I learned Plantinga's Evolutionary Argument Against Naturalism in the following form:

  1. The probability for the reliability of cognitive faculties conditional on naturalistic evolution, P(R|N&E), is low or inscrutable.
  2. If P(R|N&E) is low or inscrutable, then the believer in N&E has an undefeatable defeater for R (that one's cognitive faculties are reliable).
  3. If the believer in N&E has an undefeatable defeater for R, he/she has an undefeatable defeater for all beliefs, including N.

Therefore, it is irrational for the believer in N&E to believe in N. This concludes the self-defeating nature of naturalism Plantinga points to.

My question is regarding premise 2.

In Warrant and Proper Function, on page 228, right after Plantinga presents A Preliminary Argument against Naturalism, he says suppose you take P(R|N&E) to be low and that you think your cognitive faculties are reliable; then you have a straightforward probabilistic argument against naturalism.

I'm confused because premise 2 suggests we have an undefeatable defeater for all our beliefs; that is, all our beliefs are rendered irrational to accept. But, he starts out by saying "suppose you think your cognitive faculties are reliable".

  1. Isn't there a contradiction here?
  2. Sure P(R|N&E) is low or inscrutable, but that doesn't necessarily mean our cognitive faculties are unreliable. It would imply that if and only if that piece of information was all we knew, but in general, we know a lot more to reasonably say our cognitive faculties are reliable. So I don't see how premise 2 is even remotely logical.

    • As an example of what I mean here: consider I draw a 10 of diamonds. I can see it. But I also note the deck is fair so the P(10 D|fair deck)<<1. But, it's false for me to claim that it is irrational to believe I drew a 10 of diamonds because the conditional probability is so low. No, I see that I did in fact draw the 10 of diamonds.
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    I don't understand your second objection. If the reliability is low or inscrutable, isn't that the same as unreliable? I also don't understand why you're speaking of "that piece of information" when Plantinga is speaking about cognition in general. His argument applies to information in general as opposed to any particular piece of it. Concerning your example, that would mean you would have no means to reliably judge anything at all about the deck of cards, so there would be no grounds for assuming it to be fair.
    – user3017
    Feb 17, 2018 at 13:57
  • Argument against naturalism? Of course this guy only wants more people giving money to his church! If he was a real logician, he would have admitted that his belief is just one mythology among many. Why won't he do it?
    – Rodrigo
    Feb 18, 2018 at 0:21

1 Answer 1


For clarity let’s assume the word “rational” means the ability to construct a reason for something regardless of whether it is true or false. That is we have cognitive faculties that allow us to come up with reasons. Then “irrational” would be the opposite or the inability to construct a reason perhaps due to the lack of cognitive faculties. The point of these definitions is to not confuse “unreliable” with “irrational”.

Although our beliefs may not be perfectly reliable, being unreliable is not how we experience the beliefs of our cognitive faculties in the real world. In other words empirical evidence suggests that our cognitive faculties are generally reliable. We can even come up with logical examples where they are reliable using the ten of diamonds argument you provided.

That we can get such a non-intuitive result about the reliability of our cognitive faculties, assuming naturalism and evolution (N&E), suggests something is wrong with the beliefs leading to that conclusion. They are not describing the real world. In particular, something about the belief N&E is false. We need to take N&E apart and see what we can salvage.

The undefeatable defeator claims that our cognitive faculties are unreliable and because of that there is no way to use our cognitive faculties to defeat it. It is not easy to accept this defeator because it contradicts our experience. The real world is not like the world naturalism presents to us.

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