“If it is the function of the public realm to throw light on the affairs of men by providing a space of appearances in which they can show deed and word, for better and worse, who they are and what they can do, then darkness has come when this light is extinguished by "credibility gaps" and "invisible government" by speech that does not disclose what is but sweeps it under the carpet, by exhortations, moral and otherwise, that, under the pretext of upholding old truths, degrade all truth to meaningless triviality”