The following is lifted from John West in a piece titled C.S. Lewis and Materialism. You should go read it, but I found this part particularly good.
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Rejection of Reason and Truth
Materialism’s first deadly legacy is the rejection of reason and objective truth. Nineteenth-century materialists depicted our thoughts as the irrational products of environment or heredity or brain chemistry. As a consequence, the intellectual classes became convinced that only the reality was material, and thus the only true explanations were reductive. If you wanted to explain a flower, you described its cell structure, not its beauty. If you wanted to explain human beings, you looked not to their greatest achievements, but to the raw materials that made them up. This sort of reductionism permeates contemporary society, from politics and the social sciences to literature and the performing arts.
Lewis’s first sustained attack on reductionism came in his allegory The Pilgrim’s Regress in the early 1930s. In a section of the book titled “Through Darkest Zeitgeistheim” (literally, “through the darkest abode of the Spirit of the Age”), Lewis’s pilgrim is arrested by the flunkies of a giant who symbolizes the materialistic reductionism that was the Spirit of the Age. The pilgrim, named John, is subsequently jailed, leading to a nightmarish sequence. Lewis relates that the eyes of the giant had the property of making whatever they looked on transparent: “Consequently, when John looked around into the dungeon he retreated from his fellow prisoners in terror.… A woman was seated near him, but he did not know it was a woman, because, through the face, he saw the skull and through that the brains and the passages of the nose, and the larynx, and the saliva moving in the glands and the blood in the veins… And when John sat down and drooped his head, not to see the horrors, he saw only the working of his own inwards.…”
John is rescued from the dungeon by a towering woman in blue–Lady Reason, who slays the giant with her sword. She tells John that the giant had deceived him about the real nature of human beings: “He showed you by a trick what our inwards would look like if they were visible… But in the real world our inwards are invisible.“
“But if I cut a man open I should see them in him,” replied John.
“A man cut open,” returned the Lady, “is, so far, not a man: and if you did not sew him up speedily you would be seeing not organs, but death. I am not denying that death is ugly: but the giant made you believe that life is ugly.”
Lewis’s point was that reductionism really does not explain that which is human at all. In fact, in the name of explaining man, reductionism explains him away.
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If you’re looking for more on this, also see The Magician’s Twin: C. S. Lewis on Science, Scientism, and Society edited by John West. You may be thinking to yourself, “Sure, but SCIENCE has come a long way since C. S. Lewis was alive, and undoubtedly the questions and concerns he had have now all been answered.” Well, you shouldn’t think that to yourself because it’s simply not true. Come on. In fact, don’t be surprised if you start to notice philosophy making a comeback.