The Light of Understanding
But what precisely is the natural law inscribed into every human heart by the Creator, and what is its relationship to the Christian life? Is this law a merely positive law laid down by the divine Legislator—something that might as well have been different, thus having no essential link to what it means to be fully human? Or does it rather give expression and definition to man’s fundamental desires and needs, his genuine aspirations, his thirst for happiness and perfection?

Precepts of Natural Law
Looking into certain aspects of St. Thomas Aquinas’s teaching on natural law can help us arrive at a deeper understanding of what it is and why following it is so necessary and departing from it so grave. Drawing an analogy with the first principle of speculative reason—“the same thing cannot be affirmed and denied at the same time in the same respect”—St. Thomas presents the first principle of practical reason: “good is that which all things seek after.” From this he arrives at the first precept of natural law: “good is to be done and pursued, and evil is to be avoided.” Thus, “whatever practical reason naturally apprehends as man’s good or evil belong to the precepts of the natural law as something to be done or avoided.”7 From this starting point, St. Thomas traces out the three highest and most universal precepts which flow directly from the first precept viewed in light of man’s natural inclinations. Because man is a substance, whatever preserves his being and disables danger to his life belongs to the natural law; because man is an animal, whatever promotes the good of the species, such as the union of the sexes and the education of offspring, belongs to the natural law; third, whatever is proper to man as a rational animal, such as knowledge of the truth and living peaceably in society, belongs to the natural law. Of the last sort would be everything that pertains to man’s rational inclinations, such as shunning ignorance and avoiding offense to those among whom one lives. This elegant “tree of law,” starting with the most common nature (substance), proceeding to the generic nature (animal), and culminating in the specific nature (rational animal), enables one to place further determinations of natural law under these three headings.

To illustrate a deduction from these primary precepts, St. Thomas mentions the obligation that “one must not kill,” traceable to “one should do harm to no man.”8 That one man should not harm another is in turn an expression of the general precept that man should live according to the good of his proper nature, in which St. Thomas includes avoiding offense to those among whom one lives. Why? Because man is social and political by nature: society isn’t something optional, he needs it in order to live a good and fully human life, and must therefore respect its requirements. Similarly, any goods entrusted to a man should always be restored to their owner according to the rules of justice, unless some special circumstance arises, for instance if the owner intends to do a harmful deed with the returned property.9

Defining natural law as the rational creature’s participation in the eternal law, that is, “a share of the eternal reason, by which it has a natural inclination to its proper act and end,”10 St. Thomas (schooled by St. Paul) maintains that this law in its general determinations is naturally written into man’s heart. The weakness of a human reason blinded or handicapped by sin made it necessary for God to give a divine law which would clarify and specify the precepts governing human action.11 Hence, considered strictly in regard to moral precepts distinct from judicial or ceremonial ones, the divine law—for example, the Ten Commandments —inculcates the very same precepts that a virtuous man with a well-ordered mind would naturally discern by reflecting on the inclinations of his nature and the rights and duties they imply.

Since the moral precepts are about matters which concern good morals, and since good morals are those which accord with reason, and since also every judgment of human reason must be derived in some way from natural reason, it follows of necessity that all the moral precepts [of the Old Law] belong to the natural law—but not all in the same way. For there are certain things which the natural reason of every man, of its own accord and at once, judges ought to be done or not done, e.g. Honor thy father and thy mother, and thou shalt not kill, thou shalt not steal; and these belong to the natural law absolutely. And there are certain things which, after a more careful consideration, wise men deem obligatory. Such too belong to the law of nature, yet so that they need to be inculcated, the wiser teaching the less wise, e.g. Rise up before the hoary head and honor the aged man, and the like. And there are some things, to judge of which human reason needs divine instruction, whereby we are taught about the things of God, e.g. Thou shalt not make to thyself a graven image, thou shalt not take the name of the Lord thy God in vain.12

Here St. Thomas’s basic position can be seen. Because of the various defects to which fallen man is prone, divine law includes—repeats, if you like—moral precepts identical with those that reason can discover. Although precepts directly bearing on the respect due to God seem to be inaccessible to reason working on its own, they are still traceable to human nature as to their root-cause, even if divine instruction must further inculcate them.13 As with the precepts taught by wise men to society at large, the image suggested is one of a father instructing his children in what is naturally good for them, rather than a monarch laying down positive laws from his distant throne.

