The Law Written on their Hearts

Article ID: DL330 | By: Miguel Endara

This article first appeared in the Christian Research Journal, volume 25, number 2 (2002). For further information or to subscribe to the Christian Research Journal go to:


The natural law theory is an ethical theory that claims human beings naturally recognize moral law at least in a general way; that is, humans strive for what they think will fulfill them. Given their nature, humans are aware of their biological, social, intellectual, and spiritual needs and the natural inclinations that flow from them. Human beings further recognize that the proper fulfillment of their natural inclinations, and, ultimately, their human nature, leads to attainment of human good, which in turn leads to well-being.

According to the basic principle of natural law, therefore, human beings recognize that they ought to seek after and engage in actions that constitute human good and avoid engaging in contrary actions. Since all humans recognize this natural law, moral reasoning based on natural law principles constitutes common moral ground upon which Christians can construct critiques of culture.

One such critique involves the hotly debated issue of stem cell research. By virtue of their nature as moral beings, humans possess considerable intrinsic worth and dignity. The procedures associated with embryonic stem cell research violate this intrinsic dignity. Embryonic stem cell research, therefore, ought not be practiced. By arguing from the common ground of natural law, Christians need not appeal to the theological doctrine that man was created in God’s image, which the secular world does not accept.

For when Gentiles who do not have the Law do instinctively the things of the Law, these, not having the Law, are a law to themselves, in that they show the work of the Law written in their hearts, their conscience bearing witness, and their thoughts alternately accusing or else defending themselves, on the day when, according to my gospel, God will judge the secrets of men through Jesus Christ. (Rom. 2:14-16 NASB)

Even though the Gentile world did not know God’s Law given to Israel through Moses, they nevertheless knew right from wrong and good from evil because God wrote His moral law upon their hearts. As a consequence of their knowledge of this moral law and their failure to abide by it, they stand condemned before God, so that even those who do not possess the written Law are without excuse. What is this natural law that the Gentiles knew? How were they made aware of it?

Natural law is an old and venerable theory of ethics originally formulated by the Stoics in ancient Greece and modified by such Romans as Cicero. Prominent Christian philosophers/theologians, such as Augustine and Thomas Aquinas, also adhered to versions of natural law theory. While natural law fell out of favor during the Enlightenment, it never completely faded away. Within the past 20 years, many ethical and political philosophers have expressed renewed interest in natural law theory.1 Though the winds of natural law revival have not blown entirely in the direction of Christian evangelicalism, the evangelical community ought to consider natural law a viable moral theory, especially with an eye toward its potential for Christian apologetics. Using natural law, Christians can establish common moral ground upon which to engage the secular world in the public square over many moral issues, including sexual ethics, cloning, and stem cell research.

This article will show that certain moral principles are recognized by all humans by virtue of their nature as moral beings, thereby establishing a natural and common foundation for moral reasoning. This article concludes with an example of natural law reasoning in the field of stem cell research. By basing moral reasoning on natural law, evangelicals can argue for human dignity and worth and its importance in the stem cell research debate without directly appealing to the doctrine that man was created in God’s image (Imago Dei).2


All human beings recognize certain truths.3 The ancient Greek philosopher Aristotle discussed a certain truth he thought was self-evident: “It is impossible for anything at the same time to be and not to be.”4 According to Aristotle, this principle is undeniable and forms the basis for every type of scientific demonstration.5 In contemporary philosophy this principle, termed the principle of contradiction, tells us that “a statement and its negation cannot both be true.”6 In more practical terms, adherence to the principle of contradiction is necessary in order for communication and knowledge to be possible. Human beings are aware of this principle in that they, at least implicitly, abide by it. Even children abide by this principle. For example, when my three-year old niece, Sofia, demands her bottle, she implicitly recognizes that she cannot have and not have the bottle at the same time in the same respect.7 Human beings thus universally recognize certain logical truths such as the principle of contradiction.

