In recent years, Christian apologists such as Alvin Plantinga have advanced arguments purporting to prove that evolutionary naturalism is a self-refuting worldview. According to these people, if evolution is true and there is no intelligent creator-god, then humans' sensory and rational faculties were created by a blind process that is not concerned with truth or falsity, and therefore those faculties themselves could not reliably detect truth or falsity. The conclusion, as Plantinga and others would have it, is that if we believe evolutionary naturalism is true, we must distrust our own conclusions, including the belief in evolutionary naturalism. In this post, I will show that this argument is not just wrong, it is obviously wrong. An atheist has more than sufficient grounds to believe that their sensory and cognitive faculties are reliable, and it is not just probable but inevitable that a process of naturalistic evolution would result in this.
Over the past century and a half, our scientific study of the world has led to the conclusion that the human species, as well as all other life on this planet, was created by a process of evolution. Briefly described, evolution is a process by which living things that are better suited to survival in their environment tend to reproduce more abundantly, while living things less well suited reproduce less abundantly. The result of this is that genes which have a negative impact on survival tend to fade away, while those which contribute to survival are passed on and become more common, making them liable for further improvement in the next generation.
Our minds and senses, like all other adaptations of living species, were designed by evolution. And like all other adaptations, they could only have persisted to the degree that they aided our survival. If they did nothing but generate false beliefs, then at best, they would not harm our chances of survival, and far more likely would substantially decrease them. In either case, they would soon be eliminated by natural selection - in the latter case because they were an impediment to survival, in the former case because they were simply a waste of energy that could more usefully be spent elsewhere (like the eyes of blind cave fish). (The human brain consumes a substantial fraction of the body's total oxygen and energy consumption. Natural selection could never maintain such a costly adaptation unless it conferred substantial survival benefits.)
Clearly, then, in order for these faculties to persist, they must confer some survival benefit, and it is not difficult to see what that benefit is. What Christian apologists have ignored is that the ability to accurately perceive one's environment and respond appropriately is essential to survival. In this respect, evolution is concerned with the truth or falsity of a creature's beliefs, because while the evolutionary process is blind with respect to method, it is most definitely not blind with respect to results. A creature that could not respond correctly to its environment, or that did so only imperfectly, would be at a significant survival disadvantage compared to one that could perceive more accurately. Therefore, it should be obvious that, all else being equal, evolution will always favor greater accuracy of sensory perception - both the ability to sense the environment with greater fidelity and the disposition to respond correctly to those sensory impressions.
Consider a simple example: a bacterium trying to swim toward a source of nutrition. Suppose this bacterium has chemical receptors on its surface that can detect molecules drifting through the liquid medium all around it. To gain the maximum amount of nutrition, the bacterium needs to be able to sense the gradient - the direction in which nutrient molecules are more concentrated - since that will probably be the direction in which their source is located. Which, then, will have a greater chance of reproducing and passing on its genes - the bacterium that can accurately sense the gradient and move in that direction, or one that is blind to the gradient and strikes out in a random direction?
A bacterium has none of a human being's rich mental life, of course, and apologists such as Plantinga argue that while evolution would select for correct actions, it would not necessarily select for correct beliefs. But though this could be true for creatures whose actions are decoupled from their beliefs, human beings are not like this. If a creature will face more situations in its lifetime than its genes can explicitly program it for - if it cannot live solely by the autopilot of instinct, as human beings cannot and do not - then that creature must perceive its environment correctly in order to respond correctly. Accurate belief is the only sure way to produce correct action.
