Plantinga’s Probability Arguments Against Evolutionary Naturalism

Branden Fitelson and Elliott Sober∗
Philosophy Department
University of Wisconsin–Madison
November 20, 1997

Abstract.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) .”1 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.H is “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.H is
“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.P lantinga
further develops the main argument in his unpublished paper “Naturalism Defeated”
(Plantinga 1994).W e 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.T o do this, Plantinga uses Bayes’ Theorem, which says that this
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.A lthough
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.P lantinga says (p. 220, footnote 7) that his probabilities can be
interpreted either “epistemically” or “objectively,” but that he prefers the objective
interpretation.H owever, Bayesians have never been able to make sense
of the idea that prior probabilities have an objective basis.T he 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.T he alternative to
which Bayesians typically retreat is to construe probabilities as indicating an
agent’s subjective degree of belief.T he problem with this approach is that
it deprives prior probabilities (and the posterior probabilities that depend on
them) of probative force.I f 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.

1.1 Proposition R
For the sake of clarity, it is worth spelling out proposition R more precisely.
What does it mean for our psychological mechanisms for forming beliefs to
be “generally” reliable? In his unpublished manuscript, Plantinga says that R
means that “the great bulk” of our beliefs are true (Plantinga 1994, p.2) . Aside
from questions about how beliefs are to be counted, we don’t want to challenge
the truth of this summary statement.Ho wever, it drastically underspecifies
the data that need to be explained.F or the fact of the matter is that our
cognitive mechanisms are reliable on some subjects, unreliable on others, and of
unknown reliability on still others.W e should divide our beliefs into categories