A Harvard professor came home from a trip to China and found his front door jammed. He forced it open. A neighbour called the police. Henry Louis Gates Jr. was arrested on his own porch, in his own house, holding his own ID. He is one of the most famous historians in America. Cambridge, Massachusetts, 16 July 2009. Gates, 58, had just flown back from China. His front door would not open. He and his driver shouldered it. A passer-by called it in as a possible break-in. Sergeant James Crowley arrived. Gates was inside his own house by then. He produced his Harvard ID and his driver's licence with that address on it. What happened next is disputed by both men and always will be. Crowley said Gates was uncooperative and loud. Gates said he was asked to step outside, understood exactly what was happening, and said so — including asking the officer for his name and badge number, repeatedly, and not getting it. He was arrested for disorderly conduct. On his own property. A fifty-eight-year-old man walking with a cane. The charge was dropped within days. Cambridge later acknowledged the arrest was regrettable. Barack Obama was asked about it at a press conference and said the police had "acted stupidly," which caused a political firestorm large enough that he convened the famous "beer summit" at the White House with Gates and Crowley to defuse it. And that is where the story usually stops, with three men and some beer on a lawn. Here is what it is actually about. Henry Louis Gates has a MacArthur fellowship, dozens of honorary degrees and a chair at Harvard. He has made television series about tracing Black ancestry. He is about as protected as a Black man in America can be. And none of it worked on his own porch. Every Black professional who has ever been told that respectability is the shield watched that arrest and understood what it meant. Gates said afterwards that the experience changed what he thought he knew about his own country, and he has spent some of the time since working on criminal justice. If it does not work for him, it does not work.