Alvin Plantinga's formulation of the argument is here. I'll try to summarize it as I understand it.
- Naturalistic evolution selects for traits that tend to lead to survival.
- Some true beliefs about the natural world lead to survival.
- Some false beliefs can also lead to survival if those beliefs lead to behavior that promotes survival.
- The probability of a given survival-promoting belief being true is low or inscrutable.
Conclusion: If naturalism is true then our cognitive faculties are unreliable, then we can't know whether naturalism is true or false. Plantinga states this is
an argument for the irrationality of N, not for its falsehood
A person holds the true belief that there is water to the south. That belief drives the person south to where the water is.
Another person in the same location, however, believes that there are monsters to the north, west, and east, and is driven south towards the water by his false belief.
Yet another person believes that to the south there is an attractive mate and a beanstalk that reaches to the heavens. He's driven south towards the water by his false beliefs.
There is no reason that natural selection should favor true beliefs over false ones when the false ones lead to greater survival.
Person A is thirsty and believes there is water to the south (true). Person A believes thirst is a sign that his body is dehydrated and needs water (true). Once he drinks, he feels energized and believes the water quenched his thirst (true).
Person B is thirsty and believes thirst is a sign from a distant alien that the alien wants to communicate (false). Person B also believes that water is the means by which he can talk back to the alien (false). When person B gets to the water and drinks, he hears kind and encouraging voices in his head that he believes are the alien (false). After drinking, he feels energized and believes the alien gave him power (false).
I know that's a ridiculous example, but it makes the point. Why should natural selection favor Person A's true beliefs over person B's false beliefs?
I suppose you could say that if Persons A and B were to build worldviews based around their experience, Person A's worldview would more closely match up with reality. But their worldviews wouldn't necessarily be mutually exclusive. Perhaps Person C could believe that thirst is a sign of possible dehydration (true) and that it's a sign from an alien (false).
Natural selection may favor the true belief inasmuch as it affects survival, but it does not always select against the false belief.