Now let me go back a bit. We began by asking whether our feeling of futility could be set aside as a merely subjective and irrelevant result which the universe has produced in human brains. I postponed answering that question until we had attempted a larger one. I asked whether in general human thought could be set aside as irrelevant to the real universe and merely subjective. I now claim to have found the answer to this larger question. The answer is that at least one kind of thought—logical thought—cannot be subjective and irrelevant to the real universe: for unless thought is valid we have no reason to believe in the real universe. We reach our knowledge of the universe only by inference. The very object to which our thought is supposed to be irrelevant depends on the relevance of our thought. A universe whose only claim to be believed in rests on the validity of inference must not start telling us that inference is invalid. That would really be a bit too nonsensical. I conclude then that logic is a real insight into the way in which real things have to exist. In other words, the laws of thought are also the laws of things: of things in the remotest space and the remotest time.[I]
This admission seems to me completely unavoidable and it has very momentous consequences.
In the first place it rules out any materialistic account of thinking. We are compelled to admit between the thoughts of a terrestrial astronomer and the behaviour of matter several light-years away that particular relation which we call truth. But this relation has no meaning at all if we try to make it exist between the matter of the star and the astronomer’s brain, considered as a lump of matter. The brain may be in all sorts of relations to the star no doubt: it is in a spatial relation, and a time relation, and a quantitative relation. But to talk of one bit of matter as being true about another bit of matter seems to me to be nonsense. It might conceivably turn out to be the case that every atom in the universe thought, and thought truly, about every other. But that relation between any two atoms would be something quite distinct from the physical relations between them. In saying that thinking is not matter I am not suggesting that there is anything mysterious about it. In one sense, thinking is the simplest thing in the world. We do it all day long. We know what it is like far better than we know what matter is like. Thought is what we start from: the simple, intimate, immediate datum. Matter is the inferred thing, the mystery.
— Clive Staples (C. S.) Lewis in his essay “De Futilitate” available in Christian Reflections, edited by Walter Hooper, (Grand Rapids, Michigan: Wm. B. Eerdmans Publishing Co., 1994), p. 63. First published in 1967.
"Now let me go back a bit"
[I] Lewis’s best and fullest treatment on the validity of human reasoning appears in the first six chapters of his book Miracles: A Preliminary Study (Bles, 1947), especially Chapter III, ‘The Self-Contradiction of the Naturalist’. He later felt that he had in chapter III confused two senses of irrational; this chapter was rewritten and appears in its corrected form in the paper-backed edition of Miracles (Fontana Books, 1960).
"If Naturalism is true"