The theory that only a monopolist can price discriminate is standard textbook fare, and it's borne out by a lot of observations.
The monopolist theory stems from many observations.
The theory doesn't exist, it's a figment of the imagination.
The two men are sitting in a library.
Five players play soccer.
Kids are in school
Soldiers are at a opening
A young boy is playing.
A larger amount of money was invested in information technology.