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"It is through conscience that human beings see and recognize the demands of the divine law. They are bound to follow their conscience faithfully in everything they do."
Religious Liberty, no 2.

Chapter 2. Medieval Christian Theologians: Augustine and Aquinas

From A Shared Morality. A Narrative Defense of Natural Law Ethics

by Craig A Boyd.
published by Brazos Press, Michigan. 2007 pp 54-59

Craig A BoydIn De Civitate Augustine argues that our ultimate end is found in the love of God. Yet since God created us with a communal nature, we should love not only God, but the neighbor and the self as well. The underlying theme throughout is that humans have a specific nature, ordered to God and others, such that some actions always lead to peaceful coexistence and others to social strife. The backdrop to the entire discussion in Book XIX, chapters 12 and 13, is an essentialist understanding of human nature.(38)

Yet no theory that attempts to give a complete account of morality without explicit reference to the Christian God will ever be accurate. Oliver O’Donovan says,

St. Augustine in the City of God, with an inconsistency that is only apparent, explains the success of the Roman empire in terms of traditional Roman virtues and, at the same time, denies that they are virtues at all, since there is no virtue without true religion. Such misknowledge may take the blatant form of idolatry, or its modern non-religious equivalent, ideology.(39)

This theme that one can never have a genuine moral perspective on life apart from God’s revelation in Christ shows up more clearly in Augustine than in any of the other natural law thinkers. Yet if God requires more from us than mere obedience to the precepts of natural law, then it follows that, at the very minimum, an account of the virtues—especially the virtue of charity—must have a role, as well as an understanding, of those revealed precepts that the natural law does not address.

By far the most influential natural law theorist was Thomas Aquinas. Although Aquinas spends relatively little space in his Summa Theologiae on natural law, his Treatise on Law has become the locus classicus for subsequent theorists. According to Aquinas, the natural law is a “participation on the part of the rational creature in the eternal law.”(40)

Aquinas argues that we can come to understand the natural law by means of a careful consideration of nature: by observing how various creatures act, and what things do, and do not, contribute to their flourishing. For Aquinas, “nature is understood primarily in terms of the natures of specific kinds of creatures, regarded as the intelligible principles of their existence and their causal powers.”(41) The human capacity to reason is an a priori capacity, but the content of the natural law must be discovered by an inquiry into human nature. Like Cicero before him, Aquinas borrows freely from Aristotle’s theory of human nature that distinguishes among the organic, animal, and rational souls. The precepts of the natural law apply universally to all human beings. The approach is the following:

The order of the precepts of the natural law is according to the order of natural inclinations. . . . First, there is in humans an inclination toward the good they share in common with all substances. . . . Second, there is in humans an inclination toward those things which are in accordance with what humans have in common with other animals. . . . Third, there is in humans an inclination to the good according to the nature of their reason, which is proper to humans. Thus, humans have a natural inclination to know the truth about God, and to live in society; and in this respect, whatever pertains to this inclination belongs to the natural law: e.g., to shun ignorance, to avoid offending those among whom one has to live and so on.(42)

One considers the natural inclinations and determines from those inclinations what kinds of actions must be avoided and what actions must be pursued. However, Aquinas does not intend to say that every natural inclination is to be acted upon in any and all circumstances. Rather, reason reflects on these natural urges and considers how they are to be pursued. For example, the natural desire for sexual reproduction is a good found in our animal nature; however, reason determines that its appropriate context is within marriage. But why? Aquinas believes that the good of the community depends on the order of society and, given the extraordinary emotional attachment associated with human sexuality, he contends that it is only within a stable, lifelong relationship that this kind of activity is appropriate.

Other creatures act on the basis of instinct alone, but the human creature is capable of self-directed activity. According to Aquinas, the natural law has the following characteristics:

1. It is discoverable by the natural light of reason.

2. It is a participation in the divine logos.

3. It is immutable.

4. It is universal.

5. It has been established by God for the common good of humanity.

By the "natural light of reason” humans come to know many things about the creation: that there is a God, that there is an order to creation, and that certain activities are right and others are wrong. This capacity that humans have to know and understand the world was given to humans by God in creation. Although human nature was indeed damaged by "The Fall,” humans still retained the ability whereby they could know moral truth. Human nature was weakened and lost its perfection, but it was not destroyed or obliterated or inca¬pacitated with regard to its ability to know the basic moral principles necessary for peaceful coexistence.

