Try to set aside (for the nonce, as we say) the various systematic formulations of natural law that you may have encountered. As I have stated before, for various reasons I prefer to talk about natural revelation, over against natural law. Those interested in more can listen to this exchange I had with Dr. J. Budziszewski.
But let me run around and push from the other direction, if I might. At a certain point, some advocates of the “Bible as the only source of morality” paint themselves into a corner. It is like those people who say that because they have a Bible, they don’t need human teachers. But the Bible says that they do need human teachers. So they must be wrong on two counts. First, the Bible establishes human teachers instead of dislodging them, and second, this fact in Scripture is so obvious that if anybody needed a human teacher, it would be the guy who doesn’t see that.
So, like I said, set aside your natural law systems, and ask yourself two questions. Does the Bible teach the reality of natural revelation? This revelation would mean that someone can get accurate information about God simply from the way things in the world are, whether inside or outside him, without reference to any special revelation from God. And second, does any element of this natural revelation bring with it any sense of moral obligation? If so, one of the things that natural revelation reveals is law, and calling it natural law doesn’t give me heartburn.
So the question is posed. There are many ways to answer it, and I will just pick one. There is much more, but this will suffice.
“For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse: Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened” (Rom. 1:20-21).
In sum, the eternal power and Godhead is known from the things that are made, and the knowledge that arises from this is knowledge that brings moral culpability with it. For the knowledge was not just that God was there somewhere, but that He was worthy of worship and honor. Worship and honor were required by the knowledge that natural revelation brought. This they refused to do, in defiance of this light from natural revelation, and this light from natural revelation can certainly be called a natural law. It has all the ingredients–a lawgiver, a standard, a subject, and moral responsibilty. Nothing is missing.
You cannot find a rock or a pebble anywhere that does not command you to worship Him. This is natural law.