This weekend I’ll be hosting another theist/atheist discussion on the topic of the evolutionary argument against naturalism (EAAN). This discussion features a number of guests, most notably Tyler McNabb (Assistant Professor of philosophy at Houston Baptist University) and Stephen Law (English philosopher and Reader in Philosophy at Heythrop College in London). The other guests joining are: Josh Parikh, Perry Hendricks, Brett Lunn, Ben Watkins, Justin Schieber, Ben Bavar, and of course myself.
The event will go live right at 11am CST this Saturday, January 28 (prior to that point all that displays at the link below is a black screen with a countdown).
All of the selected participants are familiar with the argument and as such, I won’t spend a lot of time laying it out in the discussion. We want to get right to the meat of it. Nevertheless, I’ve laid out the basic structure of the argument below for anyone interested.
This version of the argument can be found in Alvin Plantinga’s recent book Where the Conflict Really Lies. There he argues that the probability our brains would produce mostly true beliefs given Naturalism & Evolution (N&E) is low. But we normally take for granted that our brains are producing mostly true beliefs (e.g.: my belief that this computer I’m typing on is real, that I really ate breakfast this morning, that Austin is the capital of Texas, that it is currently winter in Texas, etc. etc.). However, if our brains aren’t reliable (they don’t produce mostly true beliefs), then we’ve got a defeater for all the beliefs produced by our brains, including belief in N&E. Belief in N&E therefore shoots itself in the foot and cannot rationally be accepted.
Here it is in premiss/conclusion form. N&E is shorthand for the claim that Naturalism & Evolution is true. R is shorthand for the claim that our brains produce mostly true beliefs.
(2) Anyone who accepts (believes) N&E and sees that (1) is true 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 be rationally accepted.
The success of the argument turns crucially on that first premiss, that the probability our brains produce mostly true beliefs given N&E is low. So how does Plantinga argue this?
It’s first helpful to think about what a belief is given Naturalism. Most naturalists are also materialists with respect to humans. That just means they tend to think humans are purely physical in composition. They do not believe that humans possess an immaterial soul distinct from the body. So what do beliefs amount to on this view? Presumably they will have at least two properties. First, they will have a physical property like a long-standing neural structure in the brain and second, they will have a content. My belief that Austin is the capital of Texas has as its content Austin is the capital of Texas. So, given materialism, beliefs have both a physical property (like a neural structure) and a content.
Now, even if the neurology determines what content arises, it doesn’t seem there’s any reason to think the content that arises, given N&E, would be true or even likely true. All that is required for evolution is that the resulting behavior aid in survival. It’s true that physical properties are selected for because they aid in survival, but it makes no difference whether the content associated with the underlying neurology is true or false. What really matters is the resulting behavior. So long as the behavior is adaptive, it doesn’t matter what content gets associated. The content could be true, it could be false.
Even if the neurology determines what content arises, it doesn’t seem there’s any reason to think the content that arises would be true or even likely true. All that is required for evolution is that the resulting behavior aid in survival.
If all this is true, then Plantinga’s crucial first premiss is true. Given N&E, the probability our brains produce mostly true beliefs is low. So what’s a Naturalist to do at this point? Once they accept this premiss they’ve really only got two options: they can either give up belief in Naturalism or give up belief in Evolution . They cannot sensibly believe both.
Well, that is the basic form of Plantinga’s argument. If it seems rushed, not to worry. I suggest picking up Plantinga’s book if you are interested in a fuller treatment of the argument.
 Plantinga emphasizes that the kind of probability in question is objective (not subjective).  They’ve also got to accept the other premises of the argument, however those are (arguably) less controversial than (1).