(Note: For his more recent moral argument against naturalism, see this post.)
Taking Evolution Away From Atheists and Beating Them With It
...that's the plan, anyway! Plantinga argues that — contrary to expectations — evolutionary theory is friendly to the belief that humanity was intentionally created by God in His image, and fatally undermines the belief that humanity evolved without divine assistance.
- Natural selection only cares (so to speak) about survival.
- Survival is not significantly affected by the truth of beliefs.
- It's unlikely, therefore, that purely natural design would produce minds adept at forming true beliefs.
Natural selection only cares (so to speak) about survival.
Pretty much, yeah.
Survival is not significantly affected by the truth of beliefs.
Here's where things get interesting. You might think it's better — in terms of survival — to have mostly true beliefs than mostly false beliefs. Plantinga questions this assumption by examining five possibilities of how beliefs and behavior relate to each other.
(Note: Early on, he focused on Possibility #5; over time, his focus has shifted to Possibility #1 as the core challenge to naturalistic evolution.)
Possibility #1 — Epiphenomenal Beliefs
He starts by drawing a distinction between beliefs as the neural structures that combine with desires to produce behavior and beliefs as carriers-of-propositional-content. The latter definition of belief is the sort that is true or false and it "can't be a matter of definition that there are neural structures or processes displaying both propositional content and causal efficacy with respect to behavior".2
Neural structures which guide behavior.Natural selection could have designed neural structures to work in ways that produce survival-enhancing behavior, but mental stances are just extra. "[Mental stances] are not causally connected with behavior, then they would be, so to speak, invisible to evolution; and then the fact that they arose during the evolutionary history of these beings would confer no probability on the idea that they are mostly true, or mostly nearly true, rather than wildly false."2
Mental stances which are true or false.
To put it simply: true/false mental stances run free from the neural structures which affect behavior.
Possibility #2 — Same Thing
...except here Plantinga highlights the possibility that mental stances are caused by behavior or that both mental-stances and behavior are caused by a third thing. So there is a connection, but it's still invisible to natural selection.
Possibility #3 — The Form (Not Content) Affects Behavior
I read a poem very loudly, so loudly as to break a glass; the sounds I utter have meaning, but their meaning is causally irrelevant to the breaking of the glass. In the same way it might be that these creatures' beliefs have causal efficacy, but not by way of the content of those beliefs.3Is this a real party trick he does?
Possibility #4 — Belief Content Affects Behavior, Negatively
Behavior-affecting belief content might be like sickle-cell anemia: not helpful, but genetically attached to other traits which are helpful.
Possibility #5 — Belief Content Positively Affects Behavior
The true/false mental stances we take actually do help us survive. But...true beliefs don't necessarily help more than false beliefs!
You see, beliefs have to be combined with desires to produce behavior. As Plantinga points out, "there will be any number of different patterns of belief and desire that would issue in the same action; and among those there will be many in which the beliefs are wildly false."4
True Belief + Desire A -> The Helpful Action
False Belief 1 + Desire B -> The Helpful Action
False Belief 2 + Desire C -> The Helpful Action
False Belief 3 + Desire D -> The Helpful Action
[...and so on]
Since natural selection only cares about the helpful action, it's more likely that one of the many possible false beliefs will be paired up with a matching desire, than that the single true belief will be paired up with its matching desire. Plantinga gives the example of a pre-historic man, Paul, who has the adaptive behavior of running away from tigers, but not because of the belief you might expect:
Perhaps Paul very much likes the idea of being eaten, but whenever he sees a tiger, always runs off looking for a better prospect, because he thinks it unlikely that the tiger he sees will eat him. This will get his body parts in the right place so far as survival is concerned, without involving much by way of true belief.
[...] Or perhaps he thinks the tiger is a large, friendly, cuddly pussycat and wants to pet it; but he also believes that the best way to pet it is to run away from it."4He goes on to give other possibilities of false beliefs being matched up with desires that result in Paul running away from tigers. The point is: natural selection has no reason (so to speak) to favor one scenario over another, so long as Paul's body survives to reproduce.
It's unlikely, therefore, that purely natural design would produce minds adept at forming true beliefs.
If this conclusion is successful, then a person who believes in naturalism + evolution has a defeater for the belief that her belief-forming faculties are reliable. This, in turn, constitutes a defeater for all of her beliefs...including the belief that naturalism is true.
Meanwhile, a person who believes in a creator God + evolution isn't stuck with an across-the-board defeater for his beliefs. In other words, evolution mixed with this kind of theism is stable, but evolution mixed with naturalism catastrophically destructs.
1. See Chapter 12 of his book Warrant and Proper Function. For a shorter and more recent restatement, see the Introduction of Naturalism Defeated? which is a collection of essays on this very topic, edited by James Beilby.
2. Plantinga, P. (1993). Warrant and proper function. New York, New York: Oxford University Press. p. 223.
3. Ibid. p. 224.
4. Ibid. p. 225.