Moral Discourse Presupposes Common Moral Ground

Are the truths that humans recognize, however, restricted to the realms of communication and logic, or are there common truths in the moral realm as well? In other words, do human beings also recognize certain moral truths? Rational discourse concerning moral disagreement proves at least implicit recognition of common moral truths.8 Moral disputes sometimes proceed by each party appealing to commonly accepted moral truths in their attempt to demonstrate that the other party is not adhering to, or correctly interpreting, these moral truths. The point is that whenever moral disagreement occurs, further discourse would be pointless or useless if the possibility of agreement did not exist. Otherwise, people would be talking past one another since they would be appealing to different and incompatible moral principles. C. S. Lewis has a helpful analogy in this regard: “Quarrelling means trying to show that the other man’s in the wrong. And there’d be no sense in saying that a footballer had committed a foul unless there was some agreement about the rules of football.”9 In a similar way, there has to be some agreement about moral “rules” or truths for two parties to dispute a moral issue. Natural laws, therefore, serve as common ground for moral disputes.

The fact that human beings employ commonly accepted moral terms in their moral discourse also suggests a common ground of natural laws, for commonly accepted moral terms presuppose at least general agreement on moral principles. Political philosopher Hadley Arkes states, “When we invoke the language of morals, we praise and we blame, we commend and condemn, we applaud and deride, we approve and disapprove. It would make no sense to cast these judgments on other people unless it were assumed that there are standards of judgments, accessible to others as well as ourselves, which allow people to know that what they are doing is right or wrong.”10

Common Principles of Natural Law

Just what are these common moral truths that account for the existence of common moral terms and the possibility of moral discourse in the face of disagreement? It is natural for humans to direct their activities toward goals they think will fulfill them as human beings and as individuals within the human race. Both Aristotle and Aquinas argued that human beings naturally strive toward happiness, for the goal of all human striving is happiness.11 Striving toward happiness begins with the recognition that certain basic types of activities, or a basic pattern of activity, conform to the type of beings humans are. Conforming one’s basic pattern of activity to the type of being one is usually results in one’s well-being or thriving. This is no mystery. Whether humans, animals, or plants, it is evident that their well-being depends upon what they do or what is done to them according to the type of being they are. The converse is also true, namely, lack of well-being is a consequence of activities that do not conform to the type of being a human, animal, or plant is. To illustrate this point, philosopher Janet E. Smith asks her readers to consider the well-being of tomato plants: “Tomato plants have a certain nature. In order to have good tomato plants one must act toward these plants in accord with their nature; one must water them and give them sunlight and good soil if one wants to produce good tomato plants….If one’s tomato plants fail to produce tomatoes, one knows that one is doing something wrong; if one’s tomato plants produce good tomatoes, one knows one is doing something right.”12

Basic patterns of activity that conform to human nature likewise result in human well-being, while patterns that do not conform to human nature undermine human well-being. The goal of activities that conform to human nature is the attainment of human good. In other words, human inclinations toward human good prompt human beings to engage in activities that conform to human nature and thus lead to well-being. To accomplish this, however, the intellect must properly order, classify, and arrange the natural inclinations and figure out when and how, in light of particular circumstances, it is appropriate to act on them. Blindly following the natural inclinations will not lead to well-being.

Inclinations toward human good include self-preservation and avoidance of injury, satisfaction of biological needs such as eating and sleeping, living with other human beings in a community, procreating, seeking knowledge, and knowing the truth about God.13 The converse is also true: acting contrary to natural inclinations results in the thwarting or undermining of one’s well-being. This does not mean, however, that each person who has ever lived will and should always act in concert with the fulfillment of natural inclinations and avoid the contrary. For example, I do not mean that human beings will and should always live in a community, procreate, or even preserve their own life. Due to certain factors such as early training, social context, individual temperament, age, and stage in life, human beings are inclined toward these natural human objectives in varying degrees. In either case, the natural inclinations must always be subject to rational discernment. The intellect, reflecting on human nature and particular circumstances, determines how one ought to act.14 Generally speaking, human beings ought to engage in basic patterns of activities in concert with natural inclinations. Departure from basic patterns of activity that lead toward human good results in the compromise of one’s own well-being as well as the well-being of one’s family and community.

Insofar as human beings recognize that they ought to pursue what is good for them and avoid the contrary (i.e., evil) human beings recognize moral truth. Moral terms as well as discussions regarding moral disagreements presuppose the recognition of common moral truth.

