“The only proper purpose of government is to protect the rights of the individual—meaning, to protect him from physical force. A proper government is only a policeman, acting as man’s agent of self-defense, and as such may use force only against those who initiate it. The only proper functions of government are: the police, to protect against criminals; the army, to protect against foreign invaders; and the courts, to protect property and contracts from breach or fraud, and to settle disputes by rational rules under objective law”