Abstract

Metaphysical naturalism can be taken, roughly, to be the view that there is no God and nothing beyond nature. Alvin Plantinga has argued that naturalism, in this sense, is self-defeating. More specifically, he argues that an evolutionary account of human origins gives the naturalist compelling reasons for doubting the reliability of human cognitive faculties, and thus compelling reasons for doubting the truth of any of his beliefs, including naturalism itself. In my dissertation, I examine in detail the main objections so far presented to Plantinga's argument, and show that they all fail. I use this discussion to arrive at a deeper understanding of the argument than has been found in the literature so far, and on that basis I develop a novel objection to it. ;Let N be metaphysical naturalism, E the claim that human cognitive faculties have evolved by way of the mechanisms studied in contemporary evolutionary theory, and R the claim that human cognitive faculties are generally reliable at producing true beliefs. Plantinga's argument rests on two main theses. The first, which I call the Probability Thesis, asserts that the objective conditional probability P is either low or, at best, impossible to determine. The second, which I call the Defeater Thesis, asserts that any naturalist who accepts the Probability Thesis has a rationally compelling reason to doubt R . ;After setting out and clarifying the key notions of this argument, I investigate in depth the Defeater Thesis. I provide a comprehensive survey of the main published objections to the Defeater Thesis. Using John Pollock's work on the classification of defeaters, I argue that it is essential to distinguish between two distinct interpretations of the way in which accepting the Probability Thesis is meant to give the naturalist a defeater for R. On one interpretation, the significance of the Probability Thesis is meant to be that it functions as a premise in a probabilistic inference with the conclusion that R is false. On the other interpretation, the significance of the Probability Thesis is meant to be that, if the naturalist accepts it, then he cannot rule out the possibility that the evolutionary process that resulted in the creation of humans was inadequate at filtering out unreliable cognitive faculties. I argue for this second interpretation, and I show that a number of objections to the Defeater Thesis fail precisely because they mistakenly presuppose the first interpretation