Friendship through Virtue
These points in place, it should be evident why St. Thomas concludes that “every law aims at establishing friendship either between man and man or between man and God.”14 In his defense of the reasonableness of the Old Law containing moral precepts such as “Thou shalt not kill,” “Thou shalt not steal,” he writes:

just as the principal intention of human law is to create friendship between man and man, so the chief intention of the divine law is to establish man in friendship with God. Now since likeness is the reason of love, according to Sir. 13.19, every beast loveth its like, there cannot possibly be any friendship of man to God, who is supremely good, unless man become good. Thus it is written in Lev. 19.2 (cf. 11.45): You shall be holy, for I am holy. But the goodness of man is virtue, which makes its possessor good (Ethics 2.6). Therefore it was necessary for the Old Law to include precepts about acts of virtue, and these are the moral precepts of the Law.15

The precepts are expressed in the manner of absolute command or prohibition because reason dictates that some acts must be done “as being so necessary that without them the order of virtue would be destroyed.”16 “Certain matters without which the order of virtue (which is the order of reason) cannot even exist, come under the obligation of precept.” Carrying the discussion still closer to its origins, St. Thomas relates the moral content of the divine law to the virtues perfective of man’s nature:

The community for which the divine law is ordained is that of men in relation to God, either in this life or in the life to come. The divine law, therefore, proposes precepts about all those matters whereby men are well ordered in their relations to God. Now man is united to God by his reason or mind, in which is God’s image. Thus the divine law proposes precepts about all those matters whereby human reason is well ordered. But this is effected by the acts of all the virtues, since the intellectual virtues set in good order the acts of reason in themselves, while the moral virtues set in good order the acts of reason in reference to interior passions and exterior actions. It is therefore evident that the divine law fittingly proposes precepts about the acts of all the virtues.17

Hence, St. Thomas firmly maintains that the natural law is intrinsic and unvarying, universal and necessary for mankind. The Catechism itself teaches this in no uncertain terms: “The natural law is immutable, permanent throughout history. The rules that express it [i.e., give expression to it in particular matters] remain substantially valid. It is a necessary foundation for the erection of moral rules and civil law.”18 In order to understand why St. Thomas and the Catechism speak of the fixity of natural law in such strong terms, we should turn our minds to the ultimate foundation of the natural law itself: the Divine Mind, the Eternal Law.

Exemplars in the mind of God
The natural law presupposes an “exemplary foundation” in the Eternal Law—meaning, that everything to be found in or discovered by rightly-ordered human reason is supremely exemplified (precontained, foreknown) in the wisdom of the divine Mind itself. Such a foundation implies a necessary connection between the beings God freely wills to create and the eternally true conditions God knows about the universe He does create. God foreknows what He is to create on the basis of the essences of all possible finite participations in his infinite self-subsistent being. In line with St. Augustine, St. Thomas names these archetypal or creative essences the “divine Ideas.” For St. Thomas, “a divine idea is nothing but a given way in which God views his essence as capable of being imitated by a creature. Prior to the actual creation of a given entity there is a divine idea to which that creature will correspond if it is ever brought into actual being.”19 Such an Idea captures not only the existence of the individual, but the essence of which the individual is a particular expression or instance. This is another way of saying that God knows James not only as James, but also as man, as animal, and as substance. As each of these, certain conditions and certain acts will be suitable for James’s perfection, others harmful to it. Rightness and wrongness of possible human acts will obtain regardless of whether or not James ever comes into existence. The creature’s essential foundation in the divine intellect makes it possible to speak absolutely of what is healthy and unhealthy, appropriate and inappropriate, good and evil, for that creature.

It was St. Augustine who wrote: “the Son is the art of the omnipotent God, full of all the living Ideas, and all things are one in this art.”20 St. Bonaventure, a devoted reader of St. Augustine and a friend and colleague of St. Thomas at the University of Paris, develops this insight by distinguishing between two kinds of likeness: the likeness of imitation which is the way in which a creature is a likeness of the Creator, and the exemplary likeness, the way in which the exemplary Idea in the Creator is a likeness of the creature. The former kind of likeness is what causes knowledge in the human mind, which comes to understand by means of turning to the phantasms or “data” received by the senses and drawing forth their intelligible content; such a likeness involves some degree of imperfection because it involves a sort of composition or addition in the knowing intellect. For the rational creature, things are the cause of knowledge. The latter kind of likeness, however, is what causes things to be, and does not come from outside; consequently, no imperfection is implied. Such exemplars of things subsist in the mind of God, are none other than His very nature, and are perfectly expressive of the realities they cause to be. Unlike man whose knowledge is caused by the encounter with already-existing things, the Creator’s own knowledge of things is their original and originating cause.

