From Thoreau's essay, "Civil Disobedience":
If the injustice is part of the necessary friction of the machine of government, let it go, let it go: perchance it will wear smooth--certainly the machine will wear out. If the injustice has a spring, or a pulley, or a rope, or a crank, exclusively for itself, then perhaps you may consider whether the remedy will not be worse than the evil; but if it is of such a nature that it requires you to be the agent of injustice to another, then I say, break the law. Let your life be a counter-friction to stop the machine. What I have to do is to see, at any rate, that I do not lend myself to the wrong, which I condemn.