I can't cite a source for this one (I just woke up and haven't had my tea), but I love it:
"Experience should teach us to be most on our guard to protect liberty when the government's purposes are beneficial. Men born to freedom are naturally alert to repel invasions of their liberty--by evil-minded rulers. The greater dangers to liberty lurk in insidious encroachment by men of zeal, well meaning. but without understanding."
-- Justice Louis Brandeis, U.S. Supreme Court Justice