"For in the absence of debate unrestricted utterance leads to the degradation of opinion. By a kind of Gresham's law the more rational is overcome by the less rational, and the opinions that will prevail will be those which are held most ardently by those with the most passionate will. For that reason the freedom to speak can never be maintained merely by objecting to interference with the liberty of the press, of printing, of broadcasting, of the screen. It can be maintained only by promoting debate."