To many, evolution gnaws at their sense of self. If evolution offers a lesson, it seems to be that we’re not only related to other creatures but, like them, are also the product of blind and impersonal evolutionary forces. If humans are just one of many outcomes of natural selection, maybe we aren’t so special after all. You can understand why this doesn’t sit well with many people who think that we came into being differently from other species, as the special goal of a divine intention.

No one infers a god from the simple, from the known, from what is understood, but from the complex, from the unknown, and incomprehensible. Our ignorance is God; what we know is science. – Robert Green Ingersoll.

My main thesis is narrower and, I think, more defensible: understanding reality, in the sense of being able to use what we know to predict what we don’t, is best achieved using the tools of science, and is never achieved using the methods of faith.

IDers argue that such traits, involving many parts that must cooperate for that trait to function at all, defy Darwinian explanation. Therefore, by default, they must have been designed by a supernatural agent. This is commonly called the “God of the gaps” argument, and it is an argument from ignorance. What it really says is that if we don’t understand everything about how natural selection built a train, that lack of understanding itself is evidence for super-natural creation.

The rational scrutiny of religious faith involves asking believers only two questions: How do you know that? What makes you so sure that the claims of your faith are right and the claims of other faiths are wrong?

With the notion of a theistic god and a vernacular notion of “proof” in hand, we can disprove a god’s existence in this way: If a thing is claimed to exist, and its existence has consequences, then the absence of those consequences is evidence against the existence of the thing. In other words, the absence of evidence – if evidence should be there – is indeed evidence of absence.

I argue that in a world where people must support their opinions with evidence and reason rather than faith, we would experience less conflict over issues like assisted suicide, gay rights, birth control, and sexual morality.

If nearly two-thirds of Americans will accept a scientific fact only if it’s not in clear conflict with their faith, then their worldview is not fully open to the advances of science.

But the most important reason to concentrate on religion rather than other forms of irrationality is not to document a historical conflict, but because, among all forms of superstition, religion has by far the most potential for public harm. Few are damaged by belief in astrology; but, as we’ll see in the final chapter, many have been harmed by belief in a particular god or by the idea that faith is a virtue.

Science and religion, then, are competitors in the business of finding out what is true about our universe. In this goal religion has failed miserably, for its tools for discerning “truth” are useless. These areas are incompatible in precisely the same way, and in the same sense, that rationality is incompatible with irrationality.