The metaphysical and epistemological structure of Aquinas’s natural law ethics is a participation metaphysics.(43) He says, “[T]o participate is to receive as it were a part; and therefore, when anything receives in a particular manner that which belongs to another in a universal [or total] manner, it is said to participate in it.”(44) Obviously, humans receive their nature from God and “participate” in God s creative activity. It can be said that participation is the means by which Aquinas expresses the radical existential dependence of creatures on God.

Norris Clarke lists three essential elements in Aquinas’s participation structure: "(1) a source which possesses the perfection in question in a total and unrestricted manner, (2) a participant subject which posesses the same perfection in some partial or restricted way, and (3) which has received this perfection in some way from, or in dependence on, the higher source.”(45) The relation of the natural to the eternal law clearly exhibits these three characteristics. First, the eternal law possesses the perfection of law as it is, the “divine reason.” Since law is a dictate of reason, and the Word is the expressivum et operativum of the Father, it follows that God, by means of the Verbum Dei, establishes the natural law in human nature (Isa. 34:3). The human capacity to reason is therefore a mirror that reflects the divine reason.

The second requirement for participation is that there must be a participant subject which in some partial way possesses the perfection. Humans possess their being directly from God; as John Wippel explains, “In every finite substantial entity there is a participated likeness or similitude of the divine esse, that is, an intrinsic act of being (esse) which is efficiently caused in it by God.”(46) All people participate in the divine esse and receive their being from God. As a result of participating in the divine esse, all people possess the natural light of reason. Consequently, all humans know the primary precepts of natural law through the natural light of reason, since we are created in the divine image.

The final element of the participation structure is that the participant must have received the perfection from the source in question. In the case of the natural law, the perfection is reason s capacity to know. That participation enables the agent to act freely and to govern her activities in accordance with reason. The imago Dei for Aquinas bridges the human and the divine. He says, “That the human is made in the image of God . . . implies that the human agent is intelligent and free to choose and govern itself” (Iallae, prologue). This intelligence provides the continuity between the human and the divine. Since humans participate in divine reason by being created in God’s image, they are thereby enabled to understand that God commands the precepts of natural law because of the way in which God has created humanity.

Aquinas appeals to the scriptures and especially to the passage from Romans 1 in support of his idea that all humans know universally applicable moral principles.(47) The most basic precept of the natural law is that "good is to be done and pursued and evil avoided.”(48) Yet the natural light of reason enables us not only to know these basic moral truths but also to distinguish between those activities that are truly good for us and those that are not.

According to Aquinas, we can distinguish between apparent and real goods (or ends).(49) Apparent goods are those that seem to be good, but do not satisfy our natural and legitimate desires. For example, lying presents itself as an apparent good, as it might help us out of a difficult situation in the short term, but it does not satisfy our natural and legitimate desire to live with others in community. This is so because deception is a practice that breaks down the basic structure of community. On the contrary, real goods are those that truly satisfy our natural desires. As food satisfies my natural hunger, so too knowledge satisfies my natural desire to know. And I need to possess knowledge in order to survive. So we discover that there are objective goods that all people recognize as real goods. Included in the list of real goods are honesty, prudence, and non-malefescence, among others. Those actions that are always and everywhere good Aquinas calls the primary precepts of natural law.

