That's Thomas Aquinas's teaching. "Since human beings are said to be in the image of God in virtue of their having a nature that includes an intellect, such a nature is most in the image of God in virtue of being most able to imitate God" (ST, I, q. 93, a. 4). "Only in rational creatures is there found a likeness of God which counts as an image. . . . As far as a likeness of the divine nature is concerned, rational creatures seem somehow to attain a representation of that type in virtue of imitating God not only in this, that he is and lives, but especially in this, that he understands" (I, q. 93, a. 6).
One of the best arguments for theism is that this theistic doctrine of the human mind as created by God in His image provides the necessary support for the validity of human thought, including the validity of modern science. If we embrace Naturalism--the view that nothing exists except Nature, and so there is no God or nothing like God--we are caught in self-contradiction: if human thought originated not from a divine Mind but from the irrational causes of Nature, then we cannot trust our minds as reliable, and thus we cannot trust our belief in Naturalism. Naturalism destroys itself by destroying the rationality of believing in Naturalism, or anything else. Insofar as science--including evolutionary science--depends on the validity of human thought, and insofar as theism is the indispensable support for trusting in the validity of human thought, science is not only compatible with theism, science depends upon theism.
When I was a college freshman, I first learned this argument from reading C. S. Lewis's book Miracles, and it impressed me as one of the most powerful arguments for theism. 25 years later, as a college professor, I heard Alvin Plantinga present a more sophisticated version of the same argument when he gave a seminar presentation at my university on his paper "Naturalism Defeated."
I agree with Lewis and Plantinga that theistic religion and evolutionary science are compatible, because while science requires a methodological naturalism, it does not require the metaphysical naturalism that denies theism. I also agree with them that the theistic belief in the divine creation of the human mind can support our confidence in scientific reasoning, including evolutionary reasoning. But I disagree with their claim that evolutionary science is self-contradictory or incoherent without theism.
Plantinga has presented his argument in various writings, including the paper "Naturalism Defeated." A short summary of the argument is in the Introduction to Naturalism Defeated?: Essays on Plantinga's Evolutionary Argument against Naturalism, edited by James Beilby (Cornell University Press, 2002), which includes critiques of the argument by various authors and Plantinga's reply to the critiques. One of the best critiques in this volume is Evan Fales, "Darwin's Doubt, Calvin's Calvary." Plantinga's argument also comes up in his debate with Daniel Dennett in their recent book Science and Religion: Are They Compatible? (Oxford University Press, 2011).
Without going into all the technical details that are so dear to the hearts of analytic philosophers, here's the core of the argument in four steps.
(1) If we understand naturalism as the belief that there is no God--no supernatural Mind outside of Nature that created Nature--and if the naturalist is also a Darwinian who believes that evolutionary science explains the origins of all life, including human life, then the Darwinian naturalist must believe that the mental faculties of human beings originated through evolution by natural selection favoring those random mutations that were adaptive for survival and reproduction.
(2) Natural selection rewards adaptive behavior and punishes maladaptive behavior. But natural selection does not care about the truth or falsity of an animal's beliefs. If beliefs produce adaptive behavior, they will be rewarded by natural selection regardless of whether the beliefs are true or false. Therefore, the evolution of adaptive behavior in our prehistoric ancestors did not guarantee or make it probable that our cognitive faculties would be reliable in generating mostly true beliefs.
(3) From this it follows that the Darwinian naturalist has no good reason to trust his cognitive faculties as reliable. But then it follows that the Darwinian naturalist has no good reason to feel confident that his belief in naturalism is true. Consequently, Darwinian naturalism is self-defeating in that it contradicts itself.
(4) Darwinian science--and science generally--can escape this self-defeating position by rejecting naturalism and accepting theism, because theism believes that our human minds were created by God in His image such that we can understand the intelligible world He has created, and therefore we can be confident in the reliability of our divinely created cognitive faculties. This is compatible with evolutionary science, because we can assume that God has guided the evolutionary process, perhaps by causing those random mutations that He foresaw as facilitating the evolution of the human mind in its capacity for correctly understanding the world. This is also necessary for evolutionary science because it supports our confidence in the validity of human reason and escapes the incoherence of naturalism.
Like Lewis, Plantinga is a theistic evolutionist. He accepts the truth of Darwinian evolutionary science, because he believes that God could have used the evolutionary process to carry out his creative plan, which required miraculous acts by God to guide the evolutionary process towards human beings as having minds manifesting the intellectual image of God.
Dennett and other critics of Plantinga have objected that science requires naturalism, because in all scientific inquiry, the scientist must assume that everything can be explained by natural regularities or laws that are never broken by miraculous events. Otherwise, every regularity or law of science would have to be stated with the qualification unless God performs a miracle and suspends His laws. Such arbitrariness in the order of nature would make natural science impossible. Indeed, the very idea of nature as the stable order of the universe would be denied by the thought that everything is the momentary product of God's arbitrary will.
