Jeremy Jones, a University of Utah senior majoring in philosophy, is writing his honors thesis on the fuzzy distinction between treatment and enhancement. A treatment, for example, would be a drug to help Alzheimer's patients improve their failing memories. "Of course we would say 'Let's let Grandpa use it, to bring him back so he can be a functioning part of society,' " Jones says.

But what if the same drug could help a college student, as Jones says, "catch an edge"? At what point is the drug the mental equivalent of muscle-building steroids? "These conditions exist on a continuum," he says. "That's why it's so hard to draw the line."

The same dilemma will exist when we figure out how to give people a genetic tweak so they won't ever get dementia," says "Citizen Cyborg" author Hughes. On the one hand, it's a medical therapy. On the other, it's a way of fiddling with the natural aging process.

"Bio-Luddites" is what Hughes calls people who want to ban the technologies and drugs that would help humans live beyond their current potential. "There are people who are mobilizing to ban these technologies; we would do well not to underestimate them," says Hughes, who also teaches health policy at Trinity College in Hartford, Conn.

"Bioconservatives are very attached to four score and six, and the IQ, as definitions of what it means to be human," he says. "But what it means to be human is to push all those boundaries." Just look how far we've come from our agricultural ancestors, who "were flea-bitten and had short lives," he adds.

Critics of pushing boundaries come from both the political right and left, he says, pointing to conservatives such as Francis Fukuyama of the President's Council on Bioethics (which in 2003 published a critical report called "Beyond Therapy: Biotechnology and the Pursuit of Happiness") and liberal activists such as Jeremy Rifkin, president of the Foundation on Economic Trends.

The Southern Baptist Theological Seminary's R. Albert Mohler Jr. is another vocal opponent of radical enhancements. It's one thing, he says, to try to give a person with bad eyesight 20/20 vision, and it's another to try to create humans whose eyesight is superhuman. The latter, he says, uses science "to redefine the species."

"From a Christian worldview perspective," he says, "there are two problems with this. First, you have the normative definition of what it means to be a human being made in the image of God." To try to exceed normal human capacities, he says, "is to open, quite literally, a Pandora's Box of moral problems."