quality:correction, structural:reformatting, structural:expansion, quality:corruption
Universal morality

One of the main theses in Lewis's apologia is that there is a common morality known throughout humanity, which he calls " natural law ". In the first five chapters of Mere Christianity, Lewis discusses the idea that people have a standard of behaviour to which they expect people to adhere. Lewis claims that people all over the earth know what this law is and when they break it. He goes on to claim that there must be someone or something behind such a universal set of principles.

These then are the two points that I wanted to make. First, that human beings, all over the earth, have this curious idea that they ought to behave in a certain way, and cannot really get rid of it. Secondly, that they do not in fact behave in that way. They know the Law of Nature; they break it. These two facts are the foundation of all clear thinking about ourselves and the universe we live in.

Lewis also portrays Universal Morality in his works of fiction. In The Chronicles of Narnia he describes Universal Morality as the "deep magic" which everyone knew.

In the second chapter of Mere Christianity, Lewis recognizes that "many people find it difficult to understand what this Law of Human Nature... is." And he responds first to the idea "that the Moral Law is simply our herd instinct" and second to the idea "that the Moral Law is simply a social convention". In responding to the second idea Lewis notes that people often complain that one set of moral ideas is better than another, but that this actually argues for there existing some "Real Morality" to which they are comparing other moralities. Finally, he notes that sometimes differences in moral codes are exaggerated by people who confuse differences in beliefs about morality with differences in beliefs about facts:

I have met people who exaggerate the differences, because they have not distinguished between differences of morality and differences of belief about facts. For example, one man said to me, "Three hundred years ago people in England were putting witches to death. Was that what you call the Rule of Human Nature or Right Conduct?" But surely the reason we do not execute witches is that we do not believe there are such things. If we did – if we really thought that there were people going about who had sold themselves to the devil and received supernatural powers from him in return and were using these powers to kill their neighbours or drive them mad or bring bad weather, surely we would all agree that if anyone deserved the death penalty, then these filthy quislings did. There is no difference of moral principle here: the difference is simply about matter of fact. It may be a great advance in knowledge not to believe in witches: there is no moral advance in not executing them when you do not think they are there. You would not call a man humane for ceasing to set mousetraps if he did so because he believed there were no mice in the house.

Lewis also had fairly progressive views on the topic of "animal morality", in particular the suffering of animals, as is evidenced by several of his essays: most notably, On Vivisection and "On the Pains of Animals".