“Thought that is silenced is always rebellious. Majorities, of course, are often mistaken. This is why the silencing of minorities is necessarily dangerous. Criticism and dissent are the indispensable antidote to major delusions.”

Attributed to Twain, but never sourced. Suspiciously close to "A minority may be right, and the majority is always in the wrong." — Henrik Ibsen "Enemy of the People," as well as a famous quote from Danish philosopher Søren Kierkegaard
Misattributed

New York Times Magazine (30 September 1951)

Madison's own notes on Madison's remarks of debate (6 June 1787) http://avalon.law.yale.edu/18th_century/debates_606.asp
1780s, The Debates in the Federal Convention (1787)
Context: In all cases where a majority are united by a common interest or passion, the rights of the minority are in danger. What motives are to restrain them? A prudent regard to the maxim that honesty is the best policy is found by experience to be as little regarded by bodies of men as by individuals. Respect for character is always diminished in proportion to the number among whom the blame or praise is to be divided. Conscience, the only remaining tie, is known to be inadequate in individuals: In large numbers, little is to be expected from it. Besides, Religion itself may become a motive to persecution & oppression. — These observations are verified by the Histories of every Country antient & modern. In Greece & Rome the rich & poor, the creditors & debtors, as well as the patricians & plebians alternately oppressed each other with equal unmercifulness. What a source of oppression was the relation between the parent cities of Rome, Athens & Carthage, & their respective provinces: the former possessing the power, & the latter being sufficiently distinguished to be separate objects of it? Why was America so justly apprehensive of Parliamentary injustice? Because G. Britain had a separate interest real or supposed, & if her authority had been admitted, could have pursued that interest at our expence. We have seen the mere distinction of colour made in the most enlightened period of time, a ground of the most oppressive dominion ever exercised by man over man. What has been the source of those unjust laws complained of among ourselves? Has it not been the real or supposed interest of the major number? Debtors have defrauded their creditors. The landed interest has borne hard on the mercantile interest. The Holders of one species of property have thrown a disproportion of taxes on the holders of another species. The lesson we are to draw from the whole is that where a majority are united by a common sentiment, and have an opportunity, the rights of the minor party become insecure. In a Republican Govt. the Majority if united have always an opportunity. The only remedy is to enlarge the sphere, & thereby divide the community into so great a number of interests & parties, that in the 1st. place a majority will not be likely at the same moment to have a common interest separate from that of the whole or of the minority; and in the 2d. place, that in case they shd. have such an interest, they may not be apt to unite in the pursuit of it. It was incumbent on us then to try this remedy, and with that view to frame a republican system on such a scale & in such a form as will controul all the evils wch. have been experienced.

Source: Race Matters

Transcript: The Breitbart Geert Wilders Interview http://www.breitbart.com/london/2015/06/19/transcript-the-breitbart-geert-wilders-interview/ by Oliver JJ Lane, breitbart.com (19 June 2015)
2010s

"Edward Albee : An Interview", in Edward Albee : Planned Wilderness (1980) edited by Patricia De La Fuente, p. 8
Context: I've noticed that there is not necessarily a great relationship between what the majority of critics have to say and what is actually true. Some of them are so busy trying to mold the public taste according to the limits of their perceptions, and others are so busy reflecting what they consider to be the public taste — that view limited again by their perception. You find very few critics who approach their job with a combination of information and enthusiasm and humility that makes for a good critic. But there is nothing wrong with critics as long as people don't pay any attention to them. I mean, nobody wants to put them out of a job and a good critic is not necessarily a dead critic. It's just that people take what a critic says as a fact rather than an opinion, and you have to know whether the opinion of the critic is informed or uninformed, intelligent of stupid — but most people don't take the trouble.

"The Problem of Dissent" in Saturday Review, Volume 48 (December 1965), p. 81; also read into the US Congressional Record (26 June 1969)

1850s, Lecture at Brooklyn (1859)

The Jeweler’s Eye, New York: G. P. Putnam's Sons (1968) p. 68