THE CONTROVERSY ABOUT the ethics of human cloning draws strong opinions from both sides. At one extreme are those who think it should never be done, under any circumstances. "I can't think of a morally acceptable reason to clone a human being," says Richard McCormick, a Jesuit priest and professor at the University of Notre Dame.

On the other side of the question are people such as Gregory Pence, a professor of philosophy at the University of Alabama. He can think of many reasons to clone a human being, and he sees nothing wrong with most of them. "If no one is harmed by [cloning]," he says, "then it raises no moral issue."

In between these two poles is a wide-ranging argument that sometimes arouses vehement feelings in its participants. Some, such as Leon Kass, of the University...