One of the ongoing debates in Reformed scholarship is what, if any, role natural law theory played in John Calvin's thought.
The question has lots of implications, not least of which are our understandings of apologetics and politics. Whether natural law thinking--that is, arguments about ethics or God that are not based on sacred Scripture--has any place in our Christian lives.
Most famously, Karl Barth argued that for Calvin, natural law played only a negative role. That is, it showed us our culpability before God, without necessarily entailing a robust action guiding ethics. More recently, however, the consensus of scholarship has moved away from this reading of Calvin toward one that thinks the natural law plays an important part of his approach toward society and politics.
I want to believe that. I really do. But I don't buy it. Yet.
In Book II, Chapter 7 of the Institutes, Calvin outlines the three functions of the 'moral law.'
The first is the properly theological usage of the law, which clearly has a 'negative' function. It reveals and seals “the wickedness and condemnation of us all.” That is, it reveals to us our inability to act in accordance with the heavenly kingdom.
Interestingly, the second function of the moral law is also described in negative terms, even though it is in the context not of our relationship to God, but our relationship to each other--that is, society and politics. Calvin writes: “The second function of the law is this: at least by fear of punishment to restrain certain men who are untouched by any care for what is just and right unless compelled by hearing the dire threats of the law.” While there are intimations in the passage that individuals can be “partially broken in by bearing the yoke of righteousness,” they are such not by virtue of any positive vision that the law lays out for them to abide by, but because the law restrains them. “Civil righteousness” is that which is brought about by the ‘halter’ and ‘bridle’ of the law.
This isn't the only place Calvin deploys this negative aspect of the natural law. He writes in II.8.6, “Human laws, then, are satisfied when a man merely keeps his hand from wrongdoing. On the contrary, because the heavenly law has been given for our souls, they must at the outset be constrained, that it may be justly observed.” The law functions socially not as a positive motivator of the good, but as a punishment or deterrent of wickedness.
To be fair, Calvin later suggests that the laws of nations should be in conformity to the “perpetual rule of love.” But in both that section, and the subsequent section when he contends that “equity” is the goal of the law, his descriptions are completely negative in that they are oriented not around common goods that a society ought seek, but instead preventing harm between individuals.
The third, and most proper use of the law, is a positive one--but it also "finds its place among believers in whose hearts the Spirit of God already lives and reigns." The law acts as an "instrument for them to learn more thoroughly each day the nature of the Lord's will to which they aspire, and to confirm them in their understanding of it." That is, it plays a positive, educational role--but only in the lives of believers who have already been regenerated.
There is much more to be said here, of course. But tentatively, I would argue that there is more to Barth's skepticism about Calvinist natural law theory than many commentators are ready to acknowledge. I suspect that once we locate it in the structure of Calvin's doctrine of creation, that would only become more clear.