Plantinga's response is to argue that natural science requires methodological naturalism, but not metaphysical naturalism. Metaphysical naturalism denies that there is any divine reality beyond nature, and that's the naturalism that renders evolutionary science self-defeating. But methodological naturalism is the assumption that we can explain everything in purely natural terms without invoking anything supernatural. The theist can accept this, because the theist assumes that God has created the order of nature and that He will not interrupt that order arbitrarily. Although miracles are possible, the natural scientist does not normally have to be open to miraculous events in the practice of science. This combination of methodological naturalism and theistic belief in the miraculous power of God is what Plantinga calls "Augustinian science"--the sort of science that Augustine would endorse.
While I agree that theism and evolution can be compatible, I don't agree that theism is absolutely necessary for evolutionary science, because I don't agree that combining Darwinism with metaphysical naturalism creates an incoherent position.
The weak link in Plantinga's argument for metaphysical naturalism as self-defeating is step 2, where he assumes that adaptive behavior is completely unrelated to true belief. The evidence of evolutionary history suggests that evolution produces cognitive faculties that are reliable but fallible. The mental abilities of animals, including human beings, are fallible because evolution produces adaptations that are good enough but not perfect, and this results in the mental fallibility that is familiar to us.
But despite this fallibility, the mental faculties cannot be absolutely unreliable. Even Plantinga concedes (in his debate with Dennett) that in the evolution of animals, "adaptive behavior requires accurate indicators" (70). So, for example, a frog must have sensory equipment that allows him to accurately detect flies so that he can catch them with his tongue. Similarly, the immune system of the human body must accurately indicate the presence of foreign bodies and then accurately devise responses to destroy the invaders. But then Plantinga argues that these accurate indicators don't require true beliefs. It's not clear that the frog has any beliefs. And the human being is probably not even aware of what the immune system is doing exactly.
What this shows, of course, is that much of an animal's adaptive behavior through mental activity does not require conscious reasoning at all. But for those animals who do develop some capacity for conscious reasoning--and most preeminently human beings--the accuracy of this conscious reasoning will be important for adaptation. As Evan Fales argues in response to Plantinga, the highest mental capacities of human beings are so biologically expensive in terms of the investment of energy they consume that it is implausible that evolution would have produced them unless they improved the ability of human beings to track the truth about themselves and their environment. Again, this is going to be fallible, but it's implausible that human beings could be naturally evolved for being in a state of complete and perpetual delusion.
And yet that's exactly what Plantinga asks us to imagine--that we could have been naturally evolved for a state of complete and perpetual delusion. Having taken this step of radical Cartesian skepticism, he then tells us that the only escape from such skepticism is to assume that God would never allow this to happen. But as always is the case for the Cartesian skeptic, this all depends on imagining scenarios that are utterly implausible and unsupported by even a shred of evidence.
Only those who find Cartesian skepticism plausible will find Plantinga's argument plausible. Indeed, Plantinga's argument originated with Descartes.
For example, consider this possibility for human evolution suggested by Plantinga:
So suppose Paul is a prehistoric hominid; a hungry tiger approaches. Fleeing is perhaps the most appropriate behavior: . . . this behavior could be produced by a large number of different belief-desire pairs. . . .
Perhaps Paul very much likes the idea of being eaten, but when 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. . . . or perhaps he thinks the tiger is a regularly recurring illusion, and, hoping to keep his weight down, has formed the resolution to run a mile at top speed whenever presented with such an illusion; or perhaps he thinks he is about to take part in a sixteen-hundred-meter race, wants to win, and believes the appearance of the tiger is the starting signal; or perhaps . . . . Clearly there are any number of belief-cum-desire systems that equally fit a given bit of behavior.
Well, yes, these weird stories are all logically possible, as the philosophers like to say. But they are also utterly implausible, because there is no evidence that anything like this could have happened in human evolution. Plantinga's claim that there is no clear connection between adaptive behavior and true beliefs in evolutionary history depends on fantasies of his imagination unsupported by evidence. He has to do that, because if he actually looked at the evidence of human evolutionary history bearing upon the emergence of human mental faculties, he would be faced with evidence for the evolution of human cognitive capacities for exploring the world that are generally reliable, even if fallible.
He would also see evidence that human beings can use their fallible mental capacities to correct their mistakes. After all, the very capacity to recognize our fallibility presupposes our skill for reliable reasoning about ourselves and our world. There are good reasons to believe that this can be explained as an outcome of a natural evolutionary process in which divine intervention was not necessary.
Some related posts can be found here, here, here, and here.
One might also consider here the evidence that Michelangelo's Sistine Chapel's painting of "The Creation of Adam" reflects Michelangelo's knowledge of the neuroanatomy of the brain.
Does this support Plantinga's argument? Or does it suggest that what we see as the "image of God" in the human brain is a purely natural product?