Visually, non-primes can be represented as a symmetrical shape e.g. 6 can be shown as xxxxxx but also as xxx --- xxx or x|x x|x x|x Whereas

Visually, non-primes can be represented as a symmetrical shape e.g. 6 can be shown as xxxxxx but also as xxx --- xxx or x|x x|x x|x Whereas a prime like 5 doesn't have this symmetrical layout. Since 1 does have this kind of symmetry it belongs more with the non-primes.