How my NPR colleague failed at “viewpoint diversity”

I was in San Antonio last weekend for a book festival, and took an early-morning run along the city’s River Walk. The paths go beside the water and below the streets, which cross the river on Art Deco bridges. A city looks different from underneath; you feel its layers of history.

Up at street level the book festival was underway, and in a tent filled with people I talked about Differ We Must, my biography of Lincoln as seen through his encounters with people who disagree with him. Moderator Dan Goodgame, editor of...