Lewis's approach, likely descending from his analytic gifts, is to notice where the misconception and muddled thinking is and clear that up, as one removes weeds from a garden or a board from the lawn in hopes that the health of the plant will be restored by that alone. If feeding or watering or healing from disease is still needed, at least the plant will have more vigor to help this along. Yet odd things occur along the way with that. I read The Pilgrim's Regress long before I read Lewis's own comments three decades later on the weaknesses of that first book. He spent too much time exposing the weaknesses of philosophies which were popular at the time, mentioning in particular TH Green among those at Oxford. Those were already being outcompeted, whether by other weeds or by healthy plants, and were important only as historical influences on others by the 1950s. I didn't know the half of it, and innocently believed that if the august Lewis was taking the time to refute them, there must be plenty of people out there still believing them. I suppose it is nice to have the argument against them stored in memory in case it ever comes up again, but some of what he patiently corrects are ideas no one is submitting anymore.
We see another example of the popular mistake changing somewhat between his age and ours. In both The Abolition of Man and Mere Christianity he argues against the prevailing belief that there is no ultimate truth, as they are all merely culturally conditioned opinions. At the conclusion of Abolition, he lists common moral precepts from traditions worldwide. He finds the Law of Reciprocity, or of Duty to Parents or Duty to Posterity in Vedic texts, in Confucianism, Buddhism, and Daoism, in Norse religion, Greek and Roman writers, and in Zoroastrianism, as well as the Abrahamic faiths, Judaism, Christianity, and Islam. He is making an argument from Natural Law. He calls this the Tao. I have read Christians criticising this name as too distracting and too much of a nod to the modern fascination with Eastern religions. I think that is misguided. The fashionableness for Westerners of adopting Hindu and Buddhist practices was just getting under weigh in his day, confined to a few of the wealthy and cranky. He likely chose it because it has some accuracy - as much as we might hope for when using a term from one religion to describe something common to all of them. Lastly, I suspect he was also being quite intentional in choosing a nonchristian term to drive home the point that he was not in any way relying on Christian assumptions here.
Side Note: Paul makes similar reference to Natural Law in the first few chapters of his Letter to the Romans. It should be noted that not all Christian theologians subscribe to the idea of Natural Law, however.
Lewis thinks a belief in thoroughgoing moral relativism is self-refuting in much the same way that any disbelief in ultimate truth is sawing off the branch one is sitting on. If reasoning is all so vague and unreliable, how would we know it? The assertion that there are no absolutes is itself an absolute. I have a mild disagreement here. I think that holds true for the group beliefs, where many minds over many years can contemplate the core beliefs, discarding and improving. But I am not sure we can apply that to individuals. All of us rely on an approximate reasoning, not likely useless, as whatever it is has allowed us to survive. Existence is not proof of truth, but it is preliminary evidence. Nor is the genetic ruled out, as Lewis thinks, simply because that would imply an unacceptable arbitrariness and meaninglessness. If something is common to the wisest thinkers of all cultures, then it may as easily be an inborn genetic attribute as cultural, without having to bring in any hint of general revelation. There is already evidence that reciprocity has some genetic base. In earlier constructions we imagined things in terms of a few key genes for selflessness or cooperation versus competition and selfishness, but we now think there there are many interacting genes, so that all of us have tendency to both reject and welcome strangers, to be altruistic or self-protective, interacting with cultural directives in odd ways. We seem to not only have a preloaded Plan A and a Plan B for our responses, but an Asub1, Asub2...Asub9 interacting with Bsub1, Bsub2, etc. That our fallen state might push us towarrd all the A's while God constantly intervenes to make sure some B's are always present is not logically ridiculous. To be very Lewisian at this point, I will note that the existence of neither the A's nor the B's proves anything for or against the existence of God. They are just not incompatible with belief.
Yet that is only partially the question now. Lewis tries to undeceive us on this point while it is still fairly early in its popularity. At that point, this was believes by relative few who were deeply involved with the ambiguous disciplines: philosophy, arts and literature, social sciences. The great mass of western humanity was only beginning to think that we might have anything like individual truths. We are at the other end of the fad now. Beginning with those born at about the time Lewis was writing these works in the '40s and continuing for five decades this became a majority view, that morality was 100% culturally variable. To assert, as I often did, that we in fact shared many moral beliefs but ordered them differently was regarded as impossible and puzzling outside the church. We are entering a new era, in which there are moral absolutes and they are based on political and even identity status. Not for nothing is it called political correctness. It cannot easily defend itself on grounds of "correctness" per se. It relies entirely on the power formulations of "Your truth is founded only on your power. Yet our truth is really, really true and must rule. Therefore, our first aim is to take your power. Arguing our own righteousness is irrelevant. We know these things are true and reject all your arguments out of hand."
While there were intellectuals at Oxford and among the philosophers and artists who had a fascination for the Marxism that gave birth to wokeness, that was not the part of communism they were attracted to. They liked the impressions of sharing, justice for the poor, and sheer modernness of socialism. The absolute truth part they would have rejected. Lewis argued against them, poking holes in their self-deception. We are in a different situation now. Those still exist and are still greatly the majority in our societies. But a new heresy is springing up.
I am reading A Preface to Paradise Lost, one of Lewis's academic works, and find the same approach right off the bat. Lewis believes that most readers of his day have a wrong expectation of Milton's poem because they do not understand what epic poetry is supposed to be. That garden must be weeded before we can have any expectation of understanding and appreciation. He discovers another set of weeds almost immediately, the idea (following Eliot) that only great poets can be a 'jury of judgement' of Milton or any other poet, which must be attended to before the first overarching question can even be attempted.
I intended to discuss Till We Have Faces, with is entirely concerned with self-deception, but I have already gone too long. So there will be more to come.