“For I think it is the case with genius that it is not when quiescent so very much above mediocrity as the difference between the two might lead us to think, but that it has the power and privilege of rising from that level to a height utterly far from mediocrity: in other words that its greatness is that it can be so great.”

Interview with pianist Leon Fleisher http://www.examiner.com/article/interview-with-pianist-leon-fleisher by Elijah Ho (October 1, 2014)

Pt. II, ch. 2.
The Man of Genius (1891)

31
Essays, Can Poetry Matter? (1991), The Catholic Writer Today (2013)

Speech at the Albert Hall (4 December 1924), quoted in On England, and Other Addresses (1926), pp. 70-71.
1924
Context: It is a testing time for democracy... Democracy, democratic government, calls for harder work, for higher education, for further vision than any form of government known in this world. It has not lasted long yet in the West, and it is only by those like ourselves who believe in it making it a success that we can hope to see it permanent and yielding those fruits which it ought to yield. The assertion of people's rights has never yet provided that people with bread. The performance of their duties, and that alone, can lead to the successful issue of those experiments in government which we have carried further than any other people in this world. Democracy can rise to great heights; it can also sink to great depths. It is for us so to conduct ourselves, and so to educate our own people, that we may achieve the heights and avoid the depths.

Variant: Great ideas often receive violent opposition from mediocre minds.

Isaac D'Israeli, The Curiosities of Literature, "Men of Genius Deficient in Conversation".
Misattributed, Isaac D'Israeli

Men of Genius Deficient in Conversation.
Curiosities of Literature (1791–1834)

Source: God Emperor of Dune

Letter to Josiah Quincy (9 February 1811), Quincy. http://oll.libertyfund.org/titles/adams-the-works-of-john-adams-vol-9-letters-and-state-papers-1799-1811
1810s