In 1859, John Stuart Mill explained why freedom of opinion was good for society.
“First, if any opinion is compelled to silence, that opinion may, for aught we can certainly know, be true. To deny this is to assume our own infallibility.
Secondly, though the silenced opinion be in error, it may, and very commonly does, contain a portion of the truth; and since the general or prevailing opinion on any subject is rarely or never the whole truth, it is only by the collision of adverse opinions that the reminder of the truth has any chances of being supplied.
Thirdly, even if the received opinion be not only true, but the whole truth; unless it is suffered to be, and actually is, vigorously and earnestly contested, it will, by most of those who receive it be held in the manner of prejudice, with little comprehension or feeling of its rational grounds.”