. . . Popper said that it is reasonable to assume that sooner or later some rotten scoundrels will gain power. It’s not important who they will be precisely, but whatever your politcal views might be you must agree that a likelihood of such event is rather high. So whatever law you want to have in you country, don’t ask yourself the question “how this law can be used in good hands”. Ask the question “how this law can be used when the filthiest, dirtest, stupidest bastards will rule my country (and sooner or later they probably will)”. Only the law that cannot be used to anything wrong EVEN by the most vicious ruler is truly good. . . .
(Cited by Aaron Krowne.)
Over the years I have occasionally quoted the principle as “write your constitution as if your worst enemy were in power,” but without knowing where I picked it up. Popper, eh?