Ava DuVernay was frustrated.

The Selma director and When They See Us creator had just read Isabel Wilkerson’s bestseller Caste: The Origins of Our Discontents for the second time following its huge release in the summer of 2020 but, as she tells EW, “still didn’t understand it, really.”

The next read, though, proved that popular idiom true: “It really started to sink in. I thought, This is information that people should know, and I should make a movie about it.”

In her book, Wilkerson examines how it’s not race or class systems that have, throughout history and to this day, oppressed people and cultures around the world but rather a caste system — made of up eight pillars — that ranks humans and their importance to each other. “I was really taken by the interconnectedness of all of these things that we feel are very separate in terms of a cultural phenomenon that keeps us apart,” DuVernay says, “and the idea that, wow, the undergirding, the foundation of it all is the same.”