Christopher thinks that phenomena that seem supernatural can be explained, but no one knows how to explain them yet. His Uncle Terry once said he saw a ghost in a shoe shop, and the cashier told him it was the ghost of a friar who used to live in the monastery that sat just where the shop was. Sooner or later, though, scientists will find a way to explain people seeing ghosts.
Christopher makes a distinction here between not believing that supernatural phenomena exist and believing that they can be explained in logical ways. He believes his uncle’s story about seeing a ghost, but just thinks that there’s a scientific explanation for ghosts. Similarly, there has to be a logical explanation for the mysterious letters he’s just found.
On the other hand, some things that seem mysterious aren’t at all. For example, there’s a pond with frogs at Christopher’s school. The frog population changes year to year, apparently randomly. In fact, however, there’s a mathematical formula that shows animal population change. Christopher provides a couple of graphs to show hypothetical population change. This formula proves to him that even when things are too complicated to predict, they’re still obeying rules. Furthermore, major events like extinction can happen just because numbers work that way.
Christopher likes being able to explain the everyday happenings of the world. In the instance of the frog population, he can explain it with math, which is even more satisfactory to him. This example implies that even the unexpected turns in his life must still be obeying some rules of existence, even if he doesn’t understand them. Christopher turns to logic in this moment when he probably unconsciously realizes that his life is not as it seemed.