Global skeptical claims are those that call virtually everything we think we know into question. Two of the most common are the evil genie and the brains-in-vats scenarios. The idea behind such claims is that they introduce situations where our experiences would be exactly the same as they are even though virtually everything we experience is a massive illusion. And if our experiences would be identical, then there's no way to test whether our experiences are veridical or whether the skeptical scenario is true.
Descartes, in establishing his philosophy, proposed a hyperbolic doubt for methodological purposes -- that is, he said, "Let's doubt everything we can and see if there's some level we can't doubt". So he doubted that his senses give him reliable information about the physical world, he doubted that he was awake since it was possible that he was dreaming, etc. When he got to a priori knowledge, such as mathematics, he argued that it's possible that there be an evil genie who manipulates his thought processes whenever he tries to add two numbers together so that he comes up with the wrong answer every time. Of course Descartes wasn't suggesting this as a real possibility, or even that we could doubt such things on a practical level, he was just saying that it's logically possible to doubt it. He ultimately got to the ground level with his cogito: I can't doubt that I'm doubting. Doubting is a form of thought and thought requires a subject who is doing the thinking. As such, I must exist. I think therefore I am (cogito ergo sum).
Now this fails for a very obvious reason. If we're willing to consider the possibility of the evil genie, why can't we just say the genie is also manipulating our thought processes when we try to derive the conclusion of the cogito? Yes it seems as if I can't doubt that I'm doubting, but maybe that's just the evil genie having his way with me. Once the genie is proposed, it applies to everything, including Descartes's argument.
The brains-in-vats scenario is sometimes used in science-fiction. The idea is that we are all disembodied brains being manipulated by scientists or aliens or something to think we are interacting with objects and other people in a physical world. This was used in the Matrix movies, although there they weren't disembodied brains. John Pollock begins his Contemporary Theories of Knowledge with a cute little short story illustrating the problem here. A man finds scientists taking people's brains out of their heads and hooking them up to electrodes. The man is discovered and is told that the people don't know the difference because they're programming them to think that their lives are continuing on without interruption as they had before; they can't tell the difference. The upshot is that, just as the man thinks they're going to do the same thing to him, the scientist laughs and says, "Oh no. We did it to you three months ago." Then they let him go.
Now global skeptical claims are fascinating and they play a huge role in the theory of knowledge. Michael Williams uses them as the main method in establishing his epistemology in his brilliant Problems of Knowledge. But no one takes them seriously as actual possibilities. For whatever reasons, we just don't feel threatened by them. Yes, it's possible that my experiences of the physical world and other people are all illusory, but why should I think so? Simply pointing to the possibility doesn't really make them realistic options. They're just bizarre stories that someone made up. We can certainly use them to further our concept of knowledge, but that's all.
The reason I'm bringing this up is that one of the criticisms given to Alvin Plantinga's Evolutionary Argument against Naturalism (EAAN) is that it is just one more form of global skepticism, and so should be treated the same way. Philosophers already have ways of dealing with such claims, and even if they didn't they wouldn't need them to reject the claim as nothing more than an interesting puzzle.
There's some truth to this: Plantinga's argument is that if naturalism is true, the likelihood that our beliefs would be mostly true is low or inscrutable. Therefore, for any particular belief, regardless of how reasonable it seems, the probability that it be true is low or inscrutable. And of course, naturalism is itself a belief. Therefore, if naturalism is true, belief in naturalism would be irrational. I'll go into more detail about that in future posts, but for now I'll just point out that the unreliability of our cognitive faculties would amount to a form of global skepticism.
There's more to the story though. The problem with traditional global skepticism is that it calls everything into question a priori. Before we have the right (deontologically) to accept any particular belief, we have to show that the belief in question is not subject to the skeptical claim. To do this, however, would require some form of argument -- and that argument would be under the same cloud as the belief it seeks to defend since the skeptical claim would equally apply to it. There's no way out.
