Answer: the fact that people's lives are beyond their own control.
One of the main themes of Night is the fact that people's lives are beyond their own control, and this is exemplified throughout the story. The prisoners had no power to stop their capture, or to stop the tortures that go on inside the camps. One of the symbols that exemplifies this powerlessness is the "selection." The selection is the process of separating the weak and sick from the strong and "healthy." Although the prisoners do their best to appear healthy and fit, they ultimately have very little power over their own destiny.