"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 an error, it may, and very commonly does, contain a portion of 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 remainder of the truths has any chance 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 a prejudice, with little comprehension or feeling of its rational grounds."


-- John Stuart Mill, On Liberty, 1859