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. 
If the opinion is right, they are deprived of the opportunity of
exchanging error for truth:  if wrong, they lose, what is almost as
great a benefit, the clearer perception and livelier impression of
truth, produced by its collision with error.

~John Stuart Mill, On Liberty, 1859

Books
won't stay banned.  They won't burn.  Ideas won't go to jail.  In the
long run of history, the censor and the inquisitor have always lost. 
The only weapon against bad ideas is better ideas. 

~Alfred Whitney
Griswold, New York Times, 24 February 1959

The test of democracy is freedom of criticism. 

~David Ben-Gurion

We
are not afraid to entrust the American people with unpleasant facts,
foreign ideas, alien philosophies, and competitive values.  For a
nation that is afraid to let its people judge the truth and falsehood
in an open market is a nation that is afraid of its people.

  ~John F.
Kennedy

If we don't believe in freedom of expression for people we despise, we don't believe in it at all. 

~Noam Chomsky