In the first section of Mere Christianity, Lewis gives his argument for the existence of God. He starts off by making two basic points: first, that we all (almost all of us, at any rate) know the Law of Human Nature, that is, we all know deep down that certain things are right and certain things are just wrong, and second, we do not keep the Law of Human Nature. We do things which are wrong and we do not do things which are right; in both cases we feel compelled to make excuses to justify ourselves, a compulsion which we would not feel if there were no Law of Human Nature. Lewis then responded to possible objections, namely that this Law of Human Nature is nothing more than instinct or social convention. That is where we left off last time.
Continuing on, Lewis talks about the reality of the Law of Human Nature. Stones and trees and other such things are compelled to obey certain laws of nature, but they have no choice about it. They are what they are, and they do what they do. Thus there is nothing more to it than what we see them doing. But in our case, we have what we see men doing and we also have this Law of Human Nature which tells us we ought to do things differently. We cannot explain it away by calling it instinct or social convention. Nor can we say that it is simply a description of how we would wish others to behave for our own convenience, because very often the right thing is the inconvenient thing. And we cannot say that it is for the convenience of society as a whole, because that ultimately becomes a circular argument, akin to saying that the whole reason for playing football is to score goals. This leaves us with the Law of Human Nature as a real thing which really is out there whether we like it or not.
So what does this tell us about the universe we live in? Ever since the dawn of time men have pondered the question of what the universe is and how it came to be here. Two views on the subject have turned up; the materialist view which says that we are nothing more than the product of chance plus time, and the religious view which says that there is something out there, most likely some sort of conscious mind, which created the universe for the purpose of producing creatures like itself, at least to the extent of having conscious minds.
So which of these views is correct? We don’t know. We can’t find out by observing the universe or anything in it–except ourselves. We have this law which directs us to behave in a certain way, which was put inside of us by something outside of us. That law had to come from somewhere, or someone. Lewis uses the example of a mailman going down the street and delivering packets. I know that these packets contain letters because whenever I get one that is addressed to me, I open it and find a letter inside. Different people on the street get different letters; thus different objects in the universe are under different laws of nature. But in every case there are letters and there is a sender of letters; thus there is a Something or Someone who directs the universe.
Lewis then goes on to repudiate the Life-Force view, which says that the evolution of man was directed by some sort of Life-Force. Is this Life-Force a conscious mind? If so, then we are back to the religious view; if not, then it makes no sense. This is nothing more than a way to have all the comforts of the religious view without any of the unpleasant consequences. “When you are feeling fit and the sun is shining and you do not want to believe that the whole universe is a mere mechanical dance of atoms, it is nice to be able to think of this great mysterious Force rolling on through the centuries and carrying you on its crest. If, on the other hand, you want to do something rather shabby, the Life-Force, being only a blind force, with no morals and no mind, will never interfere with you like that troublesome God we learned about when we were children….All of the thrills of religion and none of the cost. Is the Life-Force the greatest achievement of wishful thinking the world has yet seen?”
Lewis concludes this section by saying that our position is very grave. Looking at the Law of Human Nature, we can find out an awful lot about the Someone behind it. We find that He is very interested in right conduct, and not the least bit indulgent or sympathetic. You cannot say that you simply won’t bother about Him, because part of you is really in agreement with Him when He disapproves of human greed and deceit. You cannot ask Him to just make an exception in your case, because unless He truly detests human corruption then He is not good at all. And yet if He exists and is truly good, then He must detest most of what we do. What a terrible fix to be in.
This is a roundabout way to arrive at the existence of God, but unless you come at it in this fashion, Christianity makes no sense at all. Christianity commands people to repent, but this is useless unless people know that they have done something they need to repent of, just as it is useless to call a doctor unless you know that you are really sick. It is only when you know that there is a Someone behind the universe and that most of what you do is offensive to Him, that Christianity begins to make any sense at all.