Sophie’s World by Jostein Gaardner is a mystery novel on at least three levels. There is the mystery of who is sending the questions and pages on philosophy. There is the mystery of Hilde. There is the mystery inherent in the profound questions of philosophy. However, the three examples given below all pertain to the first mystery, the mystery of the identity of the philosopher.
In the first example below (Ch. 1), the mystery is set in motion. In the second example (Ch. 5), Sophie comes up with a plan to catch the philosopher in the act of leaving a note in her mailbox. In the third example (Ch. 6), Sophie realizes that her attempt to catch the philosopher has failed.
In Chapter 1, "The Garden of Eden," Sophie goes to her mailbox, thinking she might find a letter from her father who captains an oil tanker and is often away. But instead she finds this:
There was only one letter in the mailbox—and it was for Sophie. The white envelope read: “Sophie Amundsen, 3 Clover Close.” That was all; it did not say whom it was from. There was no stamp on it either.
As soon as Sophie had closed the gate behind her she opened the envelope. It contained only a slip of paper no bigger than the envelope. It read: Who are you?(Jostein Gaarder, Sophie’s World: Ch. 1, p. 4).
Sophie is sure that she will hear from the mysterious letter right again—and she’s right. In Chapter 2, instead of simply receiving a slip of paper with a question, Sophie receives three pages entitled: WHAT IS PHILOSOPHY (Ch. 2, “The Top Hat,” p. 14).
By Chapter 5, "Democritus," Sophie has received enough packages that she believes she knows how to unmask the mystery mailer:
Sophie began to discern a pattern in the deliveries: every afternoon she would find a big brown envelope. While she read the contents, the philosopher would sneak up the mailbox with another little white envelope.
So now Sophie would be able to find out who he was. If it was a he! She had a good view of the mailbox from her room. If she stood at the window she would see the mysterious philosopher. White envelopes don’t just appear out of thin air! (Ch. 5, “Democritus,” p. 43)
But, in Chapter 6, “Fate,” as in all good mysteries, Sophie’s original and over simplistic plan fails:
So, he had tricked her! Today of all days, when she had kept such careful watch on the mailbox, the mystery man had sneaked up to the house from a different angle and just laid the letter on the step before making off in the woods again. Drat! (Ch. 6, “Fate,” p. 49)