Mere Christianity

I really liked CS Lewis' thought progression in Mere Christianity. He begins by establishing the fact that humans have a Law of Human Nature, but they have the choice to obey it or disobey it; "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." Lewis points out his two ideas. 

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.

Lewis then moves on to objections that have been raised, such as the moral law being the same as a herd instinct. He gives the example of a cry for help from a man in danger. This instigates two desires: the desire to help the man, and the desire to keep yourself out of danger. The desire to help is the herd instinct. Lewis makes the point that "feeling a desire to help is quite different from feeling that ought to help whether you want to or not." He then gives one of my favorite quotes when comparing these desires to a piano:

The Moral Law tells us the tune we have to play: our instincts are merely the keys.

Continuing on with his analogy, Lewis writes:

The thing that says to you, 'Your herd instinct is asleep. Wake it up,' cannot itself be the herd instinct. The thing that tells you which note on the piano needs to be played louder cannot itself be that note.

Another point he makes is that there are no "good" or "bad" impulses. The same impulse can be considered good in one situation and bad in another. The Moral Law is not an impulse or a group of impulses, but rather it directs these impulses. On the same thought, Lewis writes:

The most dangerous thing you can do is to take any one impulse of your own nature and set it up as the thing you ought to follow at all costs. ... 

You might think love of humanity in general was safe, but it is not. If you leave out justice you will find yourself breaking agreements and faking evidence in trials 'for the sake of humanity,' and become in the end a cruel and treacherous man.

I also like his rationalization of a power outside the universe. He points out that if by simply observing the physical aspects of humans, we will never have any evidence of this moral law. In the same way, we will not find anything past the observed physical facts of anything. He expands on the point, saying:  

If there was a controlling power outside the universe, it could not show itself to us as one of the facts inside the universe--no more than the architect of a house could actually be a wall or staircase or fireplace in that house. The only way in which we could expect it to show itself would be inside ourselves as an influence or a command trying to get us to behave in a certain way. And that is just what we do find inside ourselves.

Lewis then gives another great example using mail. When everyone down the street is receiving mail, a man deduces that everyone else's mail packets contain letters based on the fact that his packet contains a letter.

I'm explaining the packets I'm not allowed to open by the ones I am allowed to open. ...  The only packet I am allowed to open is Man. Where I do, especially when I open that particular man called Myself, I find that I do not exist on my own, that I am under a law; that somebody or something wants me to behave in a certain way.

I find it fascinating, as he points out in his conclusion of the chapter, that he hasn't even touched on the topic of Christianity or God. He simply establishes the grounds for the existence of a Moral Law.