I recently wrote about whether or not the theory of evolution was compatible with Christianity. I argued rather briefly that the theory of evolution is compatible with Christianity. In this post I will try to address the question: Could the theory of evolution be incompatible with atheism?
The philosopher Alvin Plantinga has a very interesting response to this question. He does not argue that evolution is incompatible with every form of atheism. But, he does argue that the theory of evolution is incompatible with certain forms of naturalism commonly assumed by many atheists. I am still not sure if I am fully convinced, but It is a very interesting argument and worth examining. Before one can understand this argument they need a few conceptual tools.
Conditional Probability
The first tool is the idea of conditional probability. Conditional probability is the probability of something happening, given something else. For instance, the probability that a person named Brigham is a Mormon given that he lives in Utah is high. On the other hand, the probability that a person named Mohammed is a Mormon given that he lives in Kuwait is low.
Defeaters for belief
The second conceptual tool is the idea of a defeater. A defeater is a reason for not believing something else. So for example, if I see a spider on the wall of my grandma's house, then I will form the belief that there is a spider on the wall. But if my grandma then says that it is just a halloween decoration, then I have a defeater for my belief that there is a spider on the wall.
There can also be defeaters for other defeaters—defeater defeaters. So for example, if I believed that my grandma might be a bit senile and that it is June, then I have a defeater for the belief that the spider is a halloween decoration. This process could continue with defeater defeater defeaters and so forth.
Definition of naturalism
Naturalism is the view that that there is no God, nor is there anything like God. A person can be an atheist without being a full blown naturalist. Naturalism implies materialism. According to a materialistic view of the human mind, both behavior and beliefs are caused by neural firings in the brain. A materialist believes that consciousness is really just the neuronal firings of the brain.
So with these conceptual tools, one can understand Plantinga's argument.
Premise 1
Beliefs are part of our cognitive faculties along with memory, perception, and rationality. Beliefs are true if they correspond to reality. Beliefs are reliable when they are true most of the time. Perhaps we can say that beliefs have to be true at least 3/4ths of the time to be considered reliable. Now we can use the conditional probability tool. What is the probability that our beliefs would be reliable given evolution and naturalism?
The theory of evolution claims that all species descended from a common ancestor through a processes of descent with modification via natural selection. Natural selection is the process whereby organisms better adapted to their environment tend to survive and produce more offspring.
According to a naturalistic conception of evolution, the human brain would have evolved to produce certain survival-enhancing behaviors. If evolution and naturalism were true, then it doesn't matter if beliefs are true or false. All that matters is the behavior. If that is true, then the probability that our beliefs are reliable is at best 50/50. Beliefs could be true, they could be false, it really wouldn't matter given naturalism and evolution. Therefore, the probability that our beliefs are reliable given evolution and naturalism is low.
A year before his death, Charles Darwin expressed this same concern. He wrote:
“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. Would any one trust in the convictions of a monkey’s mind, if there are any convictions in such a mind?”
A rather influential atheist philosopher Patricia Churchland has echoed a similar point:
Boiled down to essentials, a nervous system enables the organism to succeed in the four F’s: feeding, fleeing, fighting, and reproducing. The principle chore of nervous systems is to get the body parts where they should be in order that the organism may survive…. . Improvements in sensorimotor control confer an evolutionary advantage: a fancier style of representing is advantageous so long as it is geared to the organism’s way of life and enhances the organism’s chances of survival. Truth, whatever that is, definitely takes the hindmost.
In summary, the first premise of Plantinga's argument is that the probability of our beliefs being reliable given evolution and naturalism is low. Plantinga summarizes this premise as such: P(R | N&E) is low where "P()" is probability, "R" is the proposition that our cognitive faculties are reliable, and "N&E" refers naturalism and evolution.
Premise 2
If one believes premise 1 and she is a naturalist, then she has a defeater for the belief that her beliefs are reliable. Remember that a defeater is a reason not to believe something. That means that a naturalist who also believes in evolution has a reason for believing that her cognitive faculties are not reliable.
Premise 3
If one has a defeater for the belief that beliefs are reliable, then she also has a defeater for any other belief she thinks she has including naturalism and evolution itself. According to Alvin Plantinga:
If you have a defeater for R, you will also have a defeater for any belief you take to be produced by your cognitive faculties, any belief that is a deliverance of your cognitive faculties. But all of your beliefs, as I’m sure you have discovered, are produced by your cognitive faculties. Therefore you have a defeater for any belief you have.
Premise 4
The proposition that naturalism and evolution are both true is a self-defeating proposition. Why? because it creates its own reason not to believe in naturalism and evolution.
Conclusion
Naturalism and evolution cannot be believed at the same time. In other words, if you believe in evolution, then you cannot rationally accept naturalism and vice versa.
Summary
Plantinga's argument does not try to argue that naturalism is false, nor does it try to argue that evolution is false. It simply shows that one cannot rationally believe the theory of evolution and believe in naturalism at the same time. Alvin Plantinga summarizes his whole argument as follows:
(1) P(R | N&E) is low. (2) Anyone who accepts (believes) N&E and sees that P(R/ N&E) is low has a defeater for R. (3) Anyone who has a defeater for R has a defeater for any other belief she thinks she has, including N&E itself. (4) If one who accepts N&E thereby acquires a defeater for N&E, N&E is self-defeating and can’t rationally be accepted. Conclusion: N&E can’t rationally be accepted.
Making the argument even stronger
Earlier in the post I mentioned the idea of a defeater defeater. Could there ever be an argument that defeats the defeater for the belief that our cognitive faculties are reliable? If we can't believe that our cognitive faculties are reliable given evolution and naturalism, then there could never be any other belief that could act as a defeater defeater. That means that a belief in evolution and naturalism creates an undefeated defeater for the belief that our cognitive faculties are reliable.
Concluding remarks
Some people claim that there is a conflict between science and religion. The alleged conflict between science and religion focuses on a superficial conflict between religion and evolution. In my last post on evolution I argued that there is no serious conflict between evolution and religion (at least regarding Christianity). However, according to Alvin Plantinga there is deep conflict between evolution and naturalism. Since biological evolution is a respectable field of science, the conflict really lies between science and naturalism.
If you find this interesting, I highly recommend the book Where the Conflict Really Lies: Science, Religion, and Naturalismby Alvin Plantinga.