Most people claim to be in favor of free speech, but they often mean speech from their own side (and not whatever those crazy people on the other side want to say). But from the point of view of the brain, why does free speech need to be rigorously defended? What does this have to do with internal models, printing presses, college campuses, John Stuart Mill, online indecency, cultures of honor, Robinson Crusoe, cancel culture, the importance of literature, and why free speech makes everyone safer?