So far the book is all about proving the existence of the Law of Human Nature and sort of defining why it is different from other laws of nature.
In the first place, I remain unconvinced that Lewis has made his point. I still think that what he calls the Law of Human Nature is the cultural morals that have been instilled in us.
For this chapter, Lewis starts out by going back to one of his foundational points. That humans do not always obey this Law. He says that a natural question may be why fault humans for not being perfect? Which is what it may seem like Lewis is doing when he calls people on not following the Law. His argument is that this is a good point, but he's not trying to assign *blame* which is where that point would have weight. Lewis is concerned with finding out the truth. And there, he says, the very idea of something not being perfect has consequences.
Everything is as it should be. Except for man. When we look at a tree or a rock or something else that is not doing what we wish it would do and say that it's flawed or imperfect, we don't really mean that, not in the sense that they aren't following the laws of nature that govern them. What we really mean, basically, is that the rock or whatever is not suitable for *our* plans. However, everything in nature follows the laws of nature that govern it - there's no choice. And the same is true for humans in the context of the laws of nature. Gravity, conservation of energy, etc. There is no choice in the matter, no conscious thought required.
Lewis goes on to say that some might be tempted to explain this away. Just to say that, well, saying that a human is doing something Wrong is about as useful as saying that a rock is imperfect. That both can only be the way they are, following the laws that govern them. That we only call the actions bad or wrong that are inconvenient to us.
He, of course, argues that this is not so. Take someone accidentally tripping you versus someone trying to trip you and failing. The first, while it may inconvenience, is not Wrong - it was an accident. The second, while it doesn't inconvenience you, is more likely to cause long lasting anger. So Wrong cannot just be what is inconvenient.
He does not believe that we can merely call decent behavior, Right, what is convenient to us.
Lewis briefly addresses the theory that morality is based on the common good and survival of the species. He dismisses it rather quickly, I think, by saying that it's a circular argument. You ask someone why they should be unselfish and they will respond that it's good for society. Then you ask them why you should give a rat's behind about what's good for society except where it's also good for you. And then the person responds, well you should be unselfish.
I don't think it's a good argument. Look, maybe that's some sort of grade school argument. Why should we care what is good for society? Because what is good for society is good for us, personally. The better society is, the conscious and involved in raising up the human condition, the better our lives tend to be. And, beyond the personal concern of, how is this good for *me*, there is the natural instinctive drive to preserve the species. What is good for society is good for humanity.
I'm not saying my argument is any better than Lewis', just that it makes more sense to me than his.