One night in a Concord jail, held for refusing a tax to a government that kept slaves and waged war on Mexico, Thoreau worked out the most dangerous idea an honest person can have: that obedience to an unjust law is itself a kind of wrong. He will not lend his weight, not even the few coins of it, to a machine he believes is grinding men down. The argument is plain and merciless. If the law makes you the agent of injustice, then break the law; the true place for a just man under a government that imprisons unjustly is also a prison. What gives the essay its strange, quiet force is how personal it makes the question. It does not ask what governments should do. It asks what you will do, alone, when the tax collector knocks. The cell held him one night; the sentences he wrote there reached Gandhi's salt road and a Birmingham jail, and they are still knocking.