As an example of this, consider a more complex case: a troop of apes living in a forest, where interactions between individuals are a way of life. This mode of existence would favor a whole slew of new cognitive abilities: recognizing individuals and remembering their status in the group, remembering which group members are likely to reciprocate your favors, determining whether another individual can be bribed or deceived and being resistant to deception in turn, and group cooperation in hunting and defense. These cognitive feats all require sophisticated skills, including long-term memory and the ability to infer the contents of another individual's mind, and any individual ape that did poorly at these tasks would be outcompeted and taken advantage of by those that were superior, if not exiled from the group entirely. On the other hand, even a very imperfect capability to do these things would provide a selective foothold; the better a given ape was at it, the more that ape would prosper, and so once the capability existed at all it would be liable to refinement and improvement through natural selection. It should be clear that in these circumstances no false belief would give selective advantage in the way that true belief would.
Finally, consider a case involving a characteristically human ability: the manufacture and use of tools. Tool-making was a major evolutionary advantage that conferred a significant benefit on the primitive virtuosos that were best at it. However, it also requires even more skills in one's mental toolbox: sensitivity to fine-grained details of the environment, the ability to notice correlations, infer causality, imagine possible futures, classify objects into abstract categories, detect failures, and improve one's technique through practice and testing. None of this would be possible without a sophisticated and highly accurate set of perceptual and reasoning abilities. Again, false beliefs about what the best kind of rock is to chip into tools, or whether a blunt end will be just as good for a spear as a sharp point, or indeed any step of the process, will inevitably put their possessor at a severe disadvantage compared to the hominids who got it right.
Of course, this is not to say evolution will produce perfect sensory perception. It is obvious that we possess no such thing, and there are good reasons why. Evolution is a process of tradeoffs, and takes shortcuts whenever possible; it tends to produce a "good enough" solution rather than a perfect solution. This explains many common errors in human reasoning and perception, such as the urge to anthropomorphize natural phenomena, or our susceptibility to certain kinds of optical illusions. It should be a matter of no dispute that human brains are not perfectly reliable. However, we are not helpless to correct our own perceptual mistakes. Using our superior pattern-recognition abilities, we can perceive when our efforts have failed and alter our plans accordingly. More specifically, when we recognize a defect in our perception, we can overcome it using a prosthesis that compensates for the defect. An optical illusion such as the Muller-Lyer illusion can be overcome by using a physical prosthesis, such as a ruler. More subtle defects in our perception can be corrected by using a mental prosthesis - the scientific method.
Though it is not perfect, it is more than obvious that evolution will produce at least generally reliable mental tools for environmental perception, pattern recognition and abstract reasoning in any intelligent being. In light of this, the burden of proof is now on the presuppositionalists to explain why an evolutionary naturalist should not consider their own beliefs reliable.
In any case, this argument is not uniquely applicable to atheists. Christians have their own defeaters which by any rational reading should force them to believe that their minds and senses are unreliable.
For example, if a Christian believes that the Bible is true, they must believe that there are circumstances under which God will deceive people and cause them to believe lies (2 Chronicles 18:21-22, 2 Thessalonians 2:11-12). But if this is true, how can any Christian know that they are not one of the people God is deluding? By definition, if you are one of those people, you would not know it; a deception is not a deception if the person experiencing it recognizes it as such, and an omnipotent being could easily create a deception that a person could not see through. This means that a Christian must always admit the possibility that any of their beliefs may be delusions sent by God; but this in turn means that a Christian can never have complete confidence in any of their beliefs, including the belief that God sends people delusions or even the belief that Christianity is true. The Christian worldview undermines itself just as totally as Christians claim atheism does.
This conclusion is just a special case of the more general conclusion that belief systems incorporating inscrutable, unlimited supernatural beings can never give sufficient grounds for considering your beliefs justified, since there is always the possibility that those supernatural beings are deceiving you in undetectable ways for unknowable reasons of their own. By contrast, atheism excludes such malevolent possibilities; and while this does not prove atheism true, it does mean that it is consistent and that it provides a sufficient foundation for holding evidence-based beliefs in the first place. We can be, and often are, mistaken, but atheism at least offers us a chance to discover and correct those mistakes, without fear of mischievous supernatural beings thwarting our every attempt at finding out the truth.