The issue of stem cell research — which is back before the Senate — is often described as a moral dilemma, but it simply is not. Or at least it is not the moral dilemma often used in media shorthand: the rights of the unborn vs. the needs of people suffering from diseases that embryonic stem cells might cure.

As one of those people myself (I have Parkinson's), I am not an objective analyst of what the U.S. government's continuing near-ban on stem cell research is costing our society and the world. Naturally, I think it's costing too much. No other potential therapy — including adult stem cells — is nearly as promising for my ailment and others. Evaluate that as you wish.

Against this, you have the fact that embryonic stem cells are extracted from human embryos, killing the latter in the process. If you believe that embryos a few days after conception have the same human rights as you or me, killing innocent embryos is obviously intolerable. But do opponents of stem cell research really believe that? Stem cell research tests that belief, and sharpens the basic right-to-life question, in a way abortion never has.

Here's why. Stem cells used in medical research generally come from fertility clinics, which produce more embryos than they can use. This isn't an accident — it is essential to their mission of helping people have babies. Often these are "test tube babies": the product of an egg fertilized in the lab and then implanted in a womb to develop until birth. Controversy about test-tube babies has all but disappeared. Vague science-fiction alarms have been crushed by the practical evidence, and potential political backlash, of grateful, happy parents.

In any particular case, fertility clinics try to produce more embryos than they intend to implant. Then — like the Yale admissions office (only more accurately) — they pick and choose among the candidates, looking for qualities that make for a better human being. If you don't get into Yale, you have the choice of attending a different college. If the fertility clinic rejects you, you get flushed away — or maybe frozen until the day you can be discarded without controversy.

And fate isn't much kinder to the embryos that make this first cut. Usually several of them are implanted in the hope that one will survive. Or, to put it another way, in the hope that all but one will not survive. And fertility doctors do their ruthless best to make these hopes come true.

Proponents of stem cell research like to emphasize that it doesn't cost the life of a single embryo. The embryos killed to extract their stem cells were doomed already. But this argument gives too much ground, and misses the point. If embryos are human beings, it's not OK to kill them for their stem cells just because you were going to kill them, or knowingly let them die anyway. The better point — the killer point, if you'll pardon the expression — is that if embryos are human beings, the routine practices of fertility clinics are far worse — both in numbers and in criminal intent — than stem cell research. And yet, no one objects, or objects very loudly. President Bush actually praised the work of fertility clinics in his first speech announcing restrictions on stem cells.

Even strong believers in abortion rights (I'm one) ought to acknowledge and respect the moral sincerity of many right-to-lifers. I cannot share, or even fathom, their conviction that a microscopic dot — as oblivious as a rock, more primitive than a worm — has the same human rights as anyone reading this article. I don't have their problem with the question of when human life begins. Nevertheless, abortion opponents deserve respect for more than just their right to hold and express an opinion we disagree with. Excluding, of course, the small minority who believe that their righteousness puts them above the law, sincere right-to-lifers deserve respect as that rarity in modern American politics: a strong interest group defending the interest of someone other than themselves.

Or so I always thought — until the arrival of stem cells. Moral sincerity is not impressive if it depends on willful ignorance and indifference to logic. Not every opponent of stem cell research deserves to have his or her debater's license taken away. There are a few, no doubt, who are as horrified by fertility clinics as they are by stem cell research, and a subset of this subset may even be doing something about it. But these people, if they exist, are not a political force strong enough to stop a juggernaut of medical progress that so many other people are desperate to encourage. The vast majority of people who oppose stem cell research either haven't thought it through, or have thought it through and don't care.

I wish they would think again.

Michael Kinsley, former editor of Slate, writes a weekly column for The Washington Post.