I'm rereading Mill's On Liberty in preparation for teaching it for the first time in a couple of years. Early in the chapter on the liberty of thought and discussion, he argues:
First, the opinion which it is attempted to suppress by authority may possibly be true. Those who desire to suppress it, of course, deny its truth; but they are not infallible. They have no authority to decide the question for all mankind and exclude every other person from the means of judging. To refuse a hearing to an opinion because they are sure that it is false is to assume that their certainty is the same thing as absolute certainty. All silencing of discussion is an assumption of infallibility.
Are you listening Nancy Hopkins?