Plantinga’s Probability Arguments Against Evolutionary Naturalism

In Chapter 12 of Warrant and Proper Function, Alvin Plantinga constructs two arguments against evolutionary naturalism, which he construes as a conjunction E&N . The hypothesis E says that “human cognitive faculties arose by way of the mechanisms to which contemporary evolutionary thought directs our attention (p. 220).” With respect to proposition N , Plantinga (p. 270) says “it isn’t easy to say precisely what naturalism is,” but then adds that “crucial to metaphysical naturalism, of course, is the view that there is no such person as the God of traditional theism.” Plantinga tries to cast doubt on the conjunction E&N in two ways. His “preliminary argument” aims to show that the conjunction is probably false, given the fact (R) that our psychological mechanisms for forming beliefs about the world are generally reliable. His “main argument” aims to show that the conjunction E&N is self-defeating — if you believe E&N , then you should stop believing that conjunction. Plantinga further develops the main argument in his unpublished paper “Naturalism Defeated” (Plantinga 1994). We will try to show that both arguments contain serious errors. 1 The Preliminary Argument Plantinga constructs his preliminary argument within a Bayesian framework. His goal is to establish that Pr(E&N |R) — the probability of E and N , given R — is low. To do this, Plantinga uses Bayes’ Theorem, which says that this ∗Gordon Barnes, Matt Davidson, Ellery Eells, Malcolm Forster, Patrick Maher, Ernan McMullin, Alvin Plantinga, and Dennis Stampe provided valuable criticisms and suggestions. We are grateful to them for their help. 1All page references are to Warrant and Proper Function (viz., Plantinga 1993), unless otherwise noted. 1 conditional probability is a function of three other quantities: Pr(E&N |R) = Pr(R |E&N) · Pr(E&N) Pr(R) . Plantinga says you should assign to Pr(R) a value very close to 1 on the grounds that you believe R (p. 228). He argues that Pr(R |E&N) is low. Although Plantinga doesn’t provide an estimate of the prior probability Pr(E&N), he says that it is “comparable” to the prior probability of traditional theism (TT ) (p. 229), meaning, we take it, that their values aren’t far apart. This last claim should raise eyebrows, not just among evolutionary naturalists who reject the idea that their theory and traditional theism are on an equal footing before proposition R is taken into account, but also among critics of Bayesianism, who doubt that there is an objective basis for such probability assignments. Plantinga says (p. 220, footnote 7) that his probabilities can be interpreted either “epistemically” or “objectively,” but that he prefers the objective interpretation. However, Bayesians have never been able to make sense of the idea that prior probabilities have an objective basis. The siren song of the Principle of Indifference has tempted many to think that hypotheses can be assigned probabilities without the need of empirical evidence, but no consistent version of this principle has ever been articulated. The alternative to which Bayesians typically retreat is to construe probabilities as indicating an agent’s subjective degree of belief. The problem with this approach is that it deprives prior probabilities (and the posterior probabilities that depend on them) of probative force. If one agent assigns similar prior probabilities to evolutionary naturalism and to traditional theism, this is entirely consistent with another agent’s assigning very unequal probabilities to them, if probabilities merely reflect intensities of belief. Although Plantinga’s Bayesian framework commits him to making sense of the idea that the conjunction E&N has a prior probability, his argument does not depend on assigning any particular value to this quantity. As Plantinga notes (p. 228), if Pr(R) ≈ 1 and Pr(R |E&N) is low, then Pr(E&N |R) also is low, no matter what value Pr(E&N) happens to have.