The publication in 1975 of Edward O. Wilson's Sociobiology provoked a great controversy. Our deepest intuitions of right and wrong, Wilson asserted on the first page of his book, are guided by the emotional control centers of the brain, which evolved by natural selection to help the human animal exploit opportunities and avoid threats in the natural environment. Ethics is a branch of biology. The publication last year of Wilson's Consilience renewed the controversy as he continued to argue for explaining ethics through the biology of the moral sentiments.

Wilson's critics have warned that his reductionistic explanation of human ethics as a mere expression of animal impulses promotes a degrading view of human life. Some of his Christian critics have explained this as an inevitable consequence of a scientific naturalism that denies God's moral law as the supernatural ground for our sense of right and wrong. And yet I think Wilson's position is much stronger than it might seem at first glance. Indeed, the full strength of Wilson's Darwinian ethics becomes clear only when it is seen as a form of ethical naturalism that belongs to a tradition of natural law reasoning that is compatible with Christian theology.

The biological character of the natural moral law is suggested by a famous remark by Domitius Ulpianus, an ancient Roman jurist: "Natural right is that which nature has taught all animals." As quoted at the beginning of Justinian's Institutes, this remark entered the tradition of natural law reasoning, and it was cited by Thomas Aquinas when he explained natural law as rooted in "natural inclinations" or "natural instincts" that human beings might share with other social animals. Adam Smith continued this tradition of thought in explaining ethics as expressing the moral sentiments of human nature. As influenced by Smith's ethical theory, Charles Darwin explained the moral emotions as manifesting a moral sense rooted in the biological nature of human beings as social animals. As influenced by both Smith and Darwin, Edward Westermarck defended a view of ethics as founded in moral emotions shaped by natural selection in evolutionary history. Wilson recognizes that his biological theory of the moral sentiments belongs to a tradition of ethical naturalism that includes Smith, Darwin, and Westermarck. But he does not recognize the roots of that tradition in the natural law reasoning of Aquinas and other Christian philosophers.

Uncovering the common ground between Wilson and Aquinas should make it easier for Christians to come to terms with Wilson's expansive claims for scientific knowledge. Rather than completely rejecting Wilson's Darwinian view of ethics, Christians can adopt some of Wilson's reasoning to strengthen the case for natural law by finding support for it in Darwinian science.

THE AMBIGUITY OF CONSILIENCE

Wilson's explanation of ethics as rooted in human biological nature is a crucial part of his larger project in striving for "consilience" in human knowledge. He adopts the term consilience for the idea that nature is governed by a seamless web of causal laws that cross the traditional disciplines of study. He appeals to "a belief in the unity of the sciences—a conviction, far deeper than a mere working proposition, that the world is orderly and can be explained by a small number of natural laws." Against the fragmentation of knowledge into apparently unrelated do mains, Wilson argues for linking the natural sciences, the social sciences, and the humanities in the common effort to explain everything through the universal laws of nature.

There is an ambiguity in Wilson's position, however, because he adopts at least two opposing views of consilience. Sometimes he identifies consilience as a strong form of reductionism, which seems implausible to me. At other times, he identifies consilience as emergent complexity, which seems more plausible. For consilience as strong reductionism, all phenomena would have to be explained by reduction to the laws of physics. But even as Wilson affirms this position, he concedes that it could be wrong, and it surely is an oversimplification. Complete reduction to the laws of physics cannot work, because at each higher level of organization in nature, Wilson indicates, phenomena emerge that could not be explained or predicted by laws that govern the lower levels of organization.

This ambiguity runs throughout Wilson's book. On the one hand, he insists on reduction of everything to the laws of physics as the ultimate aim. On the other hand, he insists that emergent phenomena at higher levels of organization cannot be predicted by the laws of physics. "Biology is almost unimaginably more complex than physics," Wilson writes, "and the arts equivalently more complex than biology." He observes that "it is not even possible to predict the three-dimensional structure of a protein from a complete knowledge of its constituent atoms," and he urges his fellow biologists to cure themselves of "physics envy," for "they inevitably encounter emergence, the appearance of complex phenomena not predictable from the basic elements and processes alone." Hence, although constrained by genes, culture has acquired a "life of its own," so that we cannot understand human life without understanding both genes and culture. We must reject the genetic determinism that would assume that genes dictate the specific forms of culture."

This seems clear, unambiguous, yet elsewhere in the book we find Wilson explaining "the central idea of the consilience world view[:] … that all tangible phenomena, from the birth of stars to the workings of social institutions, are based on material processes that are ultimately reducible, however long and tortuous the sequences, to the laws of physics." Here Wilson's language is vague: "based on" cannot mean "specifically determined by," because to say the latter would fall into the oversimplified determinism that he explicitly rejects. Wilson identifies human beings as "emergent animals" who have capacities that are constrained by, but not specifically determined by, the laws of physics.

This is clearly the case for the ethical experience of human beings. Wilson affirms "the fundamental principle that ethics is everything." This must be so because "human social existence, unlike animal sociality, is based on the genetic propensity to form long-term contracts that evolve by culture into moral precepts and law." Yet, again, his words "based on" cannot mean "specifically determined by." For as he indicates in his book, to explain "the biology of the moral sentiments" would require research at many levels, including the social histories of ethical systems and the individual histories of people living in a variety of cultures. Moreover, the "base" here is biology rather than physics. The "genetic propensity to form long-term contracts" is surely not a predetermined effect of the causal laws of physics.

How can Wilson simultaneously affirm reductionism and emergence? One way out of this ambiguity would be to take the side of strong reductionism, so that "emergence" would mean what we cannot now explain by reduction to the laws of physics, because of our limited computational capacity for figuring out the interaction of the many causal factors that determine complex phenomena, but for which there must in principle be a reductionistic explanation that we can find once we have sufficient computational capacity.1 I do not find this position persuasive, be cause he never explains how this could happen, and thus it remains little more than an unsupported assertion.

A more plausible way out of the ambiguity in Wilson's book would be to say that consilience means that the laws at each emergent level of organization must be consistent with the laws at the lower levels, although the laws at the lower levels are not sufficient to explain the higher levels. This seems to be the idea implicit in what Wilson says about the "epigenetic rules" of ethics and culture: the laws of physics are necessary but not sufficient to explain these biologically based rules for human development; and these rules, as biological propensities for learning some things more easily than others, are necessary but not sufficient to explain the complex patterns in ethics and culture. His favorite examples of consilience—such as explaining color vocabularies and incest avoidance—would seem to illustrate this idea of consilience by emergence. The laws of physics do not determine the color vocabulary or the incest rules of any human group. The epigenetic rules of human biology shape the broad patterns in both color vocabularies and incest rules that are universal propensities across all human societies. But within these broad patterns, the specific content of color vocabularies and incest rules will be determined by linguistic practices and social customs that are peculiar to some particular human group.

We should expect ethics to display both the universality of human nature and the particularity of human history. In his account of ethics, Wilson relies on the idea that ethics is ultimately an expression of natural moral sentiments. That idea belongs to a tradition of ethical naturalism that stretches from Aristotle and Aquinas to David Hume and Adam Smith. An important insight of that tradition is that although ethics is rooted in the universal propensities of human nature, the determination of what is right for particular people in particular circumstances depends on "prudence," the practical wisdom of those with a shrewd grasp of concrete cases and historical contingencies.

THE BIOLOGY OF THE MORAL EMOTIONS

Against the "transcendentalist" claim that ethics is rooted in absolute standards that exist outside of humanity, Wilson argues for the "empiricist" claim that ethics is rooted in natural human inclinations:

Ethics, in the empiricist view, is conduct favored consistently enough throughout a society to be expressed as a code of principles. It is driven by hereditary predispositions in mental development—the "moral sentiments" of the Enlightenment philosophers—causing broad convergence across cultures, while reaching precise form in each culture according to historical circumstances.

This suggests that consilience as strong reductionism is not possible in the scientific study of ethics, because the precise expression of ethical rules depends on contingent circumstances of cultural history that cannot be completely reduced to the laws of physics. And yet, despite the variability of ethics in response to different cultural environments, the biology of the moral emotions manifests a universal human nature.

For social animals like human beings, Wilson argues, social cooperation would have been advantageous for survival and reproduction in their evolutionary history. Natural selection would have favored those genetically heritable dispositions that promoted cooperative behavior, which would include innate propensities to social emotions such as sympathy, love, guilt, shame, and indignation. The highly developed intellectual faculties of human beings would then have al lowed them to formulate customary rules of conduct that expressed those social emotions of approval and disapproval. For example, the natural dependence of children on adults would favor the emotional attachments of parent-child bonding, which would be expressed as social rules approving of parental care and disapproving of parental neglect. Similarly, the benefits of cooperating for mutual advantage in evolutionary history would favor emotional dispositions that enforce reciprocity—emotional approval for fairness and emotional disapproval for cheating. The innate dispositions to feel such emotions would then be ex pressed as social rules that rewarded cooperators and punished cheaters.

Citing the research of neurologists like Antonio Damasio, Wilson infers that the innate propensities to moral emotions as shaped by natural selection are etched into the neural circuitry of the human brain. To live successfully as social animals, human beings must make practical decisions as guided by the emotional control centers of their brains. Our brain inclines us to feel sympathy and concern for the pleasures and pains of others, to feel love and gratitude toward those who help us, to feel anger and indignation toward those who harm us, to feel guilt and shame when we have betrayed our family and friends, and to feel pride and honor when others recognize our good deeds. Insofar as these moral emotions are felt generally across a society, they support social rules of love, loyalty, honesty, and justice. It might be, however, that a few human beings suffer from some abnormal circuitry in their brains such that they cannot feel, or cannot feel very strongly, the moral emotions that sustain morality. This seems to be the case for psychopaths, who feel no obligation to obey moral rules because they do not feel the moral emotions that support such rules. If so, then we must treat such people as moral strangers.

Thus, ethics is rooted in the innate inclinations of the human brain as cultivated by human social experience. This is an "empiricist" view of ethics, because moral rules are seen as products of the human mind. By contrast, a "transcendentalist" view would see moral rules as existing somehow independently of the human mind. A preeminent manifestation of transcendentalist ethics, Wilson believes, is the tradition of natural law as developed by Thomas Aquinas and other Christian theologians. Natural law is transcendental insofar as it is understood as an expression of God's will and thus independent of the human mind.

This would seem to confirm the suspicion of Wilson's Christian critics that his biological understanding of ethics cannot be compatible with Christian religious experience. But I think this is a mistake that comes from Wilson's failure to see how close his position is to Aquinas's natural law.

AQUINAS AND NATURAL LAW

Aquinas's natural law is actually an "empiricist" position, at least insofar as Aquinas speaks of natural law as rooted in natural inclinations that are instinctive to human beings. Although Aquinas believes that God is the ultimate cause of everything in nature, the natural inclinations that constitute the natural law can be known by natural human experience even without religious faith.

Aquinas quotes with approval Ulpian's remark that "natural right is that which nature has taught all animals." Indeed, much of Aquinas's reasoning about natural law can be understood as biological reasoning about natural human inclinations or instincts. "All those things to which a man has a natural inclination," Aquinas explains, "are naturally apprehended by reason as being good." What he here calls "natural inclination" (inclinatio naturae) he sometimes calls "natural instinct" (instinctu naturae).

Sexual bonding, parental bonding to children, and social bonding generally belong to natural law because they express the natural instincts of human beings as sexual, parental, and social animals. Of course, natural law for human beings requires reason, but pure reason could not move us to act if it were not linked somehow to inclination or instinct (that is, some affective or emotional impulse) as the motivation to action. Natural law is rooted in the natural inclinations of human biological nature.

For example, Aquinas explains marriage as belonging to natural law because marriage satisfies certain natural human instincts that are similar to the sexual and parental instincts of other animals. Nature inclines us to marriage to secure two natural ends. The primary end is parental care of children. Among animals whose offspring cannot survive without the care of both parents, there is some natural inclination for an enduring bond between the parents. Among human beings, children need intensive pa rental care for a long time, which is promoted by the marital bonding of father and mother. The secondary end of marriage is the spousal friendship of husband and wife in the household. In satisfying these natural ends, Aquinas concludes, the human laws of marriage conform to "a natural instinct within the human species."

Aquinas believes that the monogamous bonding of husband and wife secures these natural ends most fully. By contrast, the polygynous bonding of one husband and multiple wives is only partly natural, because while polygynous marriages can provide sufficient parental care, the sexual jealousy of the wives tends to disrupt the household. The polyandrous bonding of one wife and multiple husbands is completely unnatural, because the sexual jealousy of the husbands and the uncertainty of paternity will impede both parental care and spousal friendship. In justifying these conclusions, Aquinas applies Aristotle's biological writing in comparing human mating and parenting to that of other animals. In doing so, his reasoning is remarkably similar to what Wilson and other Darwinians would say about the biological nature of marriage and family life.

Christians who are suspicious of such biological reasoning object that human beings are endowed by God with reason and free will, which sets human beings apart from the animal world. Only human beings can be truly moral, because human conduct can be rationally guided and freely chosen, while the behavior of other animals is purely instinctive and determined. Even such seemingly powerful instincts as sexual mating and parental care can be denied by human beings who freely choose a life of celibacy for moral and religious reasons. Consequently, it might seem that human morality cannot be rooted in natural inclinations that human beings share with other animals.

To this objection Aquinas responds by distinguishing three levels of human biological nature: the generic, the specific, and the temperamental. (This trichotomy of the natural dispositions comes from Aristotle's biological ac count of inherited psychic traits.) In their generic nature as animals, human beings are moved by natural instincts for mating and parenting similar to those of other animals. In their specific nature as rational animals, human beings are moved by practical reasoning to devise customary and legal rules for marriage and the family. In their temperamental nature as individuals, human beings vary in their personal traits, so that a few human beings are temperamentally suited for a life of celibacy that would be unbearable for most human beings. So although morality in the strict sense is uniquely human, because it depends on a uniquely human capacity for rational deliberation, human morality is still rooted in natural instincts similar to those of other animals.

But still some Christians would insist that the uniquely human capacity for "free will" gives human beings the freedom to act outside or against nature, while Darwinian biologists like Wilson reject the idea of "free will" as an "illusion." It seems to me, however, that a biological explanation of human nature does not deny human freedom if we define that freedom as the capacity for deliberation and choice based on one's own desires. Animals with sufficiently complex nervous systems are capable of voluntary action, in the sense that they can learn to adapt their behavior to changing environmental circumstances. Because human beings have more complex nervous systems than other animals, human beings can use images and words to compare alternative courses of action through mental trial and error and then choose between them. Consequently, we are morally responsible for our actions because of our uniquely human capacity for reflecting on our motives and circumstances and acting in the light of those reflections.

Some Christians argue that moral agents must be able to transcend nature through a "free will" that violates the natural order of the world. But this assumes a Gnostic opposition between natural law and moral freedom that is not biblical. If God is the all-powerful and all-good Creator of nature, then there is no reason to believe that the natural constitution of human beings has been designed in opposition to moral goodness. To justify his moral laws, Moses suggested that all human beings in principle could recognize the prudential goodness of those moral laws that were adapted to the natural human condition (Deut. 4:6). He taught that "whoever obeys the law will find life in it" (Lev. 18:5), because the purpose of his moral law was to secure the earthly survival and prosperity of oneself and one's progeny (Deut. 30:15–20). Aquinas concluded that the moral precepts of the Mosaic law belonged to natural law, and this natural morality as rooted in human nature could be known by natural reason without divine revelation.

NATURAL RELIGION

Christians might insist, however, that the natural desires of human beings point beyond nature to the supernatural Creator of nature. The human spirit yearns for something beyond mortal life on earth. It yearns for a redemptive transformation that would restore its original union with God. And surely this transcends the natural world as explained by Wilson's natural science.

But doesn't this suggest that the longing to transcend nature is itself a natural desire, and therefore as open to scientific observation as any other natural phenomenon? In fact, Wilson speaks about the religious impulse as part of human nature, even as he acknowledges that impulse in his own life. Wilson begins Consilience by describing his early experience as a young man in Alabama who read the Bible studiously and who was "born again" as a member of a Southern Baptist church. "I knew the healing power of redemption." But then he discovered Charles Darwin in college, and his belief in evolution took the place of his belief in biblical theology.

Yet Wilson recounts his experience not as a denial of religious feelings but as an expression of those feelings. As human beings, Wilson observes, we yearn to know the ultimate meaning of things, we need to see our lives as conforming to some cosmic purpose, and this requires that we understand where we came from, why we are here, and where we are going. This is a "religious hunger" that is satisfied by belief in divine revelation: "The idea of the mystical union is an authentic part of the human spirit." But we can also satisfy this hunger by belief in the scientific unification of all knowledge. "In that sense," Wilson asserts, "science is religion liberated and writ large." The final aim of scientific consilience must be the formulation of an "evolutionary epic" that satisfies our longing to grasp the meaning of our existence.

Wilson thus acknowledges what Aquinas identifies as the "natural instinct" for religious belief. Deep in human nature is the natural desire to know the cause of any effect, which leads us to want to know the first cause of all things, which in turn is expressed in the philosophic or scientific life as devoted to understanding for its own sake in the quest for complete knowledge of how and why the world is the way it is. Thus the natural inclination to know the ultimate causes of everything leads human beings to desire to know God as the First Cause. Consequently, religion is natural for human beings, Aquinas claims, "because man feels that he is obligated by some sort of natural instinct to pay, in his own way, reverence to God."

Similarly, Wilson recognizes that it is natural for human beings to seek the ultimate causes of everything, which leads them to ask, Why is there something rather than nothing? And he acknowledges that natural science cannot refute the religious claim that this question points to God as the Creator. Science can appeal to the laws of nature as the ultimate ground of explanation, but it cannot explain where these laws come from if not from some supernatural power beyond the laws themselves.

Moreover, Wilson believes, not only does religion satisfy the natural intellectual need for an ultimate explanation of everything, it also satisfies the natural emotional need for some purpose to human existence. For science to satisfy those needs, scientific materialism would have to become a mythology more powerful than the mythology of religion, but it is not clear that that could ever happen. So Wilson must conclude that "the predisposition to religious belief is the most complex and powerful force in the human mind and in all probability an ineradicable part of human nature."

It is easy to understand why many Christians have rejected Wilson's Darwinian science of human nature, because he often seems to be arguing for scientific materialism as an alternative to traditional religion. But just as Thomas Aquinas saw a fundamental compatibility between biblical theology and Aristotelian naturalism, so should Christians today see that Wilson's biological naturalism supports many of their moral and intellectual impulses. Believing that human nature and nature as a whole manifest the rationality and goodness of the Creator, Christians can agree with Wilson that human beings are by nature both moral animals, who judge right and wrong in the light of their natural moral sentiments, and rational animals, who desire to understand the natural causes of all things.

Furthermore, Wilson shares with Christianity—particularly, evangelical Protestantism—a longing for the emotional ecstasy of grace. In his intellectual autobiography, Wilson describes his deeply felt experience at age 14 in being "born again" and being baptized by a Southern Baptist preacher. Reflecting on his subsequent turn to Darwin and disbelief, he writes,

The still faithful might say I never truly knew grace, never had it; but they would be wrong. The truth is that I found it and abandoned it. … I was enchanted with science as a means of explaining the physical world, which increasingly seemed to me to be the complete world. In essence, I still longed for grace, but rooted solidly on Earth.

Although Wilson is not a religious believer in any doctrinal sense, he looks at nature with a sense of joyful awe that Christians can recognize as a natural piety that points beyond nature to nature's God.

Larry Arnhart is professor of political science at Northern Illinois University. He is the author most recently of Darwinian Natural Right (SUNY Press).

1. Wilson has suggested this in a published interview. See Frank Miele, "The Ionian Instauration," Skeptic, Vol. 6, No. 1 (1998) pp. 79–80.

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