Sam Harris says his moment came in 2006, at a conference at the Salk Institute with Richard Dawkins, Neil deGrasse Tyson and other prominent scientists. Mr. Harris said something that he thought was obvious on its face: Not all cultures are equally conducive to human flourishing. Some are superior to others.

“Until that time I had been criticizing religion, so the people who hated what I had to say were mostly on the right,” Mr. Harris said. “This was the first time I fully understood that I had an equivalent problem with the secular left.”

After his talk, in which he disparaged the Taliban, a biologist who would go on to serve on President Barack Obama’s Commission for the Study of Bioethical Issues approached him. “I remember she said: ‘That’s just your opinion. How can you say that forcing women to wear burqas is wrong?’ But to me it’s just obvious that forcing women to live their lives inside bags is wrong. I gave her another example: What if we found a culture that was ritually blinding every third child? And she actually said, ‘It would depend on why they were doing it.’”