Recluse

Nobody knew what form of intimidation Mr. Radley employed to keep Boo out of sight, but Jem figured that Mr. Radley kept him chained to the bed most of the time. Atticus said no, it wasn’t that sort of thing, that there were other ways of making people into ghosts.” (p. 14)

Imagine if Jem said, what do you mean? Answer that question.
Explain the other ways people can be made into ghosts.