Facts are facts, we often say with certainty; but values--well, they're relative. But every day we are confronted with situations where these simple distinctions begin to blur--whether our concerns are the roots of crime and violence, the measure of intelligence, the causes of disease, the threat and promise of genetic engineering. Where do our "facts" end and our "values" begin?

Recent developments in neuroscience have begun to shed light on this confusion, by radically revising our notions of where human nature ends and human nurture begins. As Edward Hundert--a philosopher,...