Take the case of a local officer who persists in enforcing a type of ordinance which the Court has held invalid as violative of the guarantees of free speech or freedom of worship. Or a local official continues to select juries in manner which flies in the teeth of decisions of the Court. If those acts are done willfully, how can the officer possibly claim that he had no fair warning that his acts were prohibited by the statute? He violates the statute not merely because he has a bad purpose but because he acts in defiance of announced rules of law. He who defies a [325 U.S. 91, 105] decision interpreting the Constitution knows precisely what he is doing. If sane, he hardly may be heard to say that he knew not what he did. Of course, willful conduct cannot make definite that which is undefined. But willful violators of constitutional requirements, which have been defined, certainly are in no position to say that they had no adequate advance notice that they would be visited with punishment. When they act willfully in the sense in which we use the word, they act in open defiance or in reckless disregard of a constitutional requirement which has been made specific and definite. When they are convicted for so acting, they are not punished for violating an unknowable something.
SCREWS v. U.S., 325 U.S. 91 (1945)
