In Part 3, ten journal entries span about a month: from the end of the 7th month to the end of the 8th month.

Picking up on the 20th day of the seventh month, Piranesi finds an old man in the labyrinth who eventually is revealed to be Laurence Arne-Sayles. During their conversation, Arne-Sayles admits Ketterley was his student, names some of the dead, and says he isn’t the Sixteenth Person that Ketterley is worried about, but came to see the narrator. Arne-Sayles’s brief and final trip to the labyrinth, the reader later learns, is inspired by being interrogated by a police officer about Matthew Rose Sorensen’s disappearance (it is this police officer, Sarah Raphael, who is 16).

The narrator names Arne-Sayles “The Prophet” because he knows so much about the dead and the labyrinth. Arne-Sayles briefly covers how the world of the labyrinth was made and how he went to prison for activities related to the labyrinth. However, like the narrator, he believes the “wisdom of the ancients” (89) that Ketterley seeks in his ritual isn’t accessible via ceremonial magic. They also talk about the statues.