Plantinga's argument is aimed at metaphysical naturalism or roughly the view that no supernatural beings exist. Naturalism is typically conjoined with evolution as an explanation of the existence and diversity of life. Plantinga's claim is that one who holds to the truth of both naturalism and evolution is irrational in doing so. More specifically, because the probability that unguided evolution would have produced reliable cognitive faculties is either low or inscrutable, one who holds both naturalism and evolution acquires a "defeater" for every belief he/she holds, including the beliefs associated with naturalism and evolution.