[John Davies - A Visualization of Prime Numbers]
"The rule for working out prime numbers is really simple, but no one has ever worked out a simple formula for telling you whether a very big number is a prime number or what the next one will be. If a number is really, really big, it can take a computer years to work out whether it is a prime number.

[...]

Prime numbers are what is left when you have taken all the patterns away. I think prime numbers are like life. They are very logical but you could never work out the rules, even if you spent all your time thinking about them."

- Mark Haddon, The Curious Case of the Dog in the Night-Time