Answering a challange from a redditor to read this book. And here is what I found.
In rhe Preface there is an interesting tangent about words loosing their descriptive meaning, and becoming mere terms of praise or condemnation. Lewis is wrong in suggesting that Non believers would be eager to use the word 'Christian' to mean a good person /r/atheism has many a post proving that atheists do not like this usage any more then Lewis does. So far However I can say to agree with the argument.
We then get to the main thrust of this book. I can only say that after the preface I was dismayed to see Mr Lewis immediately ignore his own advice and attempt to water down the meaning of "Natural Law". A Natural Law is a law that cannot be broken, simply because it is how the universe works. There is no grounds for saying that a natural law, as applied to rocks is how they do behave but when applied to Humans means how they ought to behave. Even the claim that this definition is standard in philosophy is demonstrably false. I only have to go back as far as Sir Thomas Hobbs to get a counter example. Sir Hobbs describes the State of Nature, or how Humans behave when there is no social constrainst up on them, and it really does boil down to kill them before they kill you.
I also suspect that there is an implicit argument from design hiding here. Really the only time you can get an ought from an is, is when you envoke design. If X is a watch then X ought to tell the correct time. So if there is a way Humans ought to behave then it follows that they where designed to behave in some way. To me the fac that they don't suggest that either there was no such way, or that the designer really wasn't particularly good at his job. Refering to God as He is yet another thing that Lewis slips in with no justification. However I suspect that his sencibilites, having a strong hint of white colonialism about them, made this an intierly unconcious act.
Lewis then goes to argue the point that there is Some idea of Moral right and wrong which goes beyond our instincts and is universal to all humans across all tiems and places. And yet history seems to furnish an inexhaustible supply of counter examples. Lewis does attempt to address this on two fronts. Firstly by arguing that we often confuse custom with moral imperative and that the majority of such examples are really about custom and not morality. He supplied clothing as an example. I find this Ironic considering how strongly some religions feel about what the moral way to dress is. Indeed in Lewis' own faith, the taboo against nudity was pretty well the first sign that humans had learned about good and evil.
Lewis' second approach is to argue that other examples are not confusion about the moral things to do but confusion about the facts of the world leading to actions that in retrospect appear evil, but seemed right at the time. By this example our witch burning ancestors where not acting immorally, but where simple confused about the facts, and believed in witches. As opposed to the Nazi's who where truly evil. This to me is rather extreme special pleading. One could just as easily argue that the Nazi's (who where predominantly Christian) where also merely confused about the facts. Indeed they justified genocide in exactly the same way as previous generations justified the burning of witching. Namely by arguing that the targeted group was responsible for every problem that the society of the day was facing, Indeed that the targets where so evil that they desrved to be destroyed.
This argument, that the enemy is so degenerate that they do not deserve to be treated as a human. Was exactly the same one that Christians used to justify the practice of slavery throughout the colonial period. And is also the exactly same argument used in the Old Testement when Moses ordered genocide in God's name. I find it interesting to Compare Moses and Hitler. They both had the same goal, namely to establish a homeland for the chosen people, and they both ordered similer sorts of atrocities. Including the clensing of the Chosen people by killing anyone who disagreed with them, followed by the extermination of other "non Chosen" people.
Along the way we have the claim that Selfishness has never been admired. That no successfully society has ever been built that uses self interest as an ideal. Well no society other than the modern free world. Indeed the idea that individuals have the right to the pursuit of happiness is what make our society different from every form of Theocracy and Dictatorship that can be found in the pages of history and in the world today. Here again I find Lewis to be wrong. He argues that fact that we can rank societies on a moral scales shows that there must be some external we are ranking them against. But in really all we are doing is comparing the apparent morality of others to our own ideal of what a moral society should be. And indeed My ranking on this issue would be very different to that of a Fundamentalist, irrespective of what ideology he happens to follow.
At one point Lewis builds a destinciton between the Matirialist and the Relgious point of view, leading to the standard position that Science can only answer question about How things work but not questions about why things are. Lewis' account of the materialist position is to me an illustration about why you should never try to argue form the current state of scientific knowledge. He spends a rather long paragraph summarising a model of solar system evolution which no modern day physicist would subscribe too. A modern account of our solar system would be different in every detail, and yet the religous verison continues to be a longwinded way of saying "God did it".
The thing is that Sicnece can have a stab at explaining why all humans seem to agree about a lot of moral questions. And that is our evolutionary heritage as a social animal, predisposed to co-operating in small groups. And to this we add social learning. And indeed among humans social learning seems to account for the bulk of our moral values. And if we look back at history we see that almost every single time two vastly different cultures have met, both have expressed dismay about the moral standards of the other.
In in all Lewis failed to convince me that his exter external moral law is an objective reallity. And having failed there the remainder of the argument falls appart even if the chain of logic was at all reasonable, which it isn't, as it does seem to make some pretty impressive leaps.