Retroactive (or ex post facto) law—i.e., a law that punishes a man for an
action which was not legally defined as a crime at the time he committed it—is
rejected by and contrary to the entire tradition of Anglo-Saxon jurisprudence.
It is a form of persecution practiced only in dictatorships and forbidden by
every civilized code of law. It is specifically forbidden by the United States
Constitution. It is not supposed to exist in the United States and it is not
applied to anyone—except to businessmen. A case in which a man cannot know
until he is convicted whether the action he took in the past was legal or
illegal, is certainly a case of retroactive law.