The Moral Law---Mere Christianity

Why do we feel that pang of guilt when we take something we know isn't ours? Why do we get offended when someone cheats on a test, or in sports, and gets away with it? Time and time again we say, "it isn't fair!" What defines if something is fair or not? It seems that to a large degree there is a universal code that we all adhere to. It is true that we often choose to ignore this code if it is inconvenient to us. But consider any time you have been wronged, you immediately jump back to the assumption that the other person broke an unspoken rule. It seems peculiar that we have this aspect in us. Even odder still that we have the power to overrule it. When we talk about the law of nature in reference to that of a rock or tree it seems silly to suggest that they ever perform uncharacteristically. A rock will always serve the purpose that a rock does. Trees will always grow as trees. Their environment can change certain aspects, but not in a way that one would not expect. They are always consistent in their behavior. So why then, do humans feel a code inside them that we also have the power to overrule?
Lewis believes that this code comes from a transcendent moral rule, known as the Law of Nature. "The idea was, that just as all bodies are governed by the law of gravitation, and organisms by biological laws, so the creature called man also had his law--with this great difference, that a body could not choose whether it obeyed the law of gravitation or not, but a man could choose either to obey the Law of Human Nature or to disobey it." It is called the Law of Nature because it seems to be a universal trait. To be clear, there are some people in the world who do not follow this law-just as there are people in the world who are colorblind. Not to mention the variations across cultures and time. However the differences are not as great as they first seem. There are definitely significant shifts depending on the culture and time, but the core concepts are generally the same, just altered in one way or another. You would be hard pressed to find a society that values cowardice, or has no sense of justice (though it may come in a different form) There can be many altercations to the basic ideas, but the core concepts can be found.
There are many objections to Lewis argument, and he takes his time in answering each. The one that I found most interesting was the concept of instinct. There are two basic desires that contribute in our decision to help people. The first is our herd instinct-trying to preserve the group. The second is our self-preservation instinct-trying to maintain our own safety. When a situation occurs that a person needs help both of these instincts kick in and battle for control of the decision.  "In addition to these two impulses, a third thing which tells you that you ought to follow the impulse to help, and suppress the impulse to run away."  Since this thing judges the two instincts, and decides which should be encouraged, it cannot be either one of these two instincts. "The Moral Law tells us the tune we have to play: our instincts are merely the keys."
I really loved Lewis' analogy of the piano when he discussed instincts-particularly that there are no right and wrong instincts, just as there are no right or wrong notes. There is just an appropriate time, and an inappropriate time for each. This seems like an obvious statement because of the beauty of it's analogy but think of many of the instincts we look at as reproachful, or as healthy. All can be used for good and for evil. The instinct for sex is a great thing, in the right setting. The instinct to seek justice can be led astray into cruelty if not properly tempered. All things in moderation (except praising God.) We need the Moral Law to guide us and to tell us when to use each instinct, if not we are walking blind.