Final answer:
Alvin Plantinga's defeater of naturalistic evolution, known as the Evolutionary Argument Against Naturalism, suggests that naturalistic evolution undermines the reliability of our cognitive faculties, casting doubt on the belief system that is built upon them.
Step-by-step explanation:
The fatal flaw, or defeater, of naturalistic evolution according to Alvin Plantinga centers on the reliability of cognitive faculties. Plantinga argues that if naturalistic evolution is true, then it gives rise to doubt about the reliability of our cognitive faculties, which are the product of this evolutionary process. Given that evolution is concerned only with adaptive behavior and survival, not with the truth or falsity of beliefs, the probability that our cognitive faculties are reliable under a naturalistic evolutionary scenario is low. This concept, known as Evolutionary Argument Against Naturalism (EAAN), suggests that, if both naturalism and evolution are true, we could never be sure that our beliefs are true because our cognitive faculties could have evolved for survival, not for truth.