Theistic Arguments Series:  Plantinga's Evolutionary Argument Against Naturalism

This is part of a series of posts on arguments commonly used by theists.  Because I am a naturalist, I believe that all arguments that purport to prove theism or disprove naturalism are flawed.  Plantigna's argument against naturalism is no exception.  Saints and Sceptics recently summarized it this way:

1)      It is improbable that human beings will have reliable cognitive faculties if both (a) scientific naturalism is true and (b) our cognitive faculties have been produced by natural selection acting on random genetic mutations scientific.

2)      Anyone who accepts that (a) naturalism is true and (b) that our cognitive faculties have been produced by evolution by natural selection has a “defeater” for the belief that humans have reliable cognitive faculties.

We can call the belief that humans have reliable cognitive faculties “R”. Call the belief that Naturalism is true “N” for short.Call the belief that our cognitive faculties have been produced by evolution by natural selection (“E” for short).

3)      Anyone who has a defeater for R has a defeater for any other belief she thinks she has, including the belief that scientific naturalism is true and the belief that the theory of evolution by natural selection is true.

4)      If one who accepts scientific naturalism and the theory of evolution (N&E) thereby acquires a defeater for those beliefs. One cannot rationally accept that both are true.
Conclusion: one cannot rationally accept that both the theory of evolution and scientific naturalism are true  (Plantinga, 2011, p.344-345).

Let's break this down.  Statement 1 is a broad brush based on the idea that under naturalism, belief-forming mechanisms in humans evolved as adaptive behavior rather than truth-seeking.  That's fine as far as it goes, but it doesn't distinguish between different kinds of beliefs or examine how beliefs are formed.  Animals use sensory input to create a cognitive model of their environment.  If sensory input indicates the presence of food or threats from predators, the cognitive model reflects those things.  The closer the cognitive model matches reality, the better the animal will be able to respond to his environment.  Human cognitive models also include beliefs that are not based directly on sensory input.  Theistic beliefs are in this category.  The associated belief-forming mechanisms may have evolved as a way to regulate complex social interactions.  Whatever the reason for having these beliefs, we know that they are less reliable than beliefs based on sensory input.  So Plantinga's statement should be taken with a grain of salt.  Beliefs that are based on observation are certainly much more reliable than those that are not.

Statement 2, therefore, is true only to the extent that beliefs are not based on observation and evidence.  Metaphysical beliefs in particular are not reliable, since they cannot easily be corroborated by observation of physical reality.  There are theists and there are naturalists.  Since theism and naturalism are mutually exclusive, either theists, or naturalists, or both are wrong in their beliefs.  It is beyond question, then, that metaphysical beliefs are not reliable in general.  But this is true whether or not naturalism is true.

Statement 3, once again, it is too broad in its scope.  If I see a closed fist moving toward my nose, I am perfectly justified in believing that I am in danger.  In fact, it would be irrational (and potentially harmful)  for me to reject the belief that I am in danger in this situation.  The notion that none of my beliefs are reliable is pure hogwash.  Nevertheless, I agree that it applies to beliefs not based on observation, and that includes theism.  By Plantinga's own logic, belief in theism is not rational if naturalism is true.

Statement 4 declares that a naturalist cannot rationally accept evolution.  Actually, by Plantinga's logic, a naturalist has no rational justification for believing anything at all.  By that, he means that all beliefs are unreliable under naturalism, and so they cannot be rationally accepted.  He assumes that any given belief is a 50-50 proposition.  But there's no justification for this arbitrary assignment of probability.  We already know that some beliefs are relatively reliable - in particular, those that are based on empirical evidence.  Evolution theory is solidly based on such evidence.  It is rational to believe it.  Plantinga couldn't be more wrong.

And therefore, his conclusion couldn't be more wrong.  But it sounds as if Plantinga is making an assumption that beliefs are reliable if theism is true, as a rational alternative to naturalism.  A teleological explanation for the development of belief-forming mechanisms would presumably avoid this problem of unreliable beliefs.  So is it Plantinga's contention, then, that under theism our beliefs are reliable?  He does not state this as part of this argument, but the implication is clearly there.  However, we have already shown that human beliefs not based on evidence are not reliable.  So the notion of the reliability of our beliefs (to the extent that they are not justified by empirical evidence) is not supported under either naturalism or theism.   It is reasonable to conclude that belief in theism is not rational in any circumstances, unless one can produce empirical evidence to support it.