The law of human nature, according to Lewis, governs human beings all over the earth. It makes them have the idea that they ought to behave in a particular manner, although it is always hard to avoid, so they keep doing wrongs. The law of nature governs natural occurrences like gravity. The law of human nature governs what is right and wrong according to humans. Just as bodies were governed by laws of gravitation and organisms by biological laws, the man had his law.
This law varied from that of gravitation because one could choose to obey it or not, but one can’t choose not to obey the law of gravity. This means that man has so many laws, but there is only one that he is free to disobey. The law was called the law of human nature because everyone thought that it was known by nature and not taught. The second point that Lewis is putting is that people behave in such a manner due to the knowledge of the law of nature. They break it whenever appropriate to them. The law relates to decent human behavior.