Moral conservatives were shocked to read a thinly veiled defense of infanticide in the New York Times a few years ago by MIT [now of Harvard] professor Steven Pinker. But they would be even more disturbed if they saw Pinker’s justification for his views in a book that appeared about the same time. In How the Mind Works, Pinker argues that the fundamental premise of ethics has been disproved by science. “Ethical theory,” he writes, “requires idealizations like free, sentient, rational, equivalent agents whose behavior is uncaused.” Yet, “the world, as seen by science, does not really have uncaused events.”...