Alvin Plantinga’s claims revolve around individuals who argue against naturalist concepts such as the rejection of the existence of supernatural beings like God, but instead, support theories such as that of evolution. Plantinga claims that such individuals have a self-undermining set of beliefs. Plantinga uses various abbreviations to illustrate his claims, whereby naturalism is represented as N, the evolutionary theory as E, evolutionary naturalism as NE, and the reliability of cognitive faculties as R. Plantinga’s arguments begin with a Probability Thesis, which is followed by the Defeater Thesis. The Probability Thesis states “the probability that R is true given the truth of N and E is either low or inscrutable” (Fales’ 434). The Defeater Thesis, on the other hand, holds that “an evolutionary naturalist who is apprised of the Probability Thesis has a defeater for her belief in R” (Fales’ 434). This paper seeks to explain Plantinga’s Defeater Thesis and the basis for his beliefs, including his claim that the naturalist has a defeater for R and other beliefs. The Probability Thesis holds that individuals are the product of an undirected evolutionary process, in the event that both N and E are true. However, the probability that a being with this sort of attribution would have reliable cognitive faculties is either low or inscrutable. Hence, the assumption that the probability of individuals’ cognitive faculties are reliable given N and E are true is low or enigmatic (Fales’ 435). Plantinga’s argument for the Defeater Thesis is based on the evaluation of instances whereby agents are observed to be contradicting their beliefs. Plantinga also bases his arguments on the correlation between such individuals and evolutionary naturalists, who are facing a defeater for their belief in R, owing to their awareness of the Probability Thesis. Plantinga uses four analogical cases to argue for the Defeater Thesis. These cases include the Widget Cases, the Freudian Theist Cases, the space Radio case, and the Brain-in-a-vat Case (Fitelson and Sober 119). According to the Widget Cases, the individual is presented with two contradicting arguments about a situation that she is familiar with, causing her to doubt her beliefs. This is illustrated when the individual walks into a factory that appears to be manufacturing red-colored widgets. However, the individual is informed by the shop superintendant that the widgets are red due to the effect of light on them, but majority of them are actually not red in color. Based on this revelation, the individual acknowledges that there is a low probability that the widgets are red in color. However, the individual later encounters the vice-president of the production line, who claims that the superintendant cannot be trusted with the information provided earlier regarding the color of the widgets. This information causes the individual to have a dilemma about the color of the widgets. Plantinga regards such a situation as causing the individual to be doubtful about her vision’s ability to provide her with accurate information regarding the color of the widgets, which causes the individual to have a defeater of color beliefs about widgets observed in an assembly line (Fitelson and Sober 120). The other cases are also based on different situations th