The divine intellect is the supreme light, the full truth, and pure act. So, as the divine power to produce things is sufficient in itself to produce everything, so the divine light and truth is sufficient in itself to express all things. And since this expression is an intrinsic act, it is eternal. Because an expression is a form of assimilation, the divine intellect—expressing all things eternally in its supreme truth—possesses from eternity the exemplary Ideas of all creatures.21

To speak of the Ideas in God as expressive of things is to speak of His causality with respect to the creature expressed, the total making of the creature according to its eternal pattern in the mind of the divine Artificer. The eternal Ideas are the productive principles of all things.22 Although the Ideas in the mind of the Creator are other than the Word (the Second Person of the Blessed Trinity), there is an intimate connection between Idea and Word. Just as the Word is the perfect expression of the Father—the Word eternally “spoken” by the Father—so too the Ideas in the mind of God, many in notion but one in divine substance, are the fullest and truest expression of all creatures. The Ideas of created things are expressed through the Word of God, and to this Word is attributed the exemplarity of creation. “In a true and proper sense, God is Word. But a word is the likeness of that which is spoken. Therefore, if the Son of God is the Word in whom all things are spoken, it is necessary that the likenesses of all things that are expressed be present in that Word.”23 Hence, as the Word is the necessary expression of the Father, so creation is the free artistic expression of the divine Ideas. Creatures are brought into being through the Art of the Word, “by which, through which, and according to which all beautiful things are formed.”24

The Word Perfectly Expresses the Creature
The Word of God is more than the source and plan of all creation, as if we were to think of Him as an architectural blueprint. He is the perfect expression of creatures, all of whom pre-exist in Him in the highest clarity and integrity. “A likeness of exemplarity and expression is found in its supreme form in the Creator with respect to the whole of creation, because that which is truth itself, being the supreme light, expresses all things in the most perfect way.”25 Creatures are like refracted light. The Eternal Art by which the Father creates, “that Eternal Art which is the form that not only produces all things, but also conserves and differentiates them, for this is the Being that contains the form in all creatures, and is the rule that directs all things,”26 is the undivided light of pure act, in whom all distinct forms subsist in absolute simplicity.

In his discussion of the two different meanings of truth (any being’s existence on the one hand, and the light in intellectual knowledge on the other), St. Bonaventure shows the way in which the truth of a thing is found more fully in the exemplary Idea than in its existence in the world:

In the first instance [‘truth is whatever exists’], truth is the remote principle of knowledge. In the second instance [‘truth is a rightness perceptible only to the mind’], it is the proximate and immediate principle of knowledge. Therefore, when it is said that truth is found more fully in the real existence of a being than in its likeness, this is true if truth is taken in the first sense, but not if it is taken in the second sense. But that truth which is the proximate and immediate principle of knowledge is found more fully in that likeness which resides in the intellect. It is found particularly and in the supreme degree in that likeness which is the exemplar of creation. Such a likeness expresses the creature more perfectly than the created being itself can.27

Only through some contact with the original Idea can we embrace the full truth of the Idea’s concrete expression, the creature. “God is truly the eternal mirror which makes possible the knowledge of every intelligible being.”28 In the domain of principles governing man’s activity, the relationship of natural law to Eternal Law exactly parallels the relationship of creature to exemplar—a point absolutely central to the traditional view of the natural moral law as something that can be discerned by reason.29 This is the next link that has to be established.

The Unchanging Law of Human Nature
God’s knowledge is the transcendent cause both of the creature’s individual being and of its specific nature. The divine knowledge of possible things and the divine willing of their actual existence accounts for the two most elementary data of the creature: that it exists, and what it is. Just as the Idea of the creature (both in its individuality and in its species or kind) is identical to the divine nature because there is no composition in God of thinker and thought, so too the natural law, wherein man discerns the principles according to which his life should be regulated, is a participation in the Eternal Law by which God governs the universe. Through the power of reason, we are able to reach up from this changing world to the unchanging pattern of creation and the law governing it. The pattern and its law are rooted in God’s foreknowledge and foreordination, which are identical to his wisdom and love.

As a consequence, St. Thomas argues to the necessity of the precepts of the moral law from man’s essence eternally known in God’s mind, not from God’s free will. Undoubtedly, man exists only because of God’s free decision to create him, for human nature is one of the infinite ways in which the divine perfection can be imitated. Granting man to be such as God knows him, however, a moral law of behavior naturally follows. The essence of man in the divine mind is self-identical, i.e., as one way of participating in the divine being it has its definition and determination in God no less than a circle, a cherry tree, or a cherub. The truth regarding a definite nature, insofar as this truth is eternally known by God, cannot be altered or negated. Natural law is derived from, and judged in terms of, the necessity built into the creature’s essence, its exemplary likeness in the mind of God. Whatever necessities follow directly from the nature itself are either first principles of natural law or more particular applications that can be deduced from them. God is said to have no power over this law, not because He is constrained by an outside rule, but rather because He cannot not will the self-identity of the nature and the fixed character of its true good as these are eternally known to Him. Indeed, because whatever God thinks is identical with Himself, to hold that the law of human nature is subject to change is to hold that human nature as such may be utterly changed—which, in the last analysis, is to hold that the divine mind is changeable in its thoughts and variable in its willing, that its thinking is reformulable and its willing arbitrary. Such a claim would necessitate that God unthink His own thought in which the archetypes of all possible beings are precontained, and unwill His own willings by which some possible beings are brought into actual being; and this would be nothing other than God undermining his own being.

As the Apostle says (2 Tim. 2.13), God “continueth faithful, He cannot deny Himself.” But He would deny Himself if He were to do away with the very order of His own justice, since He is justice itself. Thus God cannot dispense a man so that it be lawful for him not to direct himself to God, or not to be subject to His justice, even in those matters in which men are directed to one another.30

Is a man, then, naturally obligated to look out for the good of his neighbor and for the common good of society? St. Thomas reasons that if one loves God, one will love one’s neighbor and will want him to love God. To love God, both naturally and supernaturally, demands that we obey the law inscribed in our heart, which is our participation in the Eternal Law, the preexistent “art of the Son,” according to which we are constituted as rational beings whose final end necessitates habits of right action towards self and others. To love God, one must properly love oneself and one’s neighbor. Indeed, it is only by doing so that one lives a truly human life. It should be evident, then, that when we speak of “natural law,” “natural” refers to the nature or essence of man as image of God, as participant in His wisdom and providence. Precisely as the acts of a creature, human or moral acts are naturally governed by certain principles which flow from the Creator’s knowledge of man. By heeding and applying these principles he makes progress in virtue and becomes more fully human; by failing to heed and apply them he sins and falls away from his own good. That is why St. Thomas can make the striking statement: “God is not offended by us except when we act against our own good.”31

In spite of the clouds of ignorance and the weight of sins, human reason always in some way, to some degree, perceives and holds onto the universal unchanging natural law. “Even when it is rejected in its very principles, it cannot be destroyed or removed from the heart of man.”32 Yet even if we have not set ourselves in open rebellion against God, who among us has not wrestled with uncertainties, who has not committed sin (seven times or seventy times seven times a day!), who does not stand in constant need of divine help? “In the present situation, sinful man needs grace and revelation so that moral and religious truths may be known ‘by everyone with facility, with firm certainty and with no admixture of error.’”33 “The Law of Moses contains many truths naturally accessible to reason. God has revealed them because men did not read them in their hearts.”34 “The Law of the Gospel fulfills and surpasses the Old Law and brings it to perfection: its promises, through the Beatitudes of the Kingdom of heaven; its commandments, by reforming the heart, the root of human acts.”35 It is the Holy Spirit who reforms our hearts, shaping them after the Heart of Jesus, in whom Eternal Law and natural law are hypostatically united. May the same Spirit lead us to the glory of heaven where the New Law is perfectly lived: “a law of love, a law of grace, a law of freedom.”36

Peter Kwasniewski is Assistant Professor of Philosophy at the International Theological Institute in Gaming, Austria.