One of the great things about arguments for gods and the supernatural, is that you can always look back at them and find new problems. Alvin Plantinga’s arguments are especially lovely in this regard. Having been recently been thinking of Wittgenstein’s private language arguments, it occurred to me that somewhere in there is a rebuttal to Plantinga’s evolutionary argument against naturalism.

The evolutionary argument against naturalism argues that if both evolution and naturalism are true, then we cannot trust our own rational faculties, and therefore cannot trust our belief that evolution and naturalism are true. The reasoning goes that naturalistic evolution does not specifically produce true beliefs, but rather produces adaptive beliefs. An adaptive belief does not need to be true, it just needs to produce adaptive behavior. For example, rather than believing that you should run away from a tiger because it will eat you, you might believe that you should run away from a tiger because that’s the best way to pet the tiger (Plantinga’s example). The number of false beliefs that produce adaptive behavior is much larger than the number of true beliefs that produce adaptive behavior. Therefore, most beliefs are probably false.

There are numerous issues with this argument, a few of which you might be shouting at the screen. From a scientist’s perspective, Plantinga appears to be ignorant of how evolution actually works. Evolution does not necessarily produce the most adaptive traits, certainly not immediately. If you have false but adaptive beliefs at one point in time, it is questionable whether those beliefs would continue to be adaptive when your descendants find themselves in slightly different environments. Also, Plantinga ignores that brain efficiency is an adaptive trait. I would imagine that a brain which produces true beliefs via reasoning is far more efficient than a brain that produces false but adaptive beliefs via some mysterious yet reliable process.



Ah, but Plantinga is a philosopher, and specifically the kind that will ignore practical details in favor of abstraction. Plantinga can just say it’s theoretically possible that we have brains that reliably and efficiently produce false and adaptive beliefs. It’s theoretically possible (and therefore all but certain) that naturalism will produce such brains. Somewhere out there in the multiverse are countless Boltzmann brains that know the best way to respond to tigers in all situations, and yet believe that the ultimate goal of all this behavior is to pet the tiger.

Here is where private language arguments come in. If a brain somehow produces correct behavior reliably and efficiently, on what basis can we say that its beliefs are false? According to the private language argument, the content of a belief or experience is defined by the behavior it produces.

For example, you might point at a strawberry and say that it’s “red”. And suppose you’re learning English for the first time, you look at the strawberry and identify “red” with the color sensation you experience when you see a strawberry. It doesn’t really matter if I instead called it “roja” or “赤”, it’s just a word we use, which is dependent on our language. It also doesn’t matter if your experience of “red” is actually different from my experience–for instance, if your red looks like my green. What matters is behavior, such as the behavior of being able to group items together by color.

So suppose that you had a brain that reliably decides how to behave around a tiger, and believes that the purpose of this behavior is to pet the tiger. And say that you communicate this belief to someone else, saying you’re “trying to pet the tiger”. In this language, “petting the tiger” clearly does not at any point involve literally touching the tiger softly with your hands. Instead it appears to involve avoiding danger from the tiger. So if that’s what “petting the tiger” means, are we so sure that this adaptive belief is false?

I suspect that in practice, if we have false but adaptive beliefs regarding danger avoidance, it would look like a phobia. Why am I afraid of spiders? That’s an experience I can’t express. Maybe the reason I can’t express it is because I am trying to pet the spider, and believe the best way to pet it is to recoil from it and smash it with a paper towel.

Of course, private language arguments are not universally accepted among philosophers, and I suggest regarding them skeptically. However, they certainly add to the problems in Plantinga’s evolutionary argument against naturalism.

This argument was inspired in part by The Barefoot Bum, who in distant memory made a similar argument.