Chapter 151 of The Curious Incident of the Dog in the Night-Time deals not only with mysteries, but with how they can or might be logically explained. Some mysteries, such as the changes in the number of frogs in the pond at Christopher's school, can be elucidated by anyone who is sufficiently well-informed. Christopher speculates that this may also be the case with other mysteries, such as the existence of ghosts. The problem is simply that no one has enough information to explain them yet. This, however, is only a temporary situation.

Christopher is thinking over the subject of mysteries and their possible explanations because he was presented with an unusually difficult mystery in the previous chapter. While searching his father's bedroom, he discovered a letter from his mother, addressed to him, in which she refers to various matters that are completely new to Christopher. She talks about finding a new job and a new apartment, which she shares with a man called Roger. She also alludes to other letters she has written to Christopher before, which he has never received. All this would be perplexing enough; but, so far as Christopher knows, his mother is dead. This is why he begins the following chapter by musing on the topic of ghosts.