For a generation, American politics has largely been frozen in place when it comes to so-called "reproductive issues." Abortion has been the keystone holding up a number of related positions, from euthanasia to embryonic stem cell research, with self-described pro-lifers and pro-choicers locked in a permanent cold war.

But the light of science is melting the permafrost beneath them, making abortion seem like a 20th-century argument about feminism whereas the argument in the 21st century will be about humanity itself - and whether science is the source of human values.

Tellingly, in the past, both sides in the abortion wars have claimed science as their ally in the fight over when life begins. Embryonic stem cell research, however, has changed the focus of that argument because, for good reasons and bad, ESCR advocates want to stop talking about those who are pro-life and start calling their opponents "anti-science," as if being anti-science - whatever that means - is an immoral stance.

Pro-embryonic stem cell activists have given science something of a messianic role in human affairs, casting it as a deliverer from our moral plight. For example, in a pique of asininity, Sen. Arlen Specter, R-Pa., declared this month, "It is scandalous that eight years have passed since we have known about stem cell research and the potential to conquer all known maladies, and federal funds have not been available for the research."

All ... known ... maladies? Really? Before that, John Edwards all but promised that a vote for John Kerry was a vote for Christopher Reeve to walk again.

But it appears that Hermes (the Greek god of science) is proving to be a fickle ally. New research shows that there are other, perhaps more promising, sources of "pluripotent" cells (i.e., ones that can become any other cell) that don't involve destroying embryos. Wake Forest researchers found rich sources of stem cells in simple amniotic fluid. Pro-lifers are now using this research to cast themselves as the true allies of science. Hermes' sword, it seems, has a double edge.

Simply because science can do something is in no way an argument that it should (or shouldn't) do it. Science is morally neutral. Science kills and science cures. Which is why it's so disturbing that both left and right have bought into the rhetoric of science as a source of morality. Scientists themselves tend to understand the moral ambiguity of science, which is why they spend so much time arguing about professional ethics.

For example, everybody agrees that life-ending experimentation on a 5-year-old boy would be wrong. But what if such research could solve "all human maladies"? Would it be wrong then? More relevant, would it be "anti-science"?

Yes, yes, ESCR advocates reject comparing embryos to fully developed humans. But that misses the point on two scores. First, the determination that embryos have no moral worth is not a scientific conclusion but a moral one. Second, rejecting the comparison doesn't answer the question: Is it anti-science to bar certain procedures on moral grounds? Animal-rights activists don't believe they are anti-science when they oppose cruel testing on monkeys, even when it could lead to medical breakthroughs. Was it anti-science when doctors invented the "bloodless" heart bypass to accommodate the concerns of Jehovah's Witnesses who didn't want transfusions?

We need to grapple with these questions now because we are only entering the shallow rapids while the waterfalls lay ahead. But you can already hear the onrush of water.

Slate's William Saletan recently chronicled how the age of retail eugenics has arrived. Gender-selective abortion is commonplace in the developing world. In the developed West, we're more selective at the embryonic level. For example, a handful of deaf parents are deliberately selecting embryos that will become deaf - and doctors are helping. Meanwhile, researchers at Oregon State University recently revealed that hormone treatments can reverse homosexuality in sheep. Predictably, lesbian activist Martina Navratilova and others complained that the sheeps' "right" to be gay was being violated. While no one called Navratilova "anti-science," it's not hard to see the slippery slope she's concerned about.

Indeed, abortion-rights absolutism provides no defensible terrain to object to that slippery slope. Today's "pro-science" champions may soon see a world where homosexuality is eradicated in utero thanks to their hard work establishing the absolute moral sovereignty of individual choice and science.

This is the beauty and curse of science: It tends to undermine the cherished positions and assumptions of everyone, even those who claim to be its champions. Perhaps that's one reason we shouldn't derive our values from such a moving target in the first place.