My dear Wormwood,
I note what you say about guiding your patient’s reading and taking care that he sees a good deal of his materialist friend. But are you not being a trifle naïve? It sounds as if you supposed that argument was the way to keep him out of the Enemy’s clutches. That might have been so if he had lived a few centuries earlier. At that time the humans still knew pretty well when a thing was proved and when it was not; and if it was proved they really believed it. They still connected thinking with doing and were prepared to alter their way of life as the result of a chain of reasoning. But what with the weekly press and other such weapons we have largely altered that. Your man has been accustomed, ever since he was a boy, to have a dozen incompatible philosophies dancing about together inside his head. He doesn’t think of doctrines as primarily ‘true’ or ‘false’, but as ‘academic’ or 'practical', ‘outworn’ or ‘contemporary’, ‘conventional’ or ‘ruthless’. Jargon, not argument, is your best ally in keeping him from the Church. Don’t waste time trying to make him think that materialism is true! Make him think that it is strong, or stark, or courageous—that it is the philosophy of the future. That's the sort of thing he cares about....
Your affectionate uncle
C.S. Lewis, The Screwtape Letters (originally 1942; this edition: Harper Collins, 1996) 1-2.