Christopher explains that his chapters are numbered with prime numbers only, simply because he likes prime numbers. To figure out which numbers are prime, one writes down all positive whole numbers, and then takes away all the numbers that can be divided by anything other than themselves or 1. There’s no formula to tell whether very large numbers are prime. Prime numbers are valuable, and even classed as military secrets in the United States. Christopher thinks that prime numbers are similar to life, in that they’re logical, but no one can figure out the rules that govern them.
Christopher demonstrates his love of math, which permeates his life to such an extent that he delineates the events in his books with the mathematical logic of prime numbers rather than by the social convention of successive numbers. Furthermore, the numbers that head every chapter symbolize Christopher’s viewpoint: he knows that society acts as it does for certain reasons, but those reasons remain incomprehensible to him, no matter how much he tries to understand them.