Yet in order to avoid an important misunderstanding here, we should note that the natural law does not exhaust the entirety of Aquinas’s views on ethics. The natural law lays down universal principles for human action that all humans recognize as binding. However, ethics properly understood concerns not only “doing the right thing” but acting for the right reason and in the right manner. It is the acquisition of virtue that enables us to act accordingly.(50) For Aquinas, “each person’s reason dictates that he should act virtuously.” “Reason’s dictate” here is the activity of the natural law. The virtues thus provide the agent with a stable character that enable her to act in a habitually good fashion, fulfilling the general precepts of the natural law. Natural law precepts, when considered in isolation from an account of the virtues, result in a hollow, deontological approach that prescinds from human nature, leaving us with a list of “dos” and “don’ts" that have little bearing on who we are as persons who exist in real relationships with God and one another. Yet these primary precepts of the natural law serve the all-important function of delineating what kinds of behavior contribute to human flourishing, and which do not.

In contrast to his later, more voluntaristic critics (e.g., John Duns Scotus or William Ockham), who emphasized the divine will and the mutability of the moral order, Aquinas held that the primary precepts of the natural law are immutable and universal. They do not change over time or from culture to culture; rather, they apply to all humans as humans. Accordingly, humans must always honor their parents, practice prudence, and treat others justly. Conversely, one may never commit adultery or murder, as these actions frustrate the natural human telos to live together peacefully. But the end of living peacefully with others is only part of the natural law; the purpose of human existence transcends the purely terrestrial goods of peaceful communal life and aims at a life of communion with God. In the words of Claude Tresmontant,

The purpose of creation was not merely to bring about a group of spirits living together peacefully before God in a just and happy society. The purpose of creation, the supernatural goal of creation, according to God’s plan as it actually is, is a union, a marriage, a fundamental transformation, a divinization, of human nature.(51)

As the natural law prompts us to pursue the good, we discover that the good to be pursued is the good in itself, God. So it is that the natural law directs us toward God as the goal of all our activity, our ultimate telos.(52) This union with God, Aquinas understands, is a kind of participation in the divine nature that truly perfects, or completes, the human soul.


38. The Latin naturam is used no fewer than 14 times in chapter 13 alone suggesting that, for all Augustine’s criticisms of Cicero’s secularist account of ethics, the notion of nature remained an important holdover from the great Roman jurist’s thought.

39. Oliver O’Donovan, Resurrection and Moral Order: An Outline for Evangelical Ethics (Grand Rapids: Eerdmans, 1986), 89.

40. Aquinas IaIIae.91.2.

41. Jean Porter, Nature as Reason: A Thomistic Theory of Natural Law (Grand Rapids: Eerdmans, 2005), 69.

42. Aquinas IaIIae.94.2.

43. Craig A. Boyd, "Participation Metaphysics in Aquinas’ Theory of Natural Law," American Catholic Philosophical Quarterly 79, no. 3 (2005): 431-45.

44. Aquinas Commentary on the De Hebdomadibus of Boethius, lect. 2.

45. Norris Clarke, “The Meaning of Participation,” Proceedings from the American Catholic Philosophical Association (1952): 152.

46. John Wippel, The Metaphysical Thought of Thomas Aquinas: From Finite Being to Uncreated Being (Washington, DC: Catholic University of America Press, 2000), 131.

47. Fergus Kerr, After Aquinas: Versions of Thomism (London: Blackwell Publishing, 2002), says “Thomas’s concept of natural law is thoroughly theological, and his appeal is to Scripture (Rom. 2:14; Ps. 4:6), not to philosophical authorities or considerations,” 106.

48. Aquinas Iallae. 94, 2. "Bonum est faciendum et prosequendum, et malum vitandum.”

49. A more current natural law argument is offered by Mortimer Adler, Six Great Ideas (New York: Macmillan, 1981), especially chapter 11.1 follow Adlers argument here.

50.1 will address this issue in greater detail in chapter 7.

51. Claude Tresmontant, The Origins of Christian Philosophy, trans. Mark Pontifex (New York: Hawthorn Books, 1963), 108.

52. Ibid. "From the standpoint of Christian ontology and anthropology, what defines man precisely is the supernatural destiny offered him by divine grace, for which he is preadapted by creation. . . . The philosopher should be able to discover by philosophical analysis, in concrete human nature as shown to us today, traces and signs which reveal that characteristic preadaptation to a supernatural destiny," 110-11.