Can You Trust a Brain Produced by Blind Evolution?

Darwin's Troubling Doubt

In 1881, just a year before his death, Charles Darwin wrote a letter to William Graham that contained a remarkable confession:

Darwin had stumbled upon a profound problem—one that contemporary philosopher Alvin Plantinga has developed into what may be the most powerful argument against naturalistic atheism: the Evolutionary Argument Against Naturalism (EAAN). If our cognitive faculties evolved purely to promote survival rather than to discover truth, why should we trust them to tell us anything true about reality—including the truth of evolution itself?

This lesson explores Darwin's doubt and its devastating implications for the combination of naturalism and evolution. We'll see that if atheistic naturalism is true, we have a powerful reason to distrust our own reasoning—including our reasoning in favor of naturalism. It is, as Plantinga puts it, a "defeater" for naturalism.

Setting Up the Argument

Defining Our Terms

Before we can understand the argument, we need to clarify several key concepts:

Naturalism is the philosophical view that nature is all there is—there is no supernatural realm, no God, no immaterial souls. Everything that exists is part of the natural, physical world studied by science.

Evolution, as we're using it here, refers specifically to the claim that our cognitive faculties (our belief-forming mechanisms) arose through unguided natural processes—primarily random genetic mutation and natural selection—without any supernatural guidance or design.

Reliability of our cognitive faculties means they tend to produce true beliefs rather than false ones. A reliable thermometer tells you the actual temperature; reliable cognitive faculties tell you true things about yourself and your world.

The crucial question is: Given naturalism and unguided evolution, what is the probability that our cognitive faculties are reliable?

What Natural Selection Actually Selects For

Natural selection is, by definition, blind to truth. It doesn't care whether an organism's beliefs are true—only whether its behavior promotes survival and reproduction. As Darwin noted, natural selection operates on "any modification which is profitable to an individual under any conditions of life."

Consider a simple example. Imagine two early humans, Adam and Eve:

• Adam believes (correctly) that tigers are dangerous and should be avoided. He flees when he sees a tiger.
• Eve believes (falsely) that tigers are gods who must be served by running away from them as fast as possible. She also flees when she sees a tiger.

Both Adam and Eve exhibit the same survival-promoting behavior—running away. From evolution's perspective, both sets of beliefs are equally "fit." Natural selection cannot distinguish between them because selection operates on behavior, and the behavior is identical.

This point is crucial: there are vastly more false beliefs that could produce adaptive behavior than true beliefs. For almost any true belief that leads to survival-promoting behavior, we can imagine countless false beliefs that would produce the same behavior.

Plantinga's Argument in Detail

The Formal Structure

Alvin Plantinga's Evolutionary Argument Against Naturalism can be stated as follows:

1. The probability that our cognitive faculties are reliable, given naturalism and unguided evolution, is low or inscrutable (we can't know what it is).

2. If someone believes naturalism and evolution, and sees that (1) is true, then they have a defeater for the reliability of their cognitive faculties.

3. If they have a defeater for the reliability of their cognitive faculties, they have a defeater for any belief produced by those faculties.

4. But their belief in naturalism is produced by their cognitive faculties.

5. Therefore, they have a defeater for their belief in naturalism.

6. Naturalism is self-defeating: if you believe it, you have good reason to give it up.

Why the Probability Is Low

Let's examine premise (1) more carefully. Why think the probability of reliable cognitive faculties is low given naturalism and evolution?

First, consider what beliefs are on a naturalist view. If naturalism is true, beliefs must be brain states—physical configurations of neurons. But physical systems are selected for their physical properties, not their propositional content.

A tiger causes a certain pattern of neural firing when perceived. Evolution selects for whatever neural pattern leads to flight behavior. But the content of the belief—"this tiger is dangerous"—is causally irrelevant on strict naturalism. What matters is the neurophysiology, not the meaning.

This leads to four possible relationships between belief and behavior:

Epiphenomenalism: Beliefs are byproducts of brain states but don't actually cause behavior. On this view, evolution doesn't select beliefs at all—only brain states. The content of beliefs is completely irrelevant to survival.

Semantic epiphenomenalism: Beliefs have physical properties that cause behavior, but the content (meaning) of beliefs doesn't cause anything. Evolution selects the physical properties of beliefs, not their truth or falsity.

On either epiphenomenalist view, there's no reason to think evolution would produce true beliefs rather than false ones. Truth is simply not in the job description.

Even if we reject epiphenomenalism and grant that belief content does affect behavior, we face the problem illustrated earlier: for any true belief that produces adaptive behavior, there are countless false beliefs that would produce the same behavior. Without guidance toward truth, why would evolution "choose" the true one?

