This argument (from Lewis, Plantinga, and others) cuts to the foundation of the materialist worldview. If naturalism is true, we have no good reason to trust our own thinking — including the thinking that led us to naturalism.
C.S. Lewis's classic statement
From Miracles, chapter 3: "If the solar system was brought about by an accidental collision, then the appearance of organic life on this planet was also an accident, and the whole evolution of Man was an accident too. If so, then all our present thoughts are mere accidents — the accidental by-product of the movement of atoms. And this holds for the thoughts of the materialists and astronomers as well as for anyone else's. But if their thoughts — i.e., of materialism and astronomy — are merely accidental by-products, why should we believe them to be true? I see no reason for believing that one accident should be able to give a correct account of all the other accidents."
The argument formalized
- If naturalism is true, our beliefs are produced by physical brain processes shaped by natural selection.
- Natural selection selects for survival behavior, not truth.
- Therefore we have no guarantee — and indeed reason to doubt — that our beliefs are reliably true.
- This includes the belief that naturalism is true.
- Therefore naturalism is self-defeating.
Alvin Plantinga has developed this rigorously as the "evolutionary argument against naturalism" (Warrant and Proper Function; Where the Conflict Really Lies).
Darwin himself worried
In a letter to William Graham (1881): "With me, the horrid doubt always arises whether the convictions of man's mind, which has been developed from the mind of the lower animals, are of any value or at all trustworthy." Darwin saw the problem his own theory created for itself.
The atheist counter
Natural selection would produce truth-tracking minds because false beliefs would lead to bad outcomes. But this confuses adaptive behavior with truth-content of beliefs. A creature that flees from predators because it falsely believes them to be flying rocks survives just as well as one that believes them to be predators. Selection cares about behavior, not the truth-content of beliefs.
Plantinga's example: a hominin who runs from tigers because he wants to play with them and thinks the safest way to play is to run away survives perfectly well. False beliefs, true survival.
Thomas Nagel's admission
Atheist philosopher Thomas Nagel, in Mind and Cosmos (2012), wrote that materialist reductionism is "almost certainly false" — partly because it cannot account for the reliability of reason. Nagel is not a Christian; he is an honest philosopher seeing where his own worldview struggles.
The Christian alternative
Human reason is real because humans are made in the image of a rational God. The Logos who structures the universe is the same Logos that gives humans the capacity to know. That is why our minds correspond to reality — both come from the same source.
Wigner's puzzle
Eugene Wigner's famous 1960 essay "The Unreasonable Effectiveness of Mathematics in the Natural Sciences" asked: why does abstract math, invented by human minds, describe the deep structure of reality so precisely? On materialism, this is mysterious. On Christianity, it is exactly what we should expect — Mind underwrites both reality and reason.
The Orthodox lens
The Logos doctrine (John 1) means that reality is intelligible because it comes from Intelligence, and we are knowable to ourselves because we share in the rational nature of God. Christianity is not anti-reason; it is the only worldview in which reason finally makes sense.
St. Justin Martyr called Christianity "the true philosophy." The Fathers never apologized for reason; they grounded it.
A pastoral word
When a skeptic tells you faith is irrational, you can gently turn the question around. Can reason itself be trusted on a purely naturalistic worldview? The deepest case for Christianity is that it is the only framework in which thinking, truth, science, and love make full sense. You are not asked to abandon your mind to follow Christ. You are asked to use it.