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Why It’s Irrational to be a Naturalist

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C.S. Lewis

I’m currently reading C.S. Lewis’ classic book Miracles, where he starts off by discussing why naturalism – the idea that nature is all that exist and that anything supernatural is impossible – really is an unreasonable position to hold. This is because reason itself would be fully explained by nonrational causes, and thus be unreliable. Naturalism is thus self-defeating since one cannot then reasonably subscribe to it.

“This, as it seems to me, is what Naturalism is bound to do”, Lewis writes. “It offers what professes to be a full account of our mental behaviour; but this account, on inspection, leaves no room for the acts of knowing or insight on which the whole value of our thinking, as a means to truth, depends.” (Miracles, Fontana Books 1947, p. 22).

Alvin Plantinga has developed a simliar argument where he points out that evolution, which basically all naturalists accept, presents a defeater for why one should believe in naturalism. this evolutionary argument against naturalism can be presented as such:

1. If naturalism and evolution is true, the probability that our cognitive faculties are reliable is low.
2. anyone who sees that premise 1 is true and accepts or believes naturalism and evolution has a defeater for the reliability of ones cognitive faculties, a defeater that can’t be defeated
3. anyone who has an undefeated defeater for the reliability of ones cognitive faculties has a defeater for any proposition she believes—including, of course, naturalism itself.
4. Therefore, naturalism is self- defeating; it is self-referentially incoherent, and hence rationally unacceptable.

Belief in naturalism is thus just as irrational as believing that there are no beliefs or that I am my brother or some other logical inconsistency. That’s what this argument is all about. Note that it is not a claim that we cannot differ true from false beliefs in the actual world, but that if naturalism was true that would be the case and so it’s irrational to believe in naturalism. The argument is NOT:

1. We cannot know that our beliefs are true.
2. Belief in naturalism is a belief.
3. Therefore we cannot know that our belief in naturalism is true.

As a Christian, it’s perfectly fine for me to be assured that most of my beliefs are true. But on naturalism, what’s the likelihood that our beliefs are true and not just suitable for survival? Well, there’s no guarantee at all! The probability ought to be 50/50 since it’s totally irrelevant to evolution whether the belief is correct as long as it results in suitable behavior.

In fact, a naturalist would say that all religious beliefs are false beliefs, yet people have had those for thousands of years and it doesn’t seem to have hindered our survival at all. The ancient worldview was from our modern standpoint wrong but still helped people navigate and function in orderly societies. And I can’t see why, on naturalism, this doesn’t apply to all believes, including how our brain interprets our senses and how it thinks rationally. So if you want to think straight, drop the idea that the supernatural cannot exist.


  1. […] have no reason to believe that our reasoning reflect reality, an argument I have written more about here. Lewis also uses a moral argument for the existence of a supernatural or transcendent reality, and […]

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The author

Micael Grenholm, a Swedish charismactivist, apologist and author.

Micael Grenholm, a Swedish charismactivist, apologist and author.

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