Impressed by the growing explanatory power of the natural sciences of his time, the philosopher David Hume called upon his colleagues to abandon the armchair, turn their attention to empirical evidence, and "hearken to no arguments but those which are derived from experience… [to] reject every system of ethics, however subtle or ingenious, which is not founded on fact and observation." This was over two hundred years ago, and unfortunately not much changed in academic philosophy until about the last decade or two. Pushing past a barrier also associated with Hume—the infamous is-ought or fact-value distinction—a growing number of philosophers have finally begun arguing that our theories should be informed by our best current empirical accounts of how the human minds works, and that an ethical system that posits or requires an impossible psychology should be treated with suspicion.
One of the more robust and relevant bits of knowledge about human psychology that has emerged from the cognitive sciences is that we are not rational minds housed in irrational, emotional bodies. Metaphors like that of Plato's rational charioteer bravely struggling to control his irrational, passionate horses appeal to us because they map well onto our intuitive psychology, but they turn out to be ultimately misleading. A more empirically accurate image would be that of a centaur: rider and horse are one. To the best of our knowledge, there is no ghost in the machine. We are thoroughly embodied creatures, embedded in a complex social and culturally-shaped environment, primarily guided in our daily lives not by cold calculation but hot emotion; not conscious choice but automatic, spontaneous processes; not rational concepts descended from the realm of Forms but rather modal, analogical images.
So, the ironic result of adopting a scientific stance toward human morality is to lay bare the impossibility of a purely scientific morality. The thoroughly rational, evidence-guided utilitarian is as much of a myth as the elusive Homo economicus, and equally as worthy of our disdain. Evolution may be utilitarian, guided solely by considerations of costs and benefits, but the ruthlessly utilitarian process of bio-cultural evolution has produced organisms that are, at a proximate level, incapable of functioning in a completely utilitarian fashion, and for very good design reasons. Because of rational evolutionary considerations, we cannot help but react irrationally to unfair offers in the Ultimatum Game, challenges to our honor, or perceived threat to our loved ones or cherished ideals. We are culturally-infused animals guided largely by automatic habits, barely conscious hunches, profoundly motivating emotions, and wholehearted commitment to spooky, non-empirical entities ranging from human rights to the Word of God to the coming proletarian Utopia.
Science, of course, is so powerful and important because it represents a set of institutional practices and thinking tools that allow us to, qua scientists or intellectuals, bootstrap ourselves out of our immediate perceptions and proximate psychology. We can understand that the earth goes around the sun, that wonderful design can be the product of a blind watchmaker, or that the human mind is, in an important sense, reducible to biological processes. This gives us some helpful leverage over our evolved psychology, and I join many in thinking that this more accurate knowledge about ourselves and our world might allow us to devise—and maybe come to embrace—novel ethical commitments that could lead to more satisfying lives and a more just world. But let's not lose sight of the fact that science cannot bootstrap us out of our evolved minds themselves. The desire to bring about a more equitable, fair and peaceful world is itself an emotion, an ultimately irrational drive grounded in commitment to ideals like human dignity, freedom and well-being that we've inherited—in stripped-down, theologically minimalistic form—from the cultural-religious traditions into which we've been born. In their latest, liberal iterations, these ideals are rather odd—very few cultures have embraced diversity and tolerance as ethical desiderata, for instance—and are far from being universally embraced even in our contemporary world.
So, the myth that we secular liberals have emerged into a neutral place where we stand freed of all belief and superstition, guided solely by rationality, evidence and clearly-perceived self-interest, is something that needs to be retired. It is simply not the case that secular liberalism, grounded in materialist utilitarianism, is the inevitable and default worldview of anyone who is not stupid, brainwashed or uneducated, and thinking so seriously impedes our ability to understand people in earlier historical periods, from other cultures, and even ourselves.
Acknowledging this does not entail wallowing in postmodern relativism or blindly marching to a fundamentalist beat. Scientific inquiry, in its broad sense, is so wildly effective at giving us reliable information about the world that to seriously defend any other method of inquiry as superior—or even equal—is simply perverse. There is also arguably a pragmatic case to be made that secular liberalism is the best worldview that humans have ever come up with, or at least that individuals given a choice tend to preferentially gravitate toward it. In any case, it is our value system, and the very nature of evolved human psychology makes it impossible for us not to want to defend human dignity or women's rights and, when appropriate, impose them on others. But recognizing the limitations of reason allows us to articulate and defend such values in a more effective way. It also allows us to better understand, scientifically, problems such as the causes of religious violence, the roots of persistent international conflicts, or moral challenges such as balancing our folk intuitions about personal responsibility with a neuroscientific understanding of free will. The science of morality requires us to, in the end, get beyond the myth of a perfectly objective scientific morality.