Better Understanding Plantinga's Argument Against Naturalism

Question

My advanced apologies if my question is not appropriate for this website. This is my first semester of Philosophy so I hope I'm not asking obvious, low-level questions.

I have been reading Plantinga's argument against naturalistic evolution and have come across the following understanding:

According to Plantinga, believing in naturalism and evolutionary theory is self-defeating; if one believes in the naturalistic evolution of beings, then he/she believes that the main purpose for our cognitive faculties has been to evolve to enhance survival and reproduction, without much consideration for producing true beliefs. The latter part allows us to logically deduce that all beliefs held by naturalists are, according to their own view, not reliable. Thus, their very belief in naturalism is undermined. Hence, the self-defeating idea of naturalistic evolution of beings.

Hoping my understanding is correct, I have the following questions I'm seeking clarifications on:

1. In my class, we talked about the idea of an undefeatable defeator. Is this the same as self-defeating?
2. The naturalistic evolutionary theory believes in a blind and unguided natural processes. How is it different from the evolutionary theory (standard one)?
3. When Plantiga presents possible cases for Darwin's doubt (fact that the conditional probability of cognitive reliability P(R|N&E) is low), he uses as one of the possibilities the case where beliefs are maladaptive. He claims that because of this, P(R|N&E) is too low. But, if beliefs are maladaptive, isn't it rational to argue that most of them would have disappeared through evolutionary processes, and thus what remains makes for a larger share of reliable cognitive faculties, thus a higher P(R|N&E)?
4. What does P(N&E) mean in terms of philosophical words? Is it simply the probability that a person picked at random believes in naturalistic evolution theory?
5. [Most important question] Could someone please explain Plantinga's argument when he presents Bayes's theorem and proceeds with some math? I understand conditional probabilities, so it's not the math I don't get, but rather his assumptions and development through the argument. For example, he says to give P(R) a high probability of 1, which means we highly believe that our cognitive faculties produce reliable beliefs. But then, he says to give P(R|N&E) no more than 1/2. This would be true for someone who does not believe in our cognitive reliability, whereas we just assigned P(R)~1. I'm greatly confused on this part of the development.

Update on Question 5 After consulting with my professor, it seems like I understood the probabilities incorrectly. P(R) is the standalone probability that our cognitive faculties are reliable. Based on our intuition and everyday life, it would be reasonable for us to believe we are correct in most of our beliefs, so we can assign P(R)~1. On the other hand, P(R|N&E) indicates the conditional probability of R being true given N&E only. By definition of conditional probabilities, one cannot allow any other information other than N&E determine the probability of R. That is, compared to the former case, we cannot rely on our intuition and everyday experience to assign a value to P(R|N&E) but rather we must solely rely on the conditionals, N&E, to determine P(R|N&E). Combining this with Plantiga's argument that naturalistic evolution picks for survival and reproduction traits and doesn't concern itself with the formation of true beliefs, I can see why he would give P(R|N&E) a value less than 1/2.

I think this explanation makes sense. Although I'm certain it is open to discussion if this interpretation is true. I would love some input.

Answer 1

As reference see Alvin Plantinga’s Where the Conflict Really Lies: Science, Religion, & Naturalism. The “Evolutionary Argument Against Naturalism” is contained in Part IV Deep Conflict, pages 307-350.

As preliminary let’s assume naturalism includes a belief in materialism with respect to human beings. Plantinga uses this materialism to describe belief as a neural structure. These neural structures have two properties. There is the neurophysiological property and the content of the belief property. Adaptations operate on the neurophysiological property, not the content property. Content makes a belief true or false. See pages 318-25.

There are five questions.

In my class, we talked about the idea of an undefeatable defeator. Is this the same as self-defeating?

The specific defeator that Plantinga constructs is the claim that given naturalism and evolution our cognitive faculties are unreliable. If one cannot trust one’s cognitive faculties nothing they produce is reliable. Suppose one uses one’s cognitive faculties to find a defeator for this defeator then even that defeator is unreliable and can’t be trusted. This specific defeator that one’s cognitive faculties are unreliable given naturalism and evolution is therefore undefeatable.

Not all defeators have this property of being undefeatable, but because this one is about the reliability of our cognitive faculties themselves it is a defeator that gives itself no out. It is not that it is self-defeating, but that it is a defeator for all belief statements including itself. Furthermore, if this defeator were used against itself as a self-defeator it would not change anything. One already knows one’s cognitive faculties are unreliable given naturalism and evolution. There is no point in applying the defeator twice. See pages 345-6.

The naturalistic evolutionary theory believes in a blind and unguided natural processes. How is it different from the evolutionary theory (standard one)?

Plantinga has no problem with the various evolutionary theories as long as they allow evolution to be guided. He claims he can separate an evolutionary theory into the scientific part that does not require blind and unguided processes from the metaphysics of naturalism which contains the blind and unguided processes. He has no problem with the scientific part of these theories. His problem is only with the metaphysical add-ons to these evolutionary theories that claim they are blind and unguided.

Combine the scientific evolutionary theory with the metaphysics of naturalism and you have “naturalistic evolutionary theory”. It doesn’t matter whether the evolutionary theory is standard or not. See pages 307-9.

When Plantiga presents possible cases for Darwin's doubt (fact that the conditional probability of cognitive reliability P(R|N&E) is low), he uses as one of the possibilities the case where beliefs are maladaptive. He claims that because of this, P(R|N&E) is too low. But, if beliefs are maladaptive, isn't it rational to argue that most of them would have disappeared through evolutionary processes, and thus what remains makes for a larger share of reliable cognitive faculties, thus a higher P(R|N&E)?

Plantinga addressed this objection on pages 335-9. Given naturalism and evolution, what is adaptive is the neurophysiological processes, not the content of the beliefs. It does not matter if the beliefs are true or not for adaptation to be successful.

What does P(N&E) mean in terms of philosophical words? Is it simply the probability that a person picked at random believes in naturalistic evolution theory?

Let N be naturalism. Let E be an evolutionary theory. P(N&E) is the probability someone might assign to the belief that both naturalism and that evolutionary theory are both true. There is no need to estimate how many people in a population believe N&E. For Plantinga, P(N&E) = 0 since he does not support naturalism. For someone supporting naturalism, P(N&E) = 1. Plantinga is showing that for those who find N&E believable, he can construct a defeator, "P(R|N&E) is low", to go against the belief in the reliability of our cognitive faculties.

[Most important question] Could someone please explain Plantinga's argument when he presents Bayes's theorem and proceeds with some math? I understand conditional probabilities, so it's not the math I don't get, but rather his assumptions and development through the argument. For example, he says to give P(R) a high probability of 1, which means we highly believe that our cognitive faculties produce reliable beliefs. But then, he says to give P(R|N&E) no more than 1/2. This would be true for someone who does not believe in our cognitive reliability, whereas we just assigned P(R)~1. I'm greatly confused on this part of the development.

Let R be the belief that our cognitive faculties are reliable. Initially the probability that we have reliable beliefs, P(R), is, say, greater than 2/3. However, if someone accepts N&E, then there is a defeator for R. It is "Darwin’s doubt" made more precise by using conditional probability. The defeator is “P(R|N&E) is low”. This is not just a defeator for R. It is a defeator for every belief because our cognitive faculties that generate beliefs is believed to be unreliable. This forces us (as it did Darwin) to lower the probability of R.