What literary technique is used in the sentence, "there were other ways of making people into ghosts" from To Kill a Mockingbird.

There were many rumors around Maycomb about Boo Radley.  Miss Stephanie, a notable gossip, said that Boo had been "locked in the courthouse basement" for a time.  According to her, Boo was eventually sent home and his father kept him out of sight from the neighbors.  No one ever saw Boo around the neighborhood or in town.  Jem's theory was "that Mr. Radley kept him chained to the bed most of the time."  Atticus disagreed with this outrageous idea.  He told his son that he disagreed with him, and "that there were other ways of making people into ghosts."

The phrase "making people into ghosts" is a metaphor.  Atticus was referring to the fact that Mr. Radley somehow kept his son inside the house.  No one ever saw him, so it was as if he were no longer living, like a ghost.  Mr. Radley made Boo into something like a ghost by keeping him away from neighbors and townspeople.