When I was very young I would play with my younger sister weighty moral games. I remember one of them which said ... Suppose by pushing down just here (I touched my thumb down on a spot of grass) we could kill one Chinese at the other end of the world and we'd get $1 million. Should we do it?

No, Tish said. That would be murder.

I tried to prolong the grand inquiry by pointing out that there were different kinds of murder, some more sinful than others. "It wouldn't be as though we pulled out a pistol and shot the man."

She lingered for a moment, but came back. No, she said.

Caeteris paribus , we understand President Bush to be talking about the same thing. The circumstances are different, but, he insists, there is someone down there and we can't just do him in, whatever the benefits.

We dig in and learn the first lesson, which is that there is a difference between adult stem cells and embryonic stem cells. In vetoing the one bill, while signing another one more limited in scope, Mr. Bush made the point that he could not in good faith direct public money to embryonic stem cell research. Such research accepts the temptation of using embryonic stem cells to support experimental work, never mind that such work is designed to intervene in the development of cells in such a way as might hinder, or even eliminate, malformations that produce sundry human afflictions.

Much of the public has taken to using Alzheimer's disease as the symbolic corpus vile in the picture. It became universally known that Ronald Reagan had contracted Alzheimer's when he wrote his famous public note announcing his withdrawal from public life. As a matter of rhetorical convenience, advocates of stem cell research started advertising their work as the beginning of a cure for that disease. Early on, the explicit sanction of Mrs. Ronald Reagan was solicited, and she gave it. Understandably -- one dead Chinese vs. one live Ronald Reagan?

President Bush makes several points. The first is that it was he who initiated the very idea of federal subsidies for stem cell research by scientists bent on improving human health. What he did, in 2001, was authorize the use of stem cell lines that had already been extracted from embryos.

But he distinguished sharply between the use of these cells, which had zero prospect of developing into human life, and embryos that might conceivably serve as way stations to human life.

Last year, when Congress was considering a bill similar to the one he has just vetoed, President Bush held a ceremony at the White House honoring 21 families. The babies brought into the East Room were manifestly alive and healthy. They had been adopted as frozen embryos and implanted in the wombs of their new mothers, who had successfully brought them to term. The president's point was that he would never be instrumental in the use of public funds for research that began by destroying organic material which might conceivably result in such children as were in the White House that day.

There is no law on the books, and Mr. Bush does not seek one, that would make it criminal to kill embryos in order to use their cells experimentally for scientific work. And we know that research that entails the use of embryonic stem cells is going on, not only in foreign medical centers, but here in the United States, notably California. Mr. Bush hasn't asked for a declaration of war against those scientists, but he does ask the public at large to acknowledge that there is a moral line here that requires attention. At some point, never mind the praiseworthiness of the design, scientists need to stay their hands, guided by different criteria from those that Adolf Hitler was guided by.

Critics of the president, in high fury, say numerous things, among them that embryos by the millions are fated to die as a matter of course, so that to single out those that die, so to speak, under the researcher's knife is arbitrary and morally meaningless.

Well, so the argument goes, but we can take whatever satisfaction we wish from the knowledge that there is one Chinese there, whose life has been saved.