The Poison of Subjectivism

C.S. Lewis talks about how humans should not rely on themselves and there own judgment.
This would lead to an anarchist approach to life. Basically, if one person thinks one thing, and another person thinks something else, we would see this as being perfectly fine. For example, one person thinks that rape is normal, and another does not. The person who does not think rape is right would not try and stop the other person from raping someone because he would think that he does not have the right, seeing as everyone thinks for themselves, what puts him in the position to stop him? This would tear apart society. Thus, we need a basis, some form of already constructed knowledge that we can all follow and try to obey. This is the "Moral Law" that Lewis describes in Mere Christianity.