Books
There are two kinds of people in the world, those who believe there are two kinds of people in the world and those who don't. Benchley's Law of Distinction
— Robert Benchley ‧ Laws
Behind the line May contain spoilers.
Our reading
An editorial interpretation.
The joke divides people by whether they believe in dividing people into two kinds. It uses the same binary structure that it questions. The final line is the retained law title.
Context reviewed