O.K., Diogenes the Cynic, it’s clear to me now that you’re not even trying to make sense of what Lewis is saying. You’re using whatever interpretation of his works makes him sound stupidest. Of course Lewis wasn’t asserting that God exists in The Screwtape Letters. He was, at most, only assuming that God exists. Furthermore, he was assuming that God exists only in the sense that a fictional work makes assumptions (i.e., within the book, we need to assume that God exists to make sense of it). That doesn’t mean that the author necessarily even believes himself the assumptions of his works of fiction. Indeed, Lewis even states in his preface to the book that although he believes in devils, he doesn’t consider them a necessary belief for Christians. Within the context of this book, the existence of devils is an assumption for the fictional purpose of the book.
Given that Lewis was not asserting the existence of God, the argument is not circular. Indeed, there’s no argument there at all. The Screwtape Letters is a fictional work. Fictional works do not make assertions in the way that nonfictional ones do. At most they make assumptions for the purpose of their fiction. So Lewis did not make a circular argument in The Screwtape Letters or any argument at all. He did not make a fallacious assumption either. His assumptions may or may not have been factually correct, but that’s not the same thing.
So you may understand the word “fallacious,” but you don’t understand that it can’t be applied to a fictional work, and most of what Lewis wrote is fiction. If we set aside his scholarly works like The Allegory of Love, A Preface to Paradise Lost, and English Literature in the Sixteenth Century excluding Drama, nearly all of Lewis’s well-known works are fiction. The Chronicles of Narnia, the Ransom novels, Till We Have Faces, The Screwtape Letters, The Great Divorce, and The Pilgrim’s Regress are all fictional works. Lewis believed some of the fictional assumptions that he used in those books, but he certainly didn’t believe all of them. He didn’t claim that there must be other parallel worlds like Narnia, he didn’t claim to know anything about the inhabitants of Mars and Venus, and he didn’t claim to know what devils did (or even if they definitely existed).
Lewis was making assertions only in his scholarly works and in his few apologetic works, like Mere Christianity. Even the extent of his assertions in his apologetic works are exaggerated. Mere Christianity is a much sketchier and less thought-out work than is claimed by both fans and detractors of Lewis. There’s a book called C. S. Lewis in a Time of War (or C. S. Lewis at the BBC, depending on which edition you pick up) by Justin Phillips that details the evolution of the lectures that lead the creation of Mere Christianity, which was very much ad hoc. Lewis never meant to have people consider it his complete, definite defense of Christianity.
Indeed, much of this confusion has been created by many fans of Lewis who seem to believe that Lewis was writing a Summa Theologica in his apologetics, fiction, etc. in which he was making a major defense of Christianity. Lewis never intended to do any such thing. His nonfiction (other than his major scholarly works) was much more off the cuff and limited in scope than that. Many detractors of Lewis have picked up this claim and also think that Lewis was making more definite claims than he actually did. One example of the is the misguided picking out of the Trilemma from Mere Christianity. First of all, Lewis never used the name “Trilemma” himself. He didn’t even think of it as being a single piece of argumentation. As Thudlow Boink has already partially indicated, the point of the Trilemma is generally misunderstood. Indeed, it’s my contention that it’s completely misunderstood by most people who quote it and it makes a different argument than they claim it does. I think that analyzing the Trilemma is a new thread though, so I’m not going to discuss it any further in this thread. Start a new thread for that.