In a senate hearing room, Christopher Reeve is testifying in support of embryonic stem-cell research. Sitting in his wheelchair, breathing with a ventilator, the former star of Superman films makes a sympathetic figure. And then someone raises an uncomfortable question: Is it ethical to take a life to save a life? Embryonic stem-cell research does, after all, destroy human embryos.

Reeve counters: "I thought it was the job of the government to do the greatest good for the greatest number." Senators and reporters nod in agreement.

Obviously, they didn't get it. If the government really embraced "the greatest good for the greatest number," Reeve himself might be dead. After all, Reeve's therapies, his doctors, his aides, and his motorized wheelchair cost millions—money that could, instead, fund basic medical care for hundreds of poor children.

Of course, Reeve can afford to pay for all of this himself—a fortunate fact should the government take his "greatest good" advice. But he's still asking taxpayers to spend millions on research to treat spinal cord injuries. Given that vastly more Americans need immunizations than a cure for paralysis, wouldn't a "greatest good" policy mean spending scarce research funds on immunizations instead of paralysis research?

I've used this vignette in speeches, and to my shock, I've found that even Christians nod in agreement with Reeve's reasoning—until I explain just where this thinking leads. Have we all unthinkingly become practical utilitarians?

Reeve is echoing the arguments of Princeton philosopher Peter Singer, the quintessential moral utilitarian. Singer believes morality cannot be judged by any transcendent standard. Instead, he suggests we ask whether a particular action will increase ...