If we hope to rid our country of racism, we need to understand the origins of racist thought. That’s the basis of historian Ibram X. Kendi’s research, in which he explores the intellectual justifications that lead to discriminatory policies. He talks with Krys Boyd about his book “Stamped from the Beginning: The Definitive History of Racist Ideas in America” (Nation Books), which won the 2016 National Book Award for nonfiction and is now out in paperback.