Hi guys.
John -- If you want to argue the merits of C.S. Lewis, you really ought to read and take some time to think over his arguments through the entirety of Mere Christianity, which has a number of insightful passages dealing with human nature, even if you are not a Christian (!). To dismiss him out of hand is premature on your part.
It is okay to say you aren't familiar with his writing and move on to avoid besmirching someone whose ideas you have not evaluated fully. It is hard to believe however, that you have posted so many hundreds of times on this site with a decidedly anti-Christian perspective and still have not read this 100-odd page book that is one of the most widely read works of Christian apologetics! (It is a short, easy, and interesting read even if you may not agree with parts of it.)
PaulK -- You need to take a look at that witch burning passage again in the context of the actual book. That is, read the book. You will see that it is just an introduction explaining how moral codes in society really haven't changed all that much in 300 years, even though on the surface society's morals may seem to have changed drastically. Like Funkmaster said, it has nothing to do with Lewis' beliefs concerning what one ought to do with witches, or even with Christianity. Just a portion of a simple, well-thought-out progression of a logical argument.
Funkmaster and Bamboo -- Hi there guys, you are an encouragement. But I fear you will not get far with this until John and others have actually read MC and have taken the time to understand it. It is easy to get wrapped up and discouraged in pointless argument that is weighted down in ignorance of the topic being discussed. Even then it is a difficult proposition. One problem is that MC is written for the fence-sitter, the person who thinks there is probably a God but doesn't know how to go about believing in him (to paraphrase CSL). CSL is not addressing those such as John who are in a completely different camp...
John and others on this site (like Schrafinator) implicitly claim to be apostate -- that is having had knowledge of the truth but denying it utterly, and teaching others to do so as well. This I think is why they have such a fundamental disagreement with any position you happen to take, almost as if they are playing the Devil's Advocate, which in fact they are (literally).