“I do see one large and grievous kind of ignorance, separate from the rest, and as weighty as all the other parts put together. Thinking that one knows a thing when one does not know it. Through this, I believe, all the mistakes of the mind are caused in all of us.”
229c
Sophist
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Plato 80
Classical Greek philosopher -427–-347 BCRelated quotes
“I believe that ignorance is the root of all evil.
And that no one knows the truth.”

Spoken on his return to India from England as recorded in From Colombo to Almora (1904), Calcutta, p. 221
Context: No one ever landed on English soil with more hatred in his heart for a race than I did for the English, and, on this platform, are present English friends who can bear witness to the fact, but the more I lived among them, saw how the machine is working, the English national life, mixed with them, found where the heart-beat of the nation was, the more I loved them. There is none among you here present, my brothers, who loves the English people more than I do. You have to see what is going on there, and you have to mix with them. As the philosophy, our national philosophy of the Vedanta, has summarised all misfortune, all misery from that one cause, ignorance, herein also we must understand that the difficulties that arise between us and the English people are mostly due to that ignorance; we do not know them, they do not know us.

Mahatma Gandhi. October 1927. The Collected Works, Volume 35, New Delhi, 1968, pp. 166-67. Quoted in Goel, S.R. History of Hindu-Christian Encounters (1996)
Posthumous publications (1950s and later)

Referring to the first Woody Guthrie record he ever heard, p. 243
Chronicles: Vol. One (2004)

Statement (1906) in Mark Twain in Eruption: Hitherto Unpublished Pages About Men and Events (1940) edited by Bernard DeVoto

(19 January 2005)
Unfit for Mass Consumption (blog entries), 2005
Context: All I have to do is make you see this. This one particular thing here. That's all. And sometimes it's impossible. Sometