In a paper delivered at Princeton in 2012, Jennifer Herdt examines internal shifts in Reformed understandings of natural law enabled a merger between Reformed natural law thinking and “the modern natural law enterprise of the secular science of human nature.”

The shift is several-fold. Calvin affirmed that human beings have natural knowledge of God’s commandments, and affirmed (in contrast to Aquinas) that this natural knowledge has specific content. According to Calvin, “all nations condemn murder, theft, adultery, and false witness, thus grasping the precepts of the second table of the Decalogue.” Yet “Calvin holds that sinners ignore the judgments of conscience in a variety of ways.” As a result, Calvin uses natural law in large measure as part of a “theodicy” - a defense of God’s justice in condemning sinners who do not have access to his written law. From nature they know enough to condemn them.

Thus for Calvin “while conscience theoretically offers quite specific infallible moral knowledge, practically speaking we often remain in error and self-deception. That this is culpable error and culpable self-deception does not change the fact that, practically speaking, conscience does not function to give us the very specific, infallible knowledge that it seems to hold out.” What remains “primary for Calvin is the way in which conscience brings us before the judgment seat of God.”

With later Scottish thinkers (she examines Gershom Carmichael) and Americans (John Witherspoon), the uses of natural law have changed rather drastically.

“By the 18th century,” Herdt writes, “those who invoke conscience seem increasingly to rely on it as a source of moral knowledge more than as grounds for universal moral responsibility.” In the process of detaching natural law from its place in theodicy, both Carmichael and Witherspoon reverse the relationship between revelation and natural law. For Calvin, revelation gives more certain knowledge than fallen human reason or the fallen conscience. We are to “look to revealed divine law in order better to grasp natural law.” Carmichael and Witherspoon use inquiry into natural morality as a way of confirming revelation. Witherspoon in particular aimed to construct “moral philosophy as a form of moral inquiry abstracted from revelation.”

Calvin had also distinguished between what conscience offers and what we can learn from human instincts. He believed in both, but they tend in different directions. Following the definition of natural law found in Ulpian and later civil law, Calvin “held that our natural instincts predispose us to civic order and fair dealing insofar as these are necessary for the natural well-being or advantage of creatures such as ourselves.” At the same time, “he also carefully distinguished the good of advantage from the good of justice or virtue.” Instincts do not give us any specific moral direction. Conscience gives us direct access to God’s commands, and only in this respect can we speak of natural law as something having the force of duty . But conscience is distorted after the fall, and instincts toward civility and sociability are also distorted (and for Calvin these instincts were never used to impose duties anyway).

Later thinkers blurred Calvin’s distinction: “Conscience and instinct were often brought together in a way that would have been unrecognizable to Calvin, however—the distinction between duty, grasped through conscience, and advantage, grasped through instinct, has faded, and the insistence that the Fall has left the will incapable of conforming to right reason is a mere shadow of its former self.” Later writers (like Witherspoon) acknowledge depravity, but it plays little role in their treatment of the uses of natural law. As Herdt says, “the role of conscience is to provide the key end-run around the fall.”

Herdt argues that though these later thinkers departed from Calvin, they were indebted to Calvin in several ways. His concept of conscience as a faculty providing “specific, action-guiding moral knowledge even to fallen humanity” (in contrast to Thomas Aquinas and other scholastics) made the later “optimistic anthropology believable.” Further, Calvin’s “sharp” distinction between earthly and heavenly things “prepared the ground for the emergence of a secular moral science that eventually saw no need to reflect on heavenly things.”

For Herdt, the cautionary tale of this history (I have only touched on her richly detailed discussion) is not simply that natural law is ill-suited to being “a source of substantive action-guiding moral norms” or “providing concrete ethical knowledge.” The peril is that when Reformed thinkers have looked to natural law for such norms and such knowledge, they have inadvertently “created pressures that tend to empty out the substance of key Reformed commitments that animated Calvin’s thought, leaving behind intuitionist appeals to conscience, empiricist appeals to human nature, or unstable conflations of the two.” Not only did the weakening of theological commitments encourage shifts in the understanding of natural law. The deeper peril was that certain uses of natural law corrode Reformed theological commitments. Herdt goes so far as to suggest that “the secularization of modern natural law discourse is directly linked to pressures exerted by the hope and expectation that the natural law could provide such norms.”

Herdt thinks the cautionary tale is full of contemporary relevance: “it is tempting to look to natural law discourse not simply to justify the wholehearted participation of Christians in the public square, but to adjudicate controverted moral questions of our day. The moral of the story I am telling is that Reformed natural law reflection sets its eyes on this prize only at its own peril.”