Mere Christianity

The core of the first section centers on an argument from morality, the basis of which is the "law of human nature", a "rule about right and wrong," which, Lewis maintained, is commonly available and known to all human beings. He cites, as an example, the case of Nazi Germany, writing:

"This law was called the Law of nature because people thought that every one knew it by nature and did not need to be taught it. They did not mean, of course, that you might not find an odd individual here and there who did not know it, just as you find a few people who are colour-blind or have no ear for a tune. But taking the race as a whole, they thought that the human idea of decent behaviour was obvious to every one. And I believe they were right. If they were not, then all the things we said about the war were nonsense. 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 them, we could no more have blamed them for that than for the colour of their hair."[7]

On a more mundane level, it is generally accepted that stealing is a violation of this moral law. Lewis argues that the moral law is like scientific laws (e.g. gravity) or mathematics in that it was not contrived by humans. However, it is unlike scientific laws in that it can be broken or ignored, and it is known intuitively, rather than through experimentation. After introducing the moral law, Lewis argues that thirst reflects the fact that people naturally need water, and there is no other substance which satisfies that need. Lewis points out that earthly experience does not satisfy the human craving for "joy" and that only God could fit the bill; humans cannot know to yearn for something if it does not exist.[8]

After providing reasons for his conversion to theism, Lewis goes over rival conceptions of God to Christianity. Pantheism, he argues, is incoherent, and atheism too simple. Eventually he arrives at Jesus Christ, and invokes a well-known argument now known as Lewis's trilemma. Lewis, arguing that Jesus was claiming to be God, uses logic to advance three possibilities: either he really was God, was deliberately lying, or was not God but thought himself to be (which would make him delusional and likely insane). The book goes on to say that the latter two possibilities are not consistent with Jesus' character and it was most likely that he was being truthful.[8]

The next third of the book explores the ethics resulting from Christian belief. He cites the four cardinal virtues: prudence, justice, temperance, and fortitude. After touching on these, he goes into the three theological virtues: hope, faith, and charity. Lewis also explains morality as being composed of three layers: relationships between man and man, the motivations and attitudes of the man himself, and contrasting worldviews.

Lewis also covers such topics as social relations and forgiveness, sexual ethics and the tenets of Christian marriage, and the relationship between morality and psychoanalysis. He also writes about the great sin: pride, which he argues to be the root cause of all evil and rebellion.

His most important point is that Christianity mandates that one "love your neighbour as yourself." He points out that all persons unconditionally love themselves. Even if one does not like oneself, one would still love oneself. Christians, he writes, must also apply this attitude to others, even if they do not like them. Lewis calls this one of the great secrets: when one acts as if he loves others, he will presently come to love them.