C.S. Lewis begins his discussion by applying the argument from morality. The general idea is that man in whatever circumstance he finds himself, is fully aware of a standard. For the atheist, that standard is circumstantial (or so he thinks). For the Christian, that inherent Law of Right and Wrong has been written on the hearts of man, because the Creator has established fixed laws of morality, by which man must comport.
Indeed, if there are no fixed set of laws (of morality) then we enter into the realm of the absurd. As Lewis writes:
Whenever you find a man who says he does not believe in a real Right and Wrong, you will find that same man going back on this a moment later.
No one can live consistently within a worldview that does not operate within the laws of morality. These laws guide man, so that when he does wrong, he senses and knows that he has broken some law. Of course, the unregenerate continues throughout his life to suppress this reality to the point of death (ultimate death; Romans 1).
C.S. Lewis summarizes the inescapability of man before this Cosmic Law:
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.
The Law of Nature for Lewis is a law derived from the Creator. This concept of the Law of Nature has two general meanings, a) that, it is the revealed Law of God to all people and is clear in Nature and by our experiences and b) It is the revealed Word of God found in Scriptures and witnessed in Creation. There ought not to be some unnecessary dichotomy here. Both explanations answer the problem, but one is incomplete without the other. Many during the Scholastic period claimed that the Laws of Nature were clear and therefore, we were in no need of some special, divine revelation. This is foolish, since Natural Revelation is never as clear as Special Revelation.
This was the climax of the Enlightenment that autonomous man is not in need of divine assistance. Descartes, himself, sought to see the starting point of all things in the Self. Truth is found everywhere. It is not confined to the Biblical Revelation. But when the Self of Descartes replaces those two revelations, then autonomy has once again replaced the authority of God’s unique revelation in Scripture and Nature. Hans Kung summarizes Descartes’ journey:
The two books in which medieval man had sought truth—the book of nature and that of the Bible—appear to be replaced here by those of modern man: the book of the world and that of his own self.
This is at its inception the decline of a proper view of Morality that Lewis sought to follow. How is man able to know right and wrong if he is the standard of right and wrong? A true standard must be greater than self, if it is to judge other “selves.” Returning to Lewis, we find a robust view of a standard to judge all things. In fact, this line of reasoning seems to play a significant role in his apologetic. Lewis believed that the atheist or the “inquisitor” could not escape the logic of the one Absolute standard. He argues:
The moment you say that one set of moral ideas can be better than another, you are, in fact, measuring them both by a standard, saying that one of them conforms to that standard more nearly than the other. 
Here is once again demonstrated that when any human being makes any judgment about anything, he is making a moral judgment. If he says that such and such is “distasteful,” he is making an implicit comparison with something that is tasteful. Naturally, the question arises, “By what standard is something more tasteful than other?”
In light of all this, some will find enough strength to make a notable objection. The objection is that we cannot know what is right and wrong, because society changes and the times change. Therefore, conventional wisdom dictates what is right and wrong. The common illustration used here is that of the witch trials of some centuries ago. Some make the argument that witches were killed; because back then people had a different idea of what was right and what was wrong. Lewis’ rephrases the objection and directly responds to the root of the problem:
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 should all agree that if anyone deserved the death penalty, then these filthy quislings did.
We can surmise that Lewis believed in the death penalty, but beyond that of course, Lewis is establishing once again that this is not a proper argument. Because an idea sounds absurd, it does not negate it. The faulty thinking here is that because some have possibly made mistakes in the past in acting on certain laws, then the position of a fixed right and wrong is abolished. But nay, it simply proves that humans are sinful, as Lewis will argue at a later time, or that we do not understand the entire context of a particular time or situation.
One final observation relates to the fact that man’s reasoning is always circular. For the believer that is a precious thing, because he always returns to God, but for the unbeliever, he always returns to his unanswered question. Lewis argues that if a person asks, why should I be unselfish and another answers, “Because it benefits society.” This answer does not satisfy the questioner, because what if he does not care about society except if it pays him? This leads the questions back to the beginning: “Why should I be unselfish?” There is no standard of right and wrong to determine WHY one ought to be unselfish. Undoubtedly, this leads the atheist to the same place where he started –endless inconsistency. In the end, as Van Til argued, the unbelieving man lives on borrowed capital.
 Mere Christianity, pg. 19.
 Mere Christianity, pg. 19.
 All acknowledge the difficulties of Scriptures, but at least the Bible offers clear facts about sin and salvation. This cannot be known through natural revelation alone. I should mention that John Frame adds a third category called: Existential revelation. I made quick reference to this when I spoke of “experiences.”
 Nature and the Bible.
 Kung, Hans. Does God Exist? Vintage Books, 1981, pg. 4.
 I may disagree with Lewis’ emphasis on the Law of Nature, but his general premise that all men know God because of this supreme Law is accurate.
 Mere Christianity, pg. 25.
 I will assert that many of those trials and executions were righteous, while some where incredibly foolish and naïve.
 Lewis, pg. 26.