Thanks again for your response - this has been a remarkably helpful explanation of the content of natural law. I have been reading several treatments of natural law over the past few months and not quite “got it,” I think largely because I was unclear about what exactly the content of natural law is. If natural law is simply identical to Scripture, why is it necessary? If it is not, how can we know whether a purported law is genuinely part of the natural law? There seems to me a remarkable lack of clarity in the literature on these questions. Your explanation deals with these considerations, and it does not even fall foul of my delicate protestant sensibilities about laying down moral obligations outside Scripture!
However, several of the natural law treatments I have read have included things as part of natural law which to my mind do not fit easily within the subject matter of the Ten Commandments. For example, Samuel Pufendorf in The Whole Duty of Man wrote that we have a natural duty to “regulate the dispositions of our minds, in reducing and conforming them to the dictates of right reason,” and held that "widely accepted maxims of political morality" are part of the natural law, for example that those who exercise public power should be held accountable for the way they exercise that power, although the detail will vary depending on circumstances.
While all this seems valid, useful, and wise, it's not easy to see how it fits with the Ten Commandments, even on a wide reading of them. Other examples could be given. Do you consider maxims like Pufendorf's to be a part of the natural law?
Before answering your question let me clear up another point. Although the Decalogue is a good summary of the natural law, considering what the Decalogue commands is not the method of natural law. The method of natural law in itself is not to reflect on revelation, but simply to reflect on the natural goods in the light of natural reason.
Our natural goods are those things which pertain to our well-being, those things which are necessary to the fulfillment of beings of our kind, to the proper unfolding of our potentialities. These have three dimensions. Some things are good for us simply because they fulfill the inbuilt purposes we share with all organized beings or “substances,” for example our inclinations toward preservation. Other things are good for us because they fulfill the inbuilt purposes we share with other animals. These go beyond preservation; for example, they include our inclinations toward procreation and the raising of young. Still other things are good for us because they fulfill the inbuilt purposes we have as rational creatures. These have to do with seeking and knowing the truth in partnership with others, especially the truth about God.
Of course, if the natural law thinker is a Christian, he will reflect on both the natural goods and revelation, because he believes that natural law and divine law come from the same God and co-illuminate each other. So I think you would like me to explain two different things. One is how a principle like the responsibility of rulers to those whom they rule can be known even apart from revelation, by natural reason alone; the other is how the same principle is implied by revelation itself. So let me try to do so.
As to the natural goods: Classical natural law thinkers would begin by observing that by nature we humans are social and political beings. To say that we are social beings is not to say that we have a mere instinct to get together, like cows. Rather it means we cannot flourish except in society; we are beings of such a kind that the good life is not good unless we can share it with others. To say that we are political beings is not to say that we are born into subjection or anything like that. Rather it means that we cannot flourish except under institutions of public justice, such as law and adjudication. These too are a matter of shared pursuit, for we cooperate is in seeking the common good. To put it another way, it is not good for humans to be ruled like slaves; when they have the moral capacity to take part in the organization of the community, they should be allowed to do so. That cannot always be accomplished, because it requires a certain level of virtue and public responsibility on the part of the citizens. But when it can be done, it ought to.
As to revelation: Theologians would begin by observing that Scripture embraces the same view of human beings, for God does not jerk us around; he invites us into His own wisdom. Even in the midst of his indictment of Israel for its sins in the book of the prophet Isaiah, He says “Come, let us reason together.” Similarly, the book of Wisdom -- which is not part of the Protestant Bible but which Protestants such as yourself have often held in high regard -- declares that God left man in the hands of his own counsel, not meaning that man should defy God’s laws, but that he should understand them and participate in them voluntarily. Thomas Aquinas observes that the form of government established under Moses included a monarchical element, in that one person presided, but also an aristocratic element, in that men of wisdom assisted, and a democratic element, in that these men of wisdom were chosen both by and from the people. If you want to connect all this with the Decalogue, one place to start is the prohibition of bearing false witness, which clearly presupposes institutions of public justice. The rest can be worked out by the exercise of prudence. (I don’t say that’s easy!)
One more thing: I think you are right that Pufendorf’s account is rather puzzling, but Pufendorf isn’t a representative of the classical natural law tradition. Actually he represents the early modern revisionists who broke off from that tradition. Although they tried to hold onto some of its elements, they rejected or distorted others, and they tried to hold everything together with nonsense about a so-called state of nature and a so-called social contract. This is a long story. It’s enough to say here that not everyone who says “natural law” holds the same view of how natural law works.