The United States has experienced more than two centuries of political stability. When viewed against the background of world history, this is remarkable. The First Amendment has played a singularly important role. When citizens can openly criticize their government, changes come about through orderly political processes. When grievances exist, they must be aired, if not through the channels of public debate, then by riots in the streets. The First Amendment functions as a safety valve through which the pressures and frustrations of a heterogeneous society can be ventilated and defused.