“If all mankind minus one were of one opinion, and only one person were of the contrary opinion, mankind would be no more justified in silencing that one person than he, if he had the power, would be justified in silencing mankind. Were an opinion a personal possession of no value except to the owner, if to be obstructed in the enjoyment of it were simply a private injury, it would make some difference whether the injury was inflicted only on a few persons or on many. But the peculiar evil of silencing the expression of an opinion is that it is robbing the human race, posterity as well as the existing generation-- those who dissent from the opinion, still more than those who hold it.”