However, everyone (except me) accepts that Plantinga is an externalist, and part of the strength of externalist epistemologies is their ability to avoid global skepticism. A belief constitutes knowledge if it is connected in the right way to its object. Thus, if you believe that there is a tree in front of you because there is a tree in front of you, you know it. The global skeptical questions only come into play when we ask second order questions (do you know that you know there is a tree in front of you?). But you do not have to solve that problem before you can know that there is a tree in front of you. Thus externalist epistemologies don't really solve global skepticism so much as they bypass it.
Moreover, Plantinga denies that all beliefs have to be believed for a reason before they constitute knowledge (he calls his epistemology Reidian foundationalism after Thomas Reid); rather, he maintains that some beliefs are properly basic, i.e. they are simply given and we are justified or warranted in accepting them (thus they constitute knowledge) until we have a reason to doubt them. As such, they are not beyond doubt, they can potentially be shown to be false, they are just innocent until proven guilty.
With these two points in mind, Plantinga's EAAN is significantly different from classical global skepticism. First, we do not have to have a reason for a belief if it is properly basic, and such a belief can constitute knowledge even if we don't know that we know it. We are justified, or our beliefs are warranted, up until the point where we have a reason for thinking them to be false. The EAAN provides just such a reason: if naturalism is true, then it is improbable or inscrutable that any given belief would be true. After this, the EAAN has the same effect as the more traditional global skeptical arguments: any reason you can give for a particular belief is itself subject to the EAAN and is therefore not trustworthy. There is no stopping the rot once it's started. Indeed, part of the genius of Plantinga's argument is that it amounts to a global skeptical argument that arises from within externalism. Not to mention the fact that by saying that belief in our cognitive faculties' reliability is warranted until we have some reason to deny it Plantinga is also able to ward off a tu quoque argument being constructed against theism.
Another significant difference is that the other global skeptical claims involve scenarios that are logically possible but that we don't take seriously. Plantinga's, however, involves a scenario that is actually believed by many people, namely naturalism, and even those of us who don't believe it tend to take it seriously (that is, we don't consider it as crazy as the evil genie or brains-in-vats scenarios). It's like if someone came up with an argument that if theism is true, it leads irrevocably to the evil genie scenario. If the argument were sound it would be much more than just an interesting puzzle.
A third difference cuts the other way: traditional global skepticisms posit situations where it is extremely probable, almost certain, that our cognitive faculties are unreliable. Plantinga's EAAN merely argues that it is either less than 50% probable that they are reliable, or if we feel we cannot ascribe any probability, inscrutable. We can certainly modify the traditional scenarios to make them more parallel, but the point is that in their traditional formulation they are stronger than Plantinga's argument.
A fourth difference is that the traditional global skepticisms do not allow for any way out. The brains-in-vats suggestion applies to everybody. But the EAAN allows for a way out, since it only holds if naturalism is true. We can avoid it by simply rejecting naturalism. Since naturalism entails the non-existence of God or any supernatural agency, it follows that in order for us to have knowledge of anything there must be a God or some sort of supernatural agency (although "supernatural" comes with a lot of baggage, so perhaps we could come up with another term that doesn't have as many connotations).
Two final points: first, Plantinga's argument only applies to those who have heard it; the naturalist who hasn't heard the EAAN or a similar one (such as J. R. Lucas's Gödelian Argument against physical determinism) does not have a reason to reject any particular belief. So it's not the case that one has to affirm the existence of God in order to have knowledge. The claim is that there must be some supernatural agency in order for us to have knowledge, not that we have to recognize that there is a supernatural agency. The problem here is very similar to axiological (moral) arguments for the existence of God. The point of these arguments is not that one has to consciously believe in God in order to be a moral person or believe in objective moral truths. Rather the claim is that one is being inconsistent in believing that there are objective moral truths without an objective anchor for them which transcends individual people and cultures. Of course atheists do not deny that murder is immoral, the argument just seeks to show that this is inconsistent with atheism.
Second, when we ask what the supernatural agency in question is, Plantinga immediately points to God. I think instead that the agency is the individual human being. We are supernatural agents, and rational thought is a supernatural process. I would argue further that this ultimately requires God's existence via a less direct route, but that's a post for another day.
(cross-posted at Quodlibeta)