Jeff has a post up about Carl Sagan's statement "Better by far to embrace the hard truth than a reassuring fable." As a counterexample, he offers the fact that the story of Jesus would be valuable in inspiring ethical conduct even if it were proven scientifically that Jesus never lived.
But I don't think that the example Jeff is using does what he wants it to do. It seems quite possible to extract the benefits of Jesus as an ethical model without believing that he was a historically real person -- much as we can be inspired to lead a better life by the example of the tortoise who outran the hare, or by Gandalf. Certainly many theologians would contend that Jesus' teachings are meaningless if he was not real (and not really the Son of God). But even if we grant that premise, it is apparent that on a practical level, dealing with ordinary non-theologian people, the logical contradictions wouldn't impair the pragmatic efficacy of treating the story of Jesus like any other fictional story with a moral. So one can believe that Jesus didn't exist and yet not have to reject the story as irrelevant (indeed, this is my approach to much of what's in the Bible).
The real question, then, is: if we prove that Jesus never existed, should (or may) people continue to believe otherwise? For some people, I would say yes. There's a great diversity in the human psyche, so for some, but not all, people, belief in the historical reality of Jesus may be the optimal strategy for leading a happy and moral life. Of course, taking this pragmatic angle opens up the question of how we prove that Jesus never existed. "Jesus never existed" is not so much a Truth as a theory that is (in this hypothetical scenario) most useful in making sense of historical data, and thus recommends itself to people whose need to have an explanation of a certain set of historical information outweighs their dependence (if any) on belief in Jesus for happiness and morality.
I haven't read Sagan, but anti-fable and anti-pragmatism statements like his often rest on an implicit pragmatism. Rather than showing that belief in the truth is good for its own sake, the argument is that you can't fool yourself forever, and eventually your fable will get you in trouble -- as when Stalin was caught unprepared because of his insistence on believing that Hitler would honor the non-agression pact (my honors thesis was an exploration of how this type of problem plagued Soviet policy toward the Aral Sea). Basically, it amounts to the (important) argument that truth is more useful than many proponents of crude pragmatism realize (an argument also made by one of the original Pragmatists, John Dewey). Their error lies in attributing too high a utility to truth and too easy a separation between truth and fable, often springing from mistakenly assuming that the ends people's theories about the world must serve are mostly the same for everyone.
Stentor Danielson, 11:16,
[David Western] increasingly believed humans (and their farming activities) and wild animals (and their habitat) could co-exist and benefit from each other. Western became a leading advocate for involving local communities in conservation efforts.
... Similar to what has happened in the United States, much of Kenya's grasslands has turned into thick shrub. On the other hand, as more people have acquired land titles, they have helped curb deforestation by planting trees and terracing their farms.
"It's a surprising conclusion," said Western. "More people, less erosion."
... "When you take the human element out of the [national] park, you always have a slump in diversity," he said.
This shouldn't be all that surprising. But we've focussed so much on the destructive aspects of human impacts on the environment (for good reason) that concerned people tend to see our species as inevitably a blight on the landscape, defiling the purity of nature. Exceptions are sometimes made for hunter-gatherers, who are thought of as "natural." I'd speculate that the classical liberal tradition has contributed as well by raising "leave it alone" to the status of the highest moral duty toward "it" ("it" being other humans in the classical liberal tradition of freedom to be the ideal automomous individual). This attitude has provided fodder for critics of environmentalism (the folks over at the Ayn Rand Institute are masters of this genre) to caricature environmentalists as anti-civilization and even anti-humanity.
Stentor Danielson, 17:19,
28.7.03
Henry Farrell over at Crooked Timber brings up the idea of transhumanism -- the desire to transcend the limits of human nature through chemical, biological, digital/cybernetic, etc. means. He concludes that the notion seems really creepy and wrong, but he can't say precisely why. I don't have a good argument against transhumanism (or a strong confidence in the correctness of the "that's creepy" reaction that I share with Farrell). But I thought I'd take a stab at figuring out why it seems so creepy.
The human experience thus far has been largely one of working within limits. The human psyche and cultures seem quite well adapted to coming up with incredible results while working within some set of rules. These rules are what make much of our activity meaningful -- a football game would make no sense if there were no rules as to how it should be played. Many of our most important creative works gain their appeal from going so far while constrained by sometimes artificially severe rules (my favorite example is some of the complex models, such as anatomically correct insects, that can be made in purist -- one square, no cutting -- origami).
The rules are frustrating, to be sure (and indeed, that's the point). So it's no surprise that we dream of being able to more or less set them aside, especially when they're rules we find ourselves forced to live under (such as the laws of nature). That's where transhumanism comes from -- a desire to set aside those pesky rules and quirks that constrain human nature. Yet at the same time, the notion of rulelessness evokes the feeling of creepiness in those of us who aren't filled with a desire for freedom. We don't know how to operate in a world without rules as constraints and reference points.
This feeling was explored quite well in Brave New World. The reader is creeped out, but left without much rational disagreement, with the practices of the future society. What can we say is wrong with oppressing people whose nature -- re-created through drugs and eugenics -- is to want and thrive under oppression? Huxley's answer is a psychologically satisfying cop-out. Bernard Marx's inability to fit in to the brave new world is proof -- reassurance -- that human nature ultimately can't be changed fundamentally. Our ability to reason about the events is restored by restoring the framework of constraints we've been trained by millions of years of evolution and culture to operate on.
So the creepiness that many of us sense in transhumanism is not so much a sense that it's wrong, as a sense that we can't think straight about it.
Stentor Danielson, 19:08,
debitage 
Jeff has