"If there is a dividing line between liberty and license, it is where freedom of speech is no longer respected as a procedure of the truth and becomes the unrestricted right to exploit the ignorance, and to incite the passions, of the people. Then freedom is such a hullabaloo of sophistry, propaganda, special pleading, lobbying, and salesmanship that it is difficult to remember why freedom of speech is worth the pain and trouble of defending it."