Lewis takes several words with significant histories -- and chases them through all their meanings until they reach their modern sense. Which can be quite a permutation. Did you know that "sad" originally meant "full"? As in, I hope you were all sad after Christmas dinner.
Nature, free, world, life can all mean all sorts of things in old documents. And it's fascinating to watch the words undergo the exact same semantic drift in several different languages.
This is a very useful book for writers. When we bang around words, we change them. This brings it out and lets us see what we are doing.