This is the weekly Q & A blog post by our Research Professor in Philosophy, Dr. William Lane Craig.
I have a question about Alvin Platinga's Evolutionary Argument Against Naturalism. He argues, I think, that the probability that my cognitive factulties are reliable (R) given that naturalism and evolution (N&E) are true, is low or inscrutable. And that, furthermore, if I accept that P(R/N&E) is low or inscrutable, then I have a defeater for any belief produced by my faculties, including N&E.
I have found this argument persuasive for many years now (nearly seven). But it recently occurred to me that the inference from "the P(R/N&E) is low or inscrutable" to "therefore I have a defeater for N&E" may be analogous to inferring from "the probability that my beliefs are reliably true (75% or so) given that Descartes' Demon exists, is low or inscrutable" to "therefore, I have a defeater for the belief that Descartes' Demon exists."
If Descartes' Demon existed, then I would have a reason to doubt any belief I held (except, according to Descartes, that I exist), because he may be deceiving me, right? But what about the belief that Descartes' Demon exists? Such a demon could trick me into falsely believing that he doesn't exist, but he couldn't trick me into falsely believing that he *does* exist. Thus it seems that when I consider whether Descartes' Demon exists, it may lead me to doubt the truth of all my other beliefs, but how could it lead me to doubt the existence of the demon itself, for if there were no demon, then (obviously) there would be no demon to trick me.
Similarly perhaps, if N&E are true, then it would lead me to doubt all my other beliefs, but not N&E itself.
How should we understand this? Do you think these cases are analogous? I'm familiar with a good deal of Plantinga's published work and I've watched him present this argument at least a dozen times via youtube (and once in person), and I don't recall ever hearing this objection raised. I would be very interested to hear what you have to say, for I'm not sure how to respond to this.
Dr. William Lane Craig’s Response
Plantinga’s argument is intended to show that evolutionary naturalism cannot be rationally affirmed. It might be true, but it cannot be rational to affirm it as such. Here is how he formulates the argument in his recent Where the Conflict Really Lies (Oxford University Press, 2011, pp. 344-5):
1. P (R | N&E) is low.
(The probability that our cognitive faculties are reliable, given evolutionary naturalism, is low.)
2. Anyone who accepts (believes) N&E and sees that P (R | N&E) is low has a defeater for R.
(Anyone who believes evolutionary naturalism and sees that (1) is true has a defeater for believing that our cognitive faculties are reliable.)
3. Anyone who has a defeater for R has a defeater for any other belief she thinks she has, including N&E itself.
(Anyone who has a defeater for the belief that his cognitive faculties are reliable has a defeater for any other belief he has.)
4. If one who accepts N&E thereby acquires a defeater for N&E, N&E is self-defeating and can’t rationally be accepted.
(If anyone who believes evolutionary naturalism thereby acquires a defeater for evolutionary naturalism, then evolutionary naturalism is self-defeating and cannot be rationally believed.)
Conclusion: N&E can’t rationally accepted.
(Evolutionary naturalism cannot be rationally believed.)
Most of the discussion of Plantinga’s argument revolves around (1). Your objection, Joe, is unusual, challenging instead (3). You say that just as one can’t be deceived into thinking one is deceived by an evil demon, so one can’t be deceived into thinking that evolutionary naturalism is true.
Here’s my take on your objection: Premiss (3) doesn’t stipulate why one’s cognitive faculties are unreliable. It just states that if one thinks, for whatever reason, that one’s cognitive faculties are unreliable, then one can’t have confidence in anything he believes. The defeater could be anything—the belief that one is a brain in a vat, or that one is dreaming, or that one’s beliefs are selected for survival, or what have you—the point is that if you really think your faculties are unreliable then you can’t count on them for anything you believe.
What your objection amounts to, Joe, seems to be that the belief “An evil demon exists” is highly probable relative to the fact that my beliefs are formed by an evil demon. But that is trivially true. Similarly, for my belief that naturalistic evolution is true—that’s highly probable if naturalistic evolution has formed my beliefs—and equally trivial. The point remains that I could never have a good reason to think that I am deceived by an evil demon.
What you may have overlooked is that a defeater needn’t show the targeted statement to be false. There are rebutting defeaters and there are undercutting defeaters. A rebutting defeater shows the targeted statement to be false. An undercutting defeater merely removes any warrant for thinking the targeted statement to be true. In Plantinga’s argument it’s all about undercutting defeaters. You don’t show the Cartesian demon hypothesis or evolutionary naturalism to be false; you merely show that we can have no warrant for believing in a Cartesian demon or in evolutionary naturalism.
 While in England, I received the following comments from Lydia McGrew, which came too late for me to incorporate into my answer:
“Plantinga could reply that there is a difference between the Cartesian skeptic’s argument and his argument at this point: The Cartesian skeptic does not have to assert that a Deceiver *does* exist or that we are *justified* in believing that a Deceiver exists. The Cartesian skeptic is claiming that the problem is the mere _possibility_ that the Deceiver exists. In theory, the Cartesian skeptic could be a solipsist who regards himself as merely making philosophical arguments in his head for his own entertainment. He doesn’t have to assert, as a premise of his argument, anything about the world outside his own head.
“This should really be the Cartesian skeptic’s response to a self-defeat argument, not the one you [Joe] suggest for him: ‘Thus it seems that when I consider whether Descartes’ Demon exists, it may lead me to doubt the truth of all my other beliefs, but how could it lead me to doubt the existence of the demon itself, for if there were no demon, then (obviously) there would be no demon to trick me.’ Instead, the Cartesian skeptic should say, ‘Yes, of course I doubt the existence of the Deceiver. I doubt the existence of anything outside my own head. The possibility of a Deceiver also means that I could not be justified in believing in the Deceiver himself, but so what? I’m not actually arguing that the Deceiver exists, you know. I’m just arguing that I’m not justified in believing in the external world.’
“In contrast, the naturalist has to assert definite, positive premises, such as that science works successfully without invoking anything supernatural, that scientific progress has occurred, and the like. These are usually the kinds of premises that the naturalist will use to argue empirically that naturalism is true. At a minimum, the naturalist has to believe premises such as that the external world exists, that scientific instruments are real, that the scientific articles he reads really exist and describe real experiments, and so forth. So overall, the naturalist has to be committed to a lot more definite, positive premises than the Cartesian skeptic has to be committed to.” (Personal communication, March 13, 2015)
This post and other resources are available on Dr. William Lane Craig's website: www.reasonablefaith.org