Responses and Replies

Objection: True Beliefs Are More Useful

Some argue that true beliefs are generally more useful for survival than false ones, so evolution would tend to produce true beliefs.

But this isn't obvious at all. Consider these cases:

• Overactive agency detection: Early humans who believed every rustling bush concealed a predator survived better than those who investigated carefully. False positives (believing there's a lion when there isn't) are less costly than false negatives (believing there's no lion when there is). Evolution would favor paranoid, error-prone cognition over careful, truth-tracking cognition.

• Self-deception: Research suggests that mild self-deception about our abilities, attractiveness, and prospects may be psychologically adaptive. Those who overestimate their abilities may be more likely to take risks, attract mates, and lead groups—even though their beliefs are false.

• In-group bias: Believing that your tribe is superior and other tribes are inferior promotes group cohesion and willingness to fight. These beliefs are often false but evolutionarily advantageous.

Far from showing that evolution produces true beliefs, these examples suggest that evolution often produces systematically false beliefs when falsehood promotes survival.

Objection: Science Works, So Our Faculties Must Be Reliable

Another objection holds that the success of science demonstrates the reliability of our cognitive faculties. If we could get things so right about physics and biology, our minds must be trustworthy.

But this commits what Plantinga calls "Cosmic Begging of the Question." How do you know science works? You have to use your cognitive faculties to evaluate the success of science. If those faculties are unreliable, you can't trust your assessment that science works.

It's like trying to verify that a thermometer is accurate by using that same thermometer to check its readings. The circular reasoning is inescapable.

Objection: Theism Has the Same Problem

Some argue that theists face a parallel problem: how do you know God made your cognitive faculties reliable? Maybe God is a deceiver.

But this misunderstands the argument. Plantinga isn't claiming to prove that human cognitive faculties are reliable. He's pointing out that naturalism provides a defeater for believing they are reliable—a reason internal to naturalism itself for doubting our faculties.

Classical theism doesn't face this problem. If God is perfectly good and truthful, and He made us in His image to know Him and His world, then we have positive reason to expect our cognitive faculties to be reliable—at least in the domains for which they were designed.

God's reliability gives us confidence in our own—not absolute confidence (we're finite and fallen), but sufficient confidence to trust reason as a gift from the God of truth.

Implications and Applications

The Self-Defeating Nature of Naturalism

If Plantinga's argument succeeds, naturalism is in a peculiar position: believing it gives you good reason to stop believing it. It's not that naturalism is proven false, but that it cannot rationally be believed.

This is worse than being false. A false belief could at least be rationally held if the evidence seemed to support it. But a self-defeating belief cannot be rationally held even if there were evidence for it. The moment you believe it, you have reason to give it up.

The atheist who accepts both naturalism and evolution is in the position of someone who discovers that their belief-forming mechanisms were designed by a malicious demon to produce false beliefs. Would they continue trusting those mechanisms? If not, why trust mechanisms produced by blind, purposeless processes with no interest in truth?

Using the Argument in Conversation

This argument can be presented conversationally:

The goal is not to "win" but to plant a seed of doubt about the coherence of naturalism—to show that the atheist's confidence in reason may be undermined by their own worldview.

The Christian Advantage

Christianity has no such problem. We believe our cognitive faculties were designed by an all-knowing, all-good God to enable us to know truth about ourselves, our world, and our Creator. This doesn't mean we're infallible—sin has affected every part of us, including our minds. But it does mean that the basic reliability of reason is grounded in the character of God, not in blind chance.

The universe is rationally ordered because it reflects the mind of its rational Creator. Our minds can understand the universe because both were made by the same God. The "fit" between mind and world is not a cosmic accident but a gift of grace— common grace extended to all humanity, enabling science, philosophy, art, and the pursuit of truth.

Being made in God's image means, among other things, that we have rational souls capable of knowing truth. Our minds are not accidents but reflections—finite, imperfect, but genuine reflections—of the infinite Mind who made us.

Conclusion

Darwin's doubt should trouble every naturalist. If our minds are products of blind evolution, selected for survival rather than truth, we have no reason to trust them— including our belief that naturalism is true. The combination of naturalism and evolution provides its own defeater.

This doesn't prove Christianity true. But it does show that naturalism has a serious internal problem—a crack in its foundations that cannot be repaired without moving beyond naturalism itself.

Christianity offers what naturalism cannot: a ground for trusting reason. If God exists and made us to know truth, then our confidence in reason is warranted. The theist can trust their cognitive faculties because they were designed by a trustworthy God.

In the end, the choice is stark: either our minds are gifts from God, designed for truth—or they are accidents, produced by processes indifferent to truth. Only one of these options gives us reason to trust our minds. Only one provides firm ground for the pursuit of knowledge, science, and wisdom.