Truth, rationality, and morality under naturalism, are irrelevant commodities, in and of themselves. The naturalist’s (atheistic materialist’s) concern with truth, reason and morality are stolen obligations – obligations that are not derivable from naturalism.
If minds are the computed product of physics, they output whatever they output. There is no ideal form, perfection, or “truth” outside of what physics produces in any particular instance to compare what physics produces against. Whatever any individual computation of physics outputs with the label “rational” attached is the natural limit of what can be termed “rational”. There’s nothing the individual can compare it against; they are stuck with their own ruler and no means by which to check its length. What is considered “true” can be both X and not-X.
Similarly, morality is just whatever physics says it is. Like a computer programmed to output “3” when asked “what is 1 + 1”, the computer is not in error, it is simply producing the output determined by its program. “3” is only an “error” if one assumes there is some standard outside of that program by which to judge it; under naturalism, there is not. If the physics ends up in some case saying it is moral to behead infidels, then it is moral in that case; if in some other case it says it is immoral to do so, then it is immoral in that case. Under naturalism, what is moral can be both X and not-X; there is no absolute arbiter.
Under naturalism, truth, reason and morality are all relative, subjective commodities (being entirely mental phenomena), housed in a mind produced by forces unconcerned with truth, reason and morality, generated by a process only “concerned” with reproductive success. At the very core, mind cannot be said to have anything whatsoever to do with reason, truth or morality; those are just titles we assign to various output as our particular individual physics commands as those physics pursue reproductive success.
Which brings up the question: why do atheists, materialists and naturalists care whether or not their arguments are rational? Why do they care if what others say is untrue? Why are they concerned with appearing to be “moral” or to have moral cares and considerations? Why bother with any of that at all, considering that the basis of their existence is not assumed to be about any of those things, nor is their any intrinsic reason to care about them under their paradigm?
If life is fundamentally about reproductive success, what’s the point of caring about truth, reason or morality, per se? I find it odd that under a paradigm where those things have no intrinsic or ultimate value in and of themselves, many atheists go to great lengths to demonstrate they are more moral, more rational, and more truthful than theists. Why? Who cares? Are there points being scored somewhere for being moral, truthful, or rational?
No, under atheism/materialism/naturalism, the only points being scored are for producing children, and statistics show that atheists produce less children than theists (something they are often proud of, strangely enough). However, they don’t seem to have read the memo. They still argue and act as if they have some kind of binding, necessary obligation to truth, reason, and morality.