John Stuart Mill

No one can be a great thinker who does not recognize that as a thinker, it is his first duty to follow his intellect to whatever conclusions it may lead. Truth gains more even by the errors of one who, with due study, and preparation, thinks for himself, than by the true opinions of those who only hold them because they do not suffer themselves to think.

“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.”