LECTURE #4: Plantinga on Naturalism

I. A Model of Knowledge

A. Knowledge is clearly different from mere true belief of opinion.

One who hits on the truth by a wild guess does not know what he believes.

B. A traditional model of knowledge: justified true belief.

Knowledge is true belief supported by good reasons. In an important article, "Justified True Belief", Edmund Gettier produced a number of counterexamples to this traditional model. For instance, suppose I believe that the market went up yesterday, because I read that it did in the copy of the WSJ. I have true belief, supported by good reasons. However, suppose that my actual copy of the WSJ was replaced by a prankster with a phony copy, full of misinformation (except that, by chance, the phony copy correctly states that the market went up. In this case, I do not know that the market went up -- the truth of my belief is a lucky accident.

C. Contemporary models of knowledge introduce the element of causation and reliability.

Knowledge is a true belief that has been caused by the corresponding fact in the right way, in a way that reliably produces true beliefs.

D. The most popular model adds another element: the absence of "defeaters".

Knowledge can be defeated in two ways: by a rebutting defeater, and by an undercutting defeater.

For example: an alibi is a rebutting defeater of the prosecution's case (a reason for thinking that the accused is innocent), while impugning the reliability of a key witness is an undercutting rebuttal (it doesn't give reason to believe the accused to be innocent, but it casts doubt on the reasonableness of believing him -- on the basis of this testimony -- to be guilty).

Anything that casts doubt on the reliability of one of our cognitive faculties is an undercutting defeater for beliefs produced by that faculty.

An undercutting defeater of belief B is a piece of evidence that raises reasonable doubts about whether B was caused in a reliable, knowledge-conferring way.

Defeaters can themselves be defeated: if my neighbor tells me that he substituted a fake WSJ for the real one, I have a defeater. If I learn that my neighbor has falsely claimed to have pulled this trick many times in the past, then I have a defeater for my defeater.

So, Knowledge = True belief + Reliable mode of caustion + No (undefeated) defeaters

II. The Difference between Lewis's argument and Plantinga's

A. Lewis's argument is what is known as a "transcendental argument".

If naturalism were true, then rational knowledge would be impossible. So, for rational knowledge (knowledge by rational inference) to be possible, naturalism must be false.

This doesn't prove theism (since naturalism & theism aren't the only alternatives), but it does give a reason for thinking naturalism to be false: we do have rational knowledge, so naturalism must be false.

B. Plantinga's conclusion is less amibitious.

He wants to establish: it is impossible to believe rationally in naturalism. Belief in naturalism leads to an epistemic catastrophe (in which nothing we believe is rational).

This isn't a transcendental argument. Naturalism could be true, and we could have rational beliefs, so long as we didn't believe naturalism to be true.

However, it does create obvious problems for the naturalist. At the very least, it should motivate the naturalist to look at alternative worldviews that avoid this catastrophe.

III. Plantinga's Development of the Anti-Naturalist Argument from Knowledge

Five or six reasons to doubt that natural selection favors true beliefs (= "Darwin's Doubt").

  1. Beliefs (mental acts directed toward propositions) may not be causally efficacious at all.
  2. Beliefs may have no effect on our behavior (mere "epiphenomena", inert decorations).
  3. Beliefs may be efficacious, but not in virtue of their content (and, hence, not in virtue of their truth). Example: if I break a glass by reading a poem loudly, the poem is efficacious, but not by virtue of its meaning. The computational theory of mind.
  4. Beliefs could be efficacious, but maladaptive (Stich). The result of random drift, or pleiotropy (genetic linkage to separate, adaptive features).
  5. Beliefs could be efficacious (by virtue of content) and adaptive, but still not reliably true. Belief/desire holism: beliefs do not influence our behaviors by themselves, one by one. Untrue beliefs, if paired with bizarre desires, could be more adaptive than true ones.
  6. Beliefs about abstruse, highly theoretical subjects, could be adaptive for reasons having nothing to do with their truth. Errors about such matters come with no adaptive cost.

IV. Darwin's doubt is an undercutting defeater to any belief we have.

If either (1) we judge the probability P(R/N) to be low, or (2) we are agnostic about the probability P(R/N) (we decide that we can't discover it), then we have reason to doubt the reliability of our own cognitive faculties. This doubt acts as an "undercutting defeater" to defeat any inference we draw.

Plantinga: naturalism cast doubt on the reliability of all of our faculties. Thus, any belief in naturalism results in a defeaters of all our knowledge. So, a naturalist cannot know anything at all.