysics indicating that “[a]ll human beings, by nature, desire to know” (Metaph. 980a25). Aquinas argues that this rational disposition is only developed when a human knows what is true. This “rational curiosity” is analogous to what Fuller (1964, 185) calls “communication.”9 Finnis (1998, 81) describes this structure in the following way: “The order Aquinas here has in mind is a metaphysical stratification: [1] what we have in common with all substances, [2] what, more specifically, we have in common with other animals, and [3] what is peculiar to us as human beings.” C. Ryan (1965, 28) writes that these three general aspects of the human person are “the good of individual survival, biological good, and the good of human communication.”10 Golding (1974, 242–3) refers to the living dispositions as the “basic requirements of human life,” the sensitive dispositions as the basic requirements for the “furtherance of the human species,” and the rational dispositions as the basic requirements for the “promotion of (a human person’s) good as a rational and social being.” The purpose of Aquinas’ argument here is to elucidate and understand in a general fashion those dispositional properties that are central to the concept of a human being.11

Metaphysical realism, as Simon (1965, 7–8) argues so well, is a necessary condition for an adequate theory of natural law. Aquinas’ account of natural kind is similar to what Kripke (1971, 144–6) calls the “metaphysically necessary,” which is a truth that is dependent on reality. This concept is not a mere convention of human language. Hence, the “metaphysically necessary” is coextensive with “synthetic necessity” (as discussed above). Kripke argues that the proposition “Water is H2O” is a metaphysically necessary truth because something would not be water if it were not H2O. This is the essence, or what Kripke calls the natural kind, of water. This structure is the nature of the kind of thing water is, and it is, Kripke argues, true in all possible worlds. Kripke’s concept of the metaphysically necessary seems commensurate with what Aquinas holds.12

This account of a natural kind reflects Aquinas’ claim that all human persons possess the quiddity of human nature, that is, they share the same set of fundamental properties that constitute human nature. This would hold, in principle, in all possible worlds. To help elucidate this claim, one might explain, for instance, how Aquinas refutes the Latin Averroists on the structure of human nature regarding the separated agent intellect. Aquinas argues that whatever we name by human intelligence—what Kripke would say we “rigidly designate”—is not part of what we name by a separated agent intellect. Of course, human knowers might be mistaken in their attempts to understand the specific set of properties that determine a natural kind. Aquinas often remarks about the difficulties encountered in this epistemological enterprise. That this is a difficult enterprise is, however, a different question in the philosophy of mind; it does not diminish the need for a set of causal properties in reality that serve as the foundation for the natural kind. In this regard, Aquinas is an essentialist in his theory of natural kinds. This is what underlies the frequently made claim that Aquinas is a “moderate realist” in his theory of essences.13

Aquinas’ account of natural law requires as a necessary condition an ontological theory of natural kinds because he must account for the regularity of the world. He accomplishes this through his theory of essence, which in turn is rooted in his account of substantial form (or formal cause). Once Aquinas has provided his theory of essence, he has the blocks in place needed to develop his moral theory of natural law. This suggests that Aquinas views moral theory as a “second order inquiry.” It is second order because it follows from the primary ontological theory of the natural kind of human nature.

Only gold members can continue reading. Log In or Register to continue