According to Christopher, when is a mystery not a mystery?

I'm not sure which chapter you are referring to. Generally Chris feels that mysteries can be scientifically explained. The novel is set up as a detective from the outset – a simple who-done-it. What it sets up is a premise for exploration and for investigation; for Christopher, it is an opportunity to find a concrete and definite answer to the mystery. It is mathematical in its structure and yet, as we are to learn, the puzzle is not as black and white as Christopher expects.