What many people don’t seem to realize is that the whole concepts of truth and purpose are biblical ideas that don’t fit well into a purely evolutionary system. Let me explain.
What it comes down to is that evolutionary naturalism can’t reliably lead you to truthful reasoning. If our ability to reason came about by natural selection and genetic mutation, it can’t be trusted to give us true thoughts. Any thought could just as well be false as true, because that’s how natural selection and mutation work.
The principal function or purpose of our cognitive faculties is not that of producing true or near true beliefs, but instead that of contributing to survival by getting the body parts in the right place and by perceiving survival and threat properly. If our cognitive faculties are the result of natural selection and genetic mutation, we have no basis to presume they are giving us mostly reliable information. At best it’s 50-50. And if that’s the best, then I have reason to be suspicious of every thought. It could just as well be false as true, and I have no mechanism by which to determine otherwise. I cannot rationally accept my own reasoning processes.
What evolution underwrites is only (at most) that our behavior is reasonably adaptive to the circumstances in which our ancestors found themselves, and therefore it doesn’t guarantee true or mostly true beliefs. Our beliefs might be mostly true, but there is no particular reason to think they would be: natural selection is not interested in truth, but only in appropriate behavior. Evolution without God gives us reason to doubt two things: (a) that a purpose of our cognitive systems is that of serving us with true beliefs, and (b) that they do, in fact, furnish us with mostly true beliefs.
Atheist philosophers agree. Nietzsche, Nagel, Stroud, Churchland, and Darwin, all nontheists, concur that naturalistic evolution gives every reason to doubt that human cognitive faculties produce for the most part true beliefs.
Thomas Nagel: “If we came to believe that our capacity for objective theory (e.g., true beliefs) were the product of natural selection, that would warrant serious skepticism about its results.”
Barry Stroud: “There is an embarrassing absurdity in [naturalism] that is revealed as soon as the naturalist reflects and acknowledges that he believes his naturalistic theory of the world. … I mean he cannot say it and consistently regard it as true.”
Patricia Churchland: “Boiled down to essentials, a nervous system enables the organism to succeed in the four Fs: feeding, fleeing, fighting, and reproducing. The principal chore of nervous systems it to get the body parts where they should be in order that the organism may survive. … Improvements in sensorimotor control confer an evolutionary advantage: a fancier style of representing is advantageous so long as it is geared to the organism’s way of life and enhances the organism’s chances of survival. Truth, whatever that is, definitely takes the hindmost.”
Charles Darwin: “With me the horrid doubt always arises whether the convictions of man’s mind, which has been developed from the mind of the lower animals, are of any value or at all trustworthy. Would any one trust in the convictions of a monkey’s mind, if there are any convictions in such a mind?”
Nietzsche: “Only if we assume a God who is morally our like can ‘truth’ and the search for truth be at all something meaningful and promising of success. This God left aside, the question is permitted whether being deceived is not one of the conditions of life.”
In other words, the biological and chemical processes of the brain, by themselves, give every reason to doubt that our human reasoning can be counted on. Any atheist who subscribes to evolutionary naturalism but also believes there is such a thing as truth borrows the idea from Christianity, but the atheist has no grounds to stand on because he believes we came about by selective and mutational processes geared toward survival, not abstractions such as truth.
What about purpose? Again, an evolutionary naturalist has to borrow any notions of purpose from Christianity as well. Nature is concerned with cause and effect. In naturalism, cause and effect is the only constant. In evolutionary naturalism, there is only natural selection, where there is no purpose, and mutation, where there is no purpose. These processes are not totally random, but by the same token, you can’t derive “purpose” from biology. Evolution is highly organized—it’s not random in any sense. It’s not neutral, but it’s not determined, either. But to have true purpose there has to be personal intent. The fact is that in evolutionary naturalism, science can only give a partial explanation regarding to purpose. In theism and Christian, there is sufficiency of explanation.
The bottom line is that truth and purpose belong to and are justified in a theistic system like Christianity. Without Christianity, searching for truth and assuming purpose are tentative at best, and are more accurately described as atheism borrowing Christian ideas to explain science without God. It’s ultimately self-defeating.