People do not need divine revelation to realize that pursuing human benefits such as those connected with one’s biological, intellectual, social, and spiritual needs is good and proper, while the contrary is evil. Human beings naturally recognize that it is good to engage in acts that are fulfilling and that it is wrong or evil to do the contrary. The most general propositions that spell out the activities toward which human beings ought to strive and avoid are called natural law principles. Understood in this context, the most basic common moral truth — the most general natural law principle stated in propositional form — is seek after and engage in activities that constitute the human good and do not engage in contrary activities, which is evil.15

Applying Common Moral Principles

Since natural law consists of the most general principles of ethics, it constitutes the theoretical beginnings of natural moral reasoning. Natural law, however, does not consist of practical and specific moral rules to be employed during specific times and circumstances. Practical rational reflection and prudence must bridge the gap between general moral principles and their application in specific circumstances. In short, human beings possess the intellectual capacity to apply natural law to specific circumstances. Natural law, therefore, does not provide a shortcut for moral reasoning.

Natural Inclinations and the Fall

From the theological point of view, God created the human intellect and natural human inclinations. God does not expect human beings to follow a morality contrary to, or even different from, the morality that is in concert with the kind of beings that humans are, for God will not contradict His own creation. He would not have created natural human inclinations that contradict natural human needs. God instead created humans with the natural inclination to strive for their own well-being, and He endowed human beings with the intellectual capacity to discern and order these natural inclinations.

What about the effects of the Fall? Has not the Fall marred natural human inclinations as well as the human will and intellect to such an extent that humans are deceived into thinking certain actions are good when in reality they are evil? For example, along with self-preservation and procreation, why not also include selfishness, jealousy, pride, and covetousness, just to name a few, as natural human inclinations? Yes, the Fall has affected the inclinations, the will, and the intellect. Even Aristotle, someone who had great wisdom and insight into moral philosophy, sanctioned slavery, claiming that some people are slaves by nature.16 Human beings, however, are not so marred that they do not recognize the moral law; otherwise, the apostle Paul could not have claimed in Romans 2:14–16 that the Gentiles were guilty before the moral law of God.

One possible way to understand “natural inclinations” such as selfishness, pride, jealousy, or covetousness, is to realize that they were not a part of the original creation and, as such, they have been “artificially” integrated into human nature. From the natural point of view, one may realize that such “natural inclinations” are not proper to human beings, since they undermine the human good. In other words, they do not lead to the well-being of individual human beings or to thriving human communities; instead, they lead to individual bitterness and unhappiness as well as the deterioration of human interaction and community.

Moral Reasoning

In short, natural law is a moral theory that claims that human beings naturally recognize the moral law at least to a general extent. According to natural law theory, human beings recognize that they ought to seek after and engage in actions that are good and avoid engaging in contrary actions, which are evil. Since natural law is common to all humanity, moral reasoning based on natural law principles constitutes common moral ground upon which Christians can construct their critiques of culture. One such critique involves the hotly debated issue of stem cell research.


Stem cell research represents a new frontier in science, for it potentially offers help to people suffering from an array of infirmities, including Alzheimer’s disease, Parkinson’s disease, and spinal column injuries. Even though no one questions the goodness of these potential benefits, heated controversy brews over stem cell research. This controversy specifically concerns some of the sources employed in the extraction of stem cells. There are two types of stem cells, adult and embryonic. Adult stem cells come from infants, children, adults, and even cadavers while embryonic stem cells come from human embryos and, most commonly, from aborted fetal tissue.

The extraction of adult stem cells from human organisms is not controversial. The controversy instead concerns both sources of embryonic stem cells. First, the extraction of stem cells from an embryo results in destruction of the embryo. The concern here is over the moral status of the living human embryo. Second, there is also concern over the morality of the practice of abortion, which is necessary for the attainment of aborted fetal tissue.17

Moral Argument Using God’s Image

God created human beings in His own image (Gen. 1:26–27). As bearers of God’s Image (Imago Dei), humans possess intrinsic worth and dignity. This belief is the gist of the Imago Dei theological doctrine.18 By employing the Imago Dei doctrine to establish the intrinsic worth and dignity of human beings, one may formulate arguments concerning the morally questionable sources of embryonic stem cell research.

One argument addresses stem cells extracted from human embryos. First, because all human beings bear the image of God, whether young or old, all humans should be treated with the proper dignity and respect owed to them. Second, by means of scientific data and philosophical argumentation, it is logically demonstrable that human embryos are human beings.19 Third, destroying human beings for instrumental purposes is a clear violation of their intrinsic dignity and worth. It is, therefore, morally wrong to destroy human embryos for the purpose of stem cell research.

Another argument addresses stem cells extracted from aborted fetal tissue. First, since human beings bear the image of God, they should be treated with the proper respect owed them. Second, human fetuses are human beings. Third, given the first and second premises, the practice of aborting human fetuses is morally wrong.20 Fourth, sanctioning the extraction of stem cells from aborted fetal tissue may lend abortion the facade of moral legitimacy, which might help promote abortion. Fifth, given the fourth premise, the type of embryonic stem cell research that extracts stem cells from aborted fetal tissue becomes morally questionable, since it potentially helps promote abortion. Sixth, since there is potential for morally violating the intrinsic worth and dignity of human fetuses, therefore, this second type of embryonic stem cell research should not be practiced.

The scriptural doctrine of the Imago Dei does not constitute common moral ground between the Christian and the secular world, for the secular world does not accept it as a valid way of establishing the value of human beings. It is, therefore, not always easy for Christians to argue successfully along the above lines. How, then, could a Christian argue for the intrinsic worth and dignity of human beings in order to further moral arguments against embryonic stem cell research without appealing to the theological doctrine of the Imago Dei? One way is to engage the secular world in natural law reasoning.

Moral Argument Using Natural Law

Natural law reasoning will not result in the discovery that the human being possesses the inestimable intrinsic dignity and worth claimed by the Imago Dei doctrine. Natural law, nonetheless, has the potential for successfully conveying the view that human beings possess intrinsic dignity and worth. Many lines of reasoning, moreover, are available to the natural law theorist concerning the intrinsic dignity and worth of human beings. The following is one such line of natural law reasoning.

One may ask, Why is it that only human beings discover that they ought to pursue good and avoid evil? Human beings discover this common moral truth due to the possession of intellect and will. Human beings also discover that the world possesses a moral dimension and that this dimension is open only to them. In other words, human beings naturally ascertain that they are the only earthly creatures with the potential to discover and follow, or forsake, the moral order of the world. Human acts — the intentional and free acts performed by human beings — therefore, possess a distinct moral character. Though it is true that human beings do not always act in a manner befitting them as rational creatures, when their acts are rationally ordered, they stand distinct from similar types of actions performed by other creatures.21 For example, human beings do not simply devour their food but instead dine.22 Human beings also do not merely live in social populations and form instinctively based relationships with other members of their species; they instead transform their environments in order to live in social cultures and form deep and abiding friendships with other human beings. In short, human beings possess the capacity to inhabit the moral dimension of the universe, which results in the transformation of human acts.

Insofar as human beings inhabit the moral dimension of the universe through their human acts, they sustain a superior level of life and hold a superior status not available to other earthly creatures. The unique status that human beings hold in the world warrants special moral consideration. Since this special moral consideration is ultimately due to the type of beings humans are — that is, rational and free — it would seem then that human beings possess an intrinsic dignity and worth that far surpasses other earthly creatures.

By establishing the intrinsic dignity and worth of human beings based on the common moral ground of natural law reasoning, evangelicals can argue against embryonic stem cell research along the lines suggested above without appealing to the theological Imago Dei doctrine.

It is obvious, as evidenced by the stem-cell natural law argument above, that natural law by itself can only take us so far. Christian proponents of natural law such as Augustine and Aquinas situated natural law within a larger ethical theory that encompassed divine revelation. My claim is that natural law is a useful tool by which Christians could meet nonbelievers on common moral ground; nonetheless, the redemption of society cannot take place bereft of the divine revelation of the Gospel.


1. See, e.g., Anthony Lisska, Aquinas’s Theory of Natural Law (Oxford: Oxford University Press, 1997); John N. Finnis, Natural Law and Natural Rights (Oxford: Oxford University Press, 1980); Robert George, A Critique of the New Natural Law Theory (South Bend, IN: University of Notre Dame Press, 1987); Natural Law Theory: Contemporary Essays (Oxford: Oxford University Press, 1992); Natural Law and Moral Inquiry (Washington, D.C.: Georgetown University Press, 1998); Edward B. McLean, ed., Common Truths: New Perspectives on Natural Law (Wilmington, DE: ISI Books, 2000); and Charles Covell, The Defence of Natural Law (New York: St. Martin’s Press, 1992).

2. Thomas Aquinas’s treatise on law in Summa theologiae 1a2ae.90–97 forms the basis of the argument for natural law that I espouse in this article.

3. I mean all human beings who possess at least minimal intellectual competence.

4. Aristotle Metaphysics (The Basic Works of Aristotle, ed. Richard McKeon [New York: Random House, 1941]) 4.4.1006a1–2.

5. “Scientific demonstration” is a technical term Aristotle employed to refer to certain deductive syllogisms necessary in attaining scientific knowledge. Aristotle understood science as referring to a rationally discernable and well-defined body of knowledge concerning a particular subject.

6. The Cambridge Dictionary of Philosophy, 2d ed., s.v. “principle of contradiction.”

7. Ralph McInerny makes a similar point in “Are There Moral Truths That Everyone Knows?” in Common Truths, 9.

8. I am not claiming that just because people agree on certain moral principles that they take on the status of “moral truths.” I am claiming, rather, that certain universally recognized moral truths form the basis, in many cases, for moral disputes.

9. C. S. Lewis, The Case for Christianity (New York: Macmillan, 1989), 4.

10. Hadley Arkes, First Things: An Inquiry into the First Principles of Morals and Justice (Garden City, NJ: Princeton University Press, 1986), 22.

11. See Aristotle Nicomachean Ethics 1.4.1095a15–20 and Thomas Aquinas Exposition of Aristotle’s Ethics 1.9.104–6.

12. Janet E. Smith, “Natural Law and Sexual Ethics,” in Common Truths, 201.

13. Thomas Aquinas aptly describes some of these natural human inclinations in Summa theologiae 1a2ae.94.2.

14. Just because human beings recognize what they ought to do, it does not mean that human beings will act in accord with what they know they ought to do.

15. Thomas Aquinas calls the most general moral principle the first principle of practical reason. See Summa theologiae 1a2ae.94.2.

16. Aristotle Politics 1.3–13. To be fair to Aristotle, the type of slavery he espoused was not the type of slavery practiced in the antebellum United States. He argued that human beings who possessed insufficient intellectual capacities to be self-governing should become slaves. Aristotle did not argue for a cruel type of slavery based on race; nonetheless, since any type of slavery assumes that certain human beings may be treated as property, and since this type of treatment constitutes a violation of human dignity, Aristotle was morally wrong in sanctioning the practice of slavery.

17. For more information on stem cell research and the moral quandaries associated with it, see http://www. and

18. Christians are not in agreement as to exactly what it means for human beings to bear the image of God. There is, however, uncontroversial and universal agreement that human beings possess inestimable intrinsic worth and dignity due to the Imago Dei.

19. See Patrick Lee and Robert P. George, “The Stubborn Facts of Science: Human Embryos are Human Beings,”, for an argument that demonstrates the humanity of embryos. See also Francis J. Beckwith, “Abortion, Bioethics, and Personhood: A Philosophical Reflection,”, for an argument that logically demonstrates the humanity and personhood of fetuses, including embryos.

20. I am only speaking generally here, for it is outside the scope of this article to deal with the special cases of abortion such as those that involve rape and cervical cancer.

21. Whereas apes and other higher primates seem to possess a modicum of intellect and will, there is a morally significant qualitative intellectual difference between higher primates and human beings. Human beings possess self-reflection, the intellectual capacity to reflectively examine one’s own beliefs and motives, which is a necessary component in moral acts.

22. That is, sharing a meal and genial discussion with family members and friends.