Natural law may be defined as the ethical or moral structure that God has revealed to humans in creation (including in themselves and in the providential unfolding of history) and discerned through reason and experience. Natural law is a biblical concept (Rom. 2:14-16; Rom. 1:18-32; 1 Cor. 5:1), and it seems to have two major functions. First, as with the Mosaic Law it condemns people for their sins, leaving them without excuse. Second, natural law provides an explanation for why people and nations do get moral issues right. No nation has even been wicked at every point. Natural law is an an explanation for why this is so.
Recently a natural law approach to reasoned argument in the pubic square has become popular. Among evangelicals, this is often paired with a two kingdoms theology that has its recent origins in Meredith Kline. When the natural law and two kingdoms views are combined the system that emerges typically maintains that Scripture rules over the spiritual kingdom but that natural law (and not Scripture) rules over the civil kingdom. The Christian may understand the natural law better from Scripture, but he is not to impose Scripture on the civil society.
But the natural law-two kingdoms approach suffers from some serious errors. First, the exegetical basis for the two kingdoms view is shaky. Proponents often argue that the civil kingdom is based on the Noahic Covenant and the spiritual kingdom is based on the Abrahamic Covenant. But the Noahic covenant is a redemptive covenant. It is instituted with an atoning sacrifice and it exists to ensure a stable world in which God may work out his redemptive plan (cf. Jer. 33:20-21). Second, just as theonomy makes the best sense in postmillennial context (though technically one does not need to be postmillennial to be a theonomist), so the two kingdoms approach fits best in a amillennial context (though one does not need to be an amillennialist to hold the position). This is evident in the idealistic interpretation of Revelation that undergirds some arguments for the two kingdoms view. It may also reveal itself in a seeming dismissal of the physical from God’s redemptive work and a focus on the spiritual.
Second, it is not clear that God ever intended natural revelation to function apart from special revelation. Because the world is a fallen, it is very difficult to argue from what is to what ought to be. What is is not a reliable guide. Scripture needs to serve as corrective lenses to help fallen people understand natural revelation rightly. This does not mean that natural revelation is unable to achieve the two purposes noted above, but it does indicate it will be difficult to build a civil society upon it.
Third, the natural law approach may lend aid and comfort to the secularists by conceding that religious reasoning should be kept out of the public square. This is a cost not worth paying because the non-Christians may not lend an ear to natural law arguments. Rather they will most often simply say that Christians are hypocritically attempting to hide the real religious reasoning for their position.
Fourth, attempts to reason from natural law apart from Scripture are often unconvincing. This is due in part to the fact that the unregenerate conscience is weak (1 Cor. 8:7, 12), defiled (1 Cor. 8:7; Tit. 1:15), seared (1 Tim. 4:2), and in need of purification (Heb. 9:14; 10:22). It is even further complicated by a religious pluralism in which there are real competing values at work in society. The natural law is not eradicated by the Fall, but it is distorted in such a way that competing systems are now in place.
None of this is to say that natural law argumentation has no place. It can and should be used to strike at the conscience. It can expose to some the danger that they are suppressing truth that they know.
Natural law, like the Mosaic law, is complicated topic. The following are helpful resources:
Calvin, John. Institutes of the Christian Religion
Calvin is well known for his teaching about the sensus divinitatis and a seen of religion in each man. In book 1 he introduces these concepts and he teaches the necessity of Scripture for rightly perceiving natural revelation. In book 2 he provides more detailed discussion about the content of natural law. In chapter 4 he relates natural law to government.
Preus, Robert D. The Theology of Post Reformation Lutheranism
Preus provides a helpful treatment of Lutheran thought about natural law.
Muller, Richard A. Post-Reformation Reformed Dogmatics
Muller provides a helpful survey of Reformed thinking about natural law.
Van Til, Cornelius. Introduction to Systematic Theology
Van Til combines a strong affirmation of general revelation and natural law with a strong view of the distorting effects of sin on man’s perception and use of that law.
Lewis, C. S. The Abolition of Man
Lewis’s argument for natural law is insightful, but it should be read alongside the more theological treatments noted above.
Budziszewski, J. Written on the Heart: The Case for Natural Law
This is perhaps the best recent case for natural law. It should be read along with John Frame’s critique in the The Doctrine of the Christian Life
See also the recent post on natural law by my colleague Mark Ward.