‘How not to think about numbers’ examines what numbers represent and outlines different approaches to counting, before explaining the notion of prime numbers and some simple tricks for finding them. The number system currently in use is the Hindu–Arabic system, which differs from Greek and Roman systems by placing importance in the order of digits in a number, and by using zero. We usually count in base 10, but this is simply a matter of convention. Primes are a special group of numbers that are only divisible by themselves and 1. There is a range of methods for easily determining if a number is prime or divisible by another number.