In Warrant and Proper Function, Alvin Plantinga establishes a basic set of four criteria by which one can judge a belief to have warrant, explicates those criteria with respect to various types of knowing, and defends the claim that naturalism in epistemology requires supernaturalism in metaphysics. The four criteria Plantinga gives for establishing warrant are as follows. First, Plantinga argues that one must form a given belief with “cognitive faculties” functioning properly with respect to the belief being formed, including any “defeaters” which may override faculties ordinarily aimed at forming true beliefs (e.g., survival instincts). Second, in his view, one must produce this belief in a “cognitive environment” conducive to one’s cognitive faculties. Third, Plantinga introduces the concept of a design plan, each part of which consists of a triple set of circumstance, response, and purpose (i.e., function) by which someone or something is “designed” to respond to an input. The part of one’s design plan by which one forms a given belief must have as its purpose “the production of true beliefs” (40) in order for that belief to have warrant. In other words, the belief must be produced by faculties which have true belief production as their proper function. Fourth, the probability of that part of the design plan (functioning in an amenable environment) to produce a true belief must be high. 

In chapters 3 through 9, Plantinga takes what he describes as “a whirlwind tour of some of the main modules of our epistemic establishment” (48). These chapters examine the following “modules”: knowledge of self, knowledge of others, perceptual knowledge, a priori knowledge, knowledge by induction, and the probability of knowledge. In chapter 10, Plantinga launches into a discussion of evidence supporting a given warrant (at least in cases where “evidence” is applicable, since he has argued that several types of belief are basic). He also reminds the reader that in addition to having evidence for a given belief, one must also have formed the belief according to proper function. Finally, in the last two chapters, Plantinga begins to flesh out his claim in chapter 2 that “naturalism in epistemology can flourish only in the context of supernaturalism in metaphysics” (194). After rejecting the naturalism of Pollock, Millikan, Bigelow, and Pargetter with respect to proper function on the one hand, and discounting the idea of accepting supernaturalism merely for the sake of argument on the other, Plantinga suggests that one who accepts naturalism in epistemology should also accept supernaturalism in metaphysics. Plantinga examines several evolutionary accounts of proper function, some of which reject the formation of true beliefs as part of the human design plan, and some of which contend that human survival implies the formation of true beliefs as part of the human design plan. In both cases, however, Plantinga argues that the probability of such a design plan formed solely by naturalistic processes is low, at least with respect to some major types of true belief, and concludes that supernaturalism is the more rational option.