A few days later, the F.S.A. transferred me to a group of Islamists, and I had my first lesson in how to distinguish Islamist fighters from the Free Syrian Army: The fundamentalists think of themselves as the vanguard of an emergent Islamic state. They torture you more slowly, with purpose-specific instruments. You never address them as “sir,” because this reminds everyone of the state’s secular military. When the Islamists torture you, they prefer to be addressed by a title that implies religious learning. For the younger fighters, “ya sheikhi!” (“o, my sheikh!”); for the older ones, “emir.”

The F.S.A., it turned out, had given me to the Nusra Front, or Jebhat al Nusra, which was using the Children’s Hospital in Aleppo as a headquarters and a prison.