The title comes from the original, British title of God In The Dock, essays written between 1943-1960 by Lewis and published posthumously. Jill Carattini of Ravi Zacharias Ministries gives a better description of the concept than I could, with a Jane Austen reference for those who like that. The experience of being undeceived is central to much of Lewis's thought. When a writer is so knowledgeable about self-deception and so candid about it, we begin to think of him as an expert in becoming undeceived, as having gone through it so often and attentively observed it in others that he is now largely free of that fault. He writes as a philosopher about the prevailing theories of ultimate reality in The Pilgrim's Regress, giving logical rebuttals to both general tendencies of thought and fully articulated philosophies. It is intentionally allegorical in humorous imitation of Bunyan's Pilgrim's Progress, with characters such as Mr. Enlightenment and the Spirit of the Age. Though even here, that experience of having not only a thought rebutted intellectually but finally seen through a deception, of coming out into the open air after being in shadow, recurs throughout.
Lewis does admit in one of the introductions to The Screwtape Letters that he has not learned these lessons from a theological study, but from the witness of "my own heart (I need no other)." We see undeception at work in Surprised by Joy: The Shape of My Early Life, as he observes his previous beliefs - originally atheism and then an experimentation with a number of other possibilities - and his emergence from them. The Pilgrim's Regress, written over twenty years earlier is shown to be somewhat autobiographical. What a remarkable man to have seen through so many philosophies and to have so ruthlessly examined his own heart, we think. Not so fast.
That there are examples of deceiving others is described in the first chapter of Michael Ward's Planet Narnia. Yet these have various explanations which need not be regarded as self-deceiving or even necessarily immoral. He lied to protect the feelings of others, as most of us do. He was private, even excessively so and would throw others mildly off the track to keep his life compartmentalised. Even when he was blameably lying to others, including closest family and friends, these do not necessarily imply he was deceiving himself. He admits as much in his poems "Legion" and "Postures," describing his many masks and "factions." It is an odd quirk of character that those who are open personalities are fairly often secretive in some aspects. We may naturally use openness and candor as a smokescreen.
One large item displays how even the experts and great saints can lie first to themselves and to others only as a derivative to that. Lewis married Joy Davidman late in life in 1956, first as a fiction so that she could remain in Britain (both deceiving the government in that way, and ignoring the sacramental nature of marriage), but later considering it a real marriage on the basis of their deepening romantic and supportive relationship. He hid his this from friends, which Tolkien resented when he later found out. Lewis rationalised this because he believed Tolkien would not approve or even understand because of his intensely Roman Catholic views on marriage, that she was not eligible to be married because she had been married previously, with two children as a result of it. He was correct that Tolkien would not have and never did approve, but to attribute that to some extremity of Tolkien's belief was only the beginning of Lewis's self-deception. It was in fact very close to his own belief just a few short years earlier, that divorce should only be an exception allowed in direst circumstance.
And then worse, he claimed that she had not been legitimately married the first time because her first husband was then a divorced man himself. So this supremely logical and undeceived man moved with tortured logic between entirely convenient rather than consistent views of who was married and who was not. His last novel, Till We Have Faces revolves around the accusations and counteraccusations of believing in illusions and deceiving oneself, which is ironic as Joy apparently had much to do with the composition of the book. Somehow Lewis was unable to admit he was wrong one time or the other, either by being wrong about the necessary extremity of divorce or wrong in that he had married when he should not have. He concocted a ridiculous mashup instead.
As in all real-life circumstances there are complicating factors which give one pause. It was widely believed in Lewis's circle that Davidman had come to England with the express purpose of becoming Mrs. Lewis, though she had known him only via brief correspondence. I don't know if this is true - she was not liked by most of his friends, some of whom hoped he would marry the English poet Ruth Pitter if he married anyone. He may thus have been duped a bit - though willingly. Her first husband had died by 1954, which is sometimes regarded as removing such obstacles. That they had been married as a mere arrangement may have caused him to shrug it off in conscience. It was not a real marriage, don't you see? The rules for real sacramental Christian marriage don't apply. But this likely does break down a barrier and starts the chain of rationalisation going, so that the excuse-making is post hoc - always ten times easier in the human imagination. As it gradually does become a real marriage, what does one do? One can't re-examine the original situation and say "Look, now that we are becoming really married I've decided we never should have gotten married the other way and I will have to leave you now."
All very understandable, surely. Yet better to have never have deceived oneself to begin with.
There will be more Undeceptions to come, as I haven't begun to exhaust what Lewis has written about them.