A synopsis of C.S. Lewis' "The problem of pain"

Recently I found myself praying for a number of people who were either critically ill or who had lost loved ones through illness, and I decided it was time to (re)read C.S. Lewis's The problem of pain. At least, I thought I was rereading it. In the event, it seemed so unfamiliar that I wondered whether I had ever read it or whether I had simply forgotten what it said. 

The problem of pain is a more difficult read than some of Lewis's books, and I decided to write a synopsis for my own purposes. I give it here in case it is useful to someone else. But it is Inevitably a personal summary, and no real substitute for reading the book.