I'm doing this from memory rather than having the book in front of me, so it's going to be a little short of quotes. Sorry!
The first chapter of Mere Christianity is called 'THE LAW OF HUMAN NATURE' and it deals with the idea that there is a natural sense of Right and Wrong that everyone, everywhere for all time knows. Lewis' argument is based, partially, on arguments between people. For example, he uses the 'possession' of a chair. If one person is sitting there, gets up and then comes back to find it occupied by someone else, they may say 'That was *my* seat. I was sitting there first.' and the person now sitting in the chair will argue that there was no one in the chair when they got there and no indication it was taken, etc. indicating, by the nature of their argument that there is a *right* to the chair that belongs to the person who was sitting there 'first'. It's a mutually understood concept between the two parties, even if it's unstated because if they didn't both understand and agree that the right of the first person existed, then the argument would be different. It would be an argument that didn't try to justify the 'taking' of the chair by the second party because there would be no understanding of the ownership, however temporary of the chair by the first party.
There's a mutual understanding of a shared standard of behavior that people will try to make excuses for why they fail to uphold, but they will very rarely flat out deny the existence or validity of this standard.
Lewis says that there are plenty of natural laws that humans are subject to. We don't have a choice about gravity, for example. If we jump off of a diving board, we'll fall (hopefully into a pool full of water!). Even planes, which seem to defy gravity are just working within the natural laws that define it. It's a loophole, but the laws are still there and we've got to work with them. The Law of Nature that Lewis is speaking of is really the 'Law of Human Nature' - the one law, he says, that humans can choose to obey or disobey. It's the idea that there is a basic Right and a basic Wrong and everyone knows it. They may choose to ignore or go against it, but they *know* that they are doing so whether they acknowledge this fact or not.
He brings up the Nazis (not a strange thing since these radio talks were given during WWII) and how if there was no universal Right and Wrong, no Law of Human Nature, then there was no sense in telling the enemy (the Nazis) that they were wrong. "What was the sense in saying the enemy were in the wrong unless Right is a real thing which the Nazis at bottom knew as well as we did and ought to have practised? If they had had no notion of what we mean by right, then, though we might still have had to fight the, we could no more have blamed them for that than for the colour of their hair."
He goes on to state that there are some people who do not believe in the Law of Human Nature because different civilizations have had different sets of moralities throughout the ages. Lewis says that this is not so. That while there have been differences in moralities, these basically amounted to no difference at all. He says that if someone were to take the trouble to compare the Babylonians, the Egyptians, Hindu, Chinese, Greeks and Romans that they would find how very similar they are to one another (morality wise) and how similar they are to modern (at his time) morality.
Is that really true, do you think? He doesn't go into detail, apparently he did that in another book called The Abolition of Man. But I'm thinking about cases in ancient cultures where human sacrifice was a matter of course. They didn't view it as immoral. However, there were other ancient cultures that did. And we certainly do now. Or how about in the Old Testament where a man, if he raped a virgin, married her. This was by Jewish law. How horrible is it to modern senses of morality to think of a woman, raped by a total stranger and then told by the law, by the entity that is supposed to help protect her that she has to marry him and live with him for the rest of their lives? Or what defined 'moral' sexual behavior? Business practices?
I know that I've said that there are certain core moral absolutes, the largest of which is murder. But each society and time has their own definitions of what constitutes these moral absolutes. And the devil is in the details in that case. The view of that morality makes a very big difference in how it is applied and that's the thing we have to live with. Not some grandiose vision of perfect morality, but how it is lived out in the trenches, as it were.
I think Lewis is vastly oversimplifying the differences in morality between cultures and societies. I've only read the first chapter, so I could be wrong, but that's how I'm looking at it. Reading that section really threw me because it's just so wrong from where I'm standing.