Today I will attempt to refute it.
Rather than argue as some have that we can rely on our reason because of tools developed over centuries by a process of cultural evolution, I want to show that even the primitive faculties provided to us by evolution have given us abilities in this regard.
Unlike Plantinga, I certainly do not accept the premise that evolution and naturalism imply a low probability that we can reliably form beliefs.
Let's recall Plantiga's core premise (P1), regarding the probability (P) that we are reliable (R) given naturalism (N) and evolution E).
P1: P(R|E&N) is low - (there is only a low or negligible probability that we have the ability to reliably form true beliefs, given philosophical naturalism and evolution)
Plantinga's argument hinges on the observation that evolution only cares about adaptive behaviours, not whether the beliefs that are associated with those behaviours are true or false. This is true, however it doesn't help to establish P1 unless he can show that true beliefs are no more likely to be adaptive than false beliefs.
This is where I disagree with him. I think that true beliefs are far more likely to be adaptive, and this is why evolution would indeed select for the ability to form true beliefs.
Plantinga makes two major arguments to the contrary. Firstly (and most plausibly, in my view), he outlines the argument I spelled out yesterday. There are many possible motivations for any given adaptive behaviour, but only a tiny minority of these motivations will reflect a true, relevant belief about the world. Since evolution only cares about behaviour, the odds that a true motivation should be chosen seem slim. I will answer this later.
Firstly, I want to get the other major argument out of the way, because it has been brought to my attention that he tends to use it quite a lot when his other arguments are challenged. This latter argument seems to me to be quite obviously incorrect, so I will attempt to dispense with it as finally as I can.
I do not think I am being too unfair when I characterise this argument as the position that beliefs can have no effect on behaviour in a naturalistic world.
Of Course Beliefs Affect Behaviour
Of course beliefs affect behaviour! I find it extremely difficult to conceive of how someone could argue otherwise - after all, why would they argue that beliefs don't affect behaviour (this act of arguing being a behaviour in itself) if they were not motivated by their beliefs in that regard? If Plantinga seriously believes that naturalism does not allow for beliefs to affect behaviour, then he need not bother with EAAN. This should be his argument against naturalism.
The main motivation behind these views is the problem of reconciling the non-physical mental content of our minds with the physical structures comprising our brains. In the naturalistic world view, no non-physical event could ever have an effect on the material world, so it seems to be impossible to conceive of how it could be that something as insubstantial as a belief could set in motion the neural firings that lead to action and behaviour.
It should be said at this point that Plantinga himself need not be committed to this view, and indeed I believe he is not, since he is not a naturalist. He believes that the supernatural can indeed intervene in the physical world, so there's no problem as long as one accepts that the mind is not a purely natural phenomenon. Instead, he argues that a true naturalist should be committed to this position.
One solution to this problem is the suggestion that beliefs and other mental phenomena are not causative of behaviour but merely correlated with it. It could be that physical processes in the brain give rise to both beliefs and behaviours, but that the beliefs are a side effect and not causative of the behaviours. The sense that we experience mental phenomena and then act on those phenomena is an illusion. In reality, our behaviour is already determined by physical neural activity and the corresponding mental experience is just a story we tell ourselves. We are conscious automata. Beliefs, like free will, are an illusion.
Plantinga accepts this interpretation, as it sits well with his belief that the content of beliefs would have no effect on behaviour in a naturalistic world. Since beliefs are only the story we tell ourselves to explain our automatic actions, there is no reason to expect these beliefs to be true. As long as our behaviour is adaptive, our beliefs are irrelevant.
I feel this interpretation is valid to a point, but it's not the full story. I don't believe in free will (at least not in the old fashioned libertarian sense of the world), so in a sense everything we do really is determined by physical processes. It even seems valid to me to view mental phenomena as merely correlates of physical processes, but that's just one way of looking at it.
(It's also fine in my view to see mental phenomena as actually causative. Neither interpretation is more correct than the other, it just determines on how one interprets concepts such as causation.)
But Plantinga takes it too far. He seems to think that since only the physical neural correlates of beliefs (e.g. the particular pattern of neural activity corresponding to that belief) matter from the point of view of evolution, therefore the content or meaning of that belief is irrelevant.
This is simply not true. What he seems to fail to appreciate is that the content of the belief is not independent of the particular physical pattern of neural activity, but that it is intimately bound to it. Different beliefs will correspond to different patterns of neural activity and vice versa. If evolution can select for adaptive physical brain states, then the corresponding mental phenomena are also being selected for implicitly. It really is not too much of a stretch to say that evolution is actually selecting for particular beliefs.
If you want more concrete examples of non-physical "epiphenomena" having a real impact on the physical world just look at the stock market, computer software, fashion trends. Examples abound.
The only question therefore is whether the beliefs evolution selects for are likely to be true or not.
The Adaptivity of Truth
My previous post outlined an argument that there are many more false beliefs that can lead to appropriate adaptive behaviours than true beliefs, and so we should not expect the probability that evolution selects for true beliefs to be high when it selects for the associated behaviours - far more likely that evolution will select a false belief that happens to lead to an adaptive behaviour. This argument may seem superficially convincing, but it is deeply flawed, as I will attempt to show.
When discussing the selection of behaviours, there are actually two quite distinct modes of behaviour selection to consider.
1) The direct selection of specific adaptive behaviours in response to specific stimuli
2) The indirect selection of adaptive behaviour by instead selecting for the ability to dynamically determine appropriate courses of action in response to experience and environmental conditions.
Many behaviours are due to the first kind of selection. We call these behaviours instinctive. All animals exhibit behaviours of this sort, and most exhibit only behaviours of this sort. They manifest as drives to engage in specific activities, but they are not motivated by beliefs about the world.
When a spider hunts down and kills a fly, it does not do so because it believes the fly will be tasty, much less because it believes it needs to eat to live. It does so because this behaviour has been hard-wired into it by evolution.
The same is true when you stand on a glass floor above a vertiginous drop. You may feel fear, but if I ask you whether you believe you will fall, you will deny it. Your discomfort comes from an instinctive response, not a belief. Even if you characterise this instinct as a non-propositional belief, as some do, it's entirely different from the kinds of beliefs Plantinga is talking about.
Plantinga would probably agree with this so far, but ask what this has to do with his argument. Well, I'm about to get to that.
When Plantinga discusses the possible beliefs that evolution might select for in order to produce a desired behaviour (e.g. running away from a tiger), he is making the mistake of treating these beliefs as if they are selected for directly, like instincts. Evolution requires a specific response to a specific stimulus. In order to get the appropriate result, beliefs are simply not required. This scenario is what instincts are for. You don't need a reason to run away from the tiger, you just need to run away from the tiger! You don't have to know that the tiger is going to cause you pain and death, you only need to feel fear and the impetus to get as far away from it as possible.
And yet we do have beliefs and the ability to reason. This allows us to surpass the crude stimulus/response behaviours allowed by instinct alone and so to behave far more intelligently (and so adaptively) than creatures such as spiders. Our ability to deliberate and choose our behaviour allows us to adapt to situations that could never have been foreseen by evolution.
The indirect mode of behaviour selection has come into play.
A hominid running away from a tiger quite likely does have true beliefs about the consequences of being caught. Humans and certain other animals form such beliefs every day, and their behaviour is motivated by the interplay of instinct with this dynamic and ongoing generation of beliefs. The correct question to ask is not, as Plantinga does, whether there are more true or false beliefs that could explain a particular behaviour, but rather to ask whether a reliable system for generating beliefs is more likely to lead to adaptive behaviours than an unreliable one.
Once the problem is explained in this way, it seems perfectly obvious to me that a reliable system would be more adaptive. The behaviours motivated by the beliefs generated by a completely unreliable system are likely to be effectively random, as there will be little correlation between reality and the generated beliefs. It's very unlikely that such a system would confer any evolutionary advantage whatsoever.
Yes, of course such a system could produce adaptive behaviours on occasion, such as in Plantinga's tongue-in-cheek example of the hominid that seeks to pet a fearsome tiger by running away from it, but such unlikely scenarios will certainly be outnumbered by examples of maladaptive behaviour. The same hominid is likely to run away from food, mates and other items of interest. Or suppose he is preoccupied and has no interest in petting tigers on this occasion, so he remains seated as the tiger approaches.
Only the beliefs generated by a reliable system will have any meaningful correlation with reality, and only beliefs which correlate with reality can consistently produce adaptive behaviour and so evolutionary fitness.
In conclusion, there is every reason to suppose that evolution has gifted us with some basic ability to form reliable beliefs in everyday scenarios. Given this basic foundation, we have over many generations built a great number of tools which allow us to perceive truths which were previously hidden from us. We are far wiser now than we were when we had only our evolutionary heritage to draw on, but all of our intellectual achievements to date have as their root the ability, selected for by naturalistic evolution, to form true beliefs reliably.