Since the publication of my first book, The End of Faith, I have had a privileged view of the "culture wars" -- both in the United States, between secular liberals and Christian conservatives, and in Europe, between largely irreligious societies and their growing Muslim populations. Having received tens of thousands of letters and emails from people at every point on the continuum between faith and doubt, I can say with some confidence that a shared belief in the limitations of reason lies at the bottom of these cultural divides. Both sides believe that science is powerless to answer the most important questions in human life. And how a person perceives the gulf between facts and values seems to influence his views on almost every issue of social importance -- from the fighting of wars to the education of children.

This rupture in our thinking has different consequences at each end of the political spectrum: Religious conservatives tend to believe that there are right answers to questions of meaning and morality, but only because the God of Abraham deems it so. They concede that ordinary facts can be discovered through rational inquiry, but they think that values must come from a voice in a whirlwind. Scriptural literalism, intolerance of diversity, mistrust of science, disregard for the real causes of human and animal suffering -- too often, this is how the division between facts and values expresses itself on the religious right.

Secular liberals, on the other hand, tend to imagine that no objective answers to moral questions exist. While John Stuart Mill might conform to our cultural ideal of goodness better than Osama bin Laden does, most secularists suspect that Mill's ideas about right and wrong reach no closer to the Truth. Multiculturalism, moral relativism, political correctness, tolerance even of intolerance -- these are the familiar consequences of separating facts and values on the left.

It should concern us that these two orientations are not equally empowering. Increasingly, secular democracies are left supine before the unreasoning zeal of old-time religion. The juxtaposition of conservative dogmatism and liberal doubt accounts for the decade that has been lost in the United States to a ban on federal funding for embryonic stem-cell research; it explains the years of political distraction we have suffered, and will continue to suffer, over issues like abortion and gay marriage; it lies at the bottom of current efforts to pass anti-blasphemy laws at the United Nations (which would make it illegal for the citizens of member states to criticize religion); it has hobbled the West in its generational war against radical Islam; and it may yet refashion the societies of Europe into a new Caliphate. Knowing what the Creator of the Universe believes about right and wrong inspires religious conservatives to enforce this vision in the public sphere at almost any cost; not knowing what is right -- or that anything can ever be truly right -- often leads secular liberals to surrender their intellectual standards and political freedoms with both hands.

If we were to discover a new tribe in the Amazon tomorrow, there is not a scientist alive who would assume a priori that these people must enjoy optimal physical health and material prosperity. Rather, we would ask questions about this tribe's average lifespan, daily calorie intake, the percentage of women dying in childbirth, the prevalence of infectious disease, the presence of material culture, etc. Such questions would have answers, and they would likely reveal that life in the Stone Age entails a few compromises. And yet news that these jolly people enjoy sacrificing their firstborn children to imaginary gods would prompt many (even most) anthropologists to say that this tribe was in possession of an alternate moral code, every bit as valid and impervious to refutation as our own. However, the moment one draws the link between morality and human well-being, one sees that this is tantamount to saying that the members of this tribe must be as fulfilled, psychologically and socially, as any people in human history. The disparity between how we think about physical health and mental/societal health reveals a bizarre double standard: one that is predicated on our not knowing -- or, rather, on our pretending not to know -- anything at all about human flourishing.

Imagine that there are only two people living on earth: We can call them "Adam" and "Eve." Clearly, we can ask how these two people might maximize their well-being. Are there wrong answers to this question? Of course. (Wrong answer #1: They could smash each other in the face with a large rock.) And while there are ways for their personal interests to be in conflict, it seems uncontroversial to say that a man and woman alone on this planet would be better off if they recognized their common interests -- like getting food, building shelter and defending themselves against larger predators. If Adam and Eve were industrious enough, they might realize the benefits of creating technology, art, medicine, exploring the world and begetting future generations of humanity. Are there good and bad paths to take across this landscape of possibilities? Of course. In fact, there are, by definition, paths that lead to the worst misery and to the greatest fulfillment possible for these two people -- given the structure of their brains, the immediate facts of their environment, and the laws of Nature. The underlying facts here are the facts of physics, chemistry, and biology as they bear on the experience of the only two people in existence.

As I argue in my new book, even if there are a thousand different ways for these two people to thrive, there will be many ways for them not to thrive -- and the differences between luxuriating on a peak of human happiness and languishing in a valley of internecine horror will translate into facts that can be scientifically understood. Why would the difference between right and wrong answers suddenly disappear once we add 6.7 billion more people to this experiment?

Granted, genuine ethical difficulties arise when we ask questions like, "How much should I care about other people's children? How much should I be willing to sacrifice, or demand that my own children sacrifice, in order to help other people in need?" We are not, by nature, impartial -- and much of our moral reasoning must be applied to situations in which there is tension between our concern for ourselves, or for those closest to us, and our sense that it would be better to be more committed to helping others. And yet "better" must still refer, in this context, to positive changes in the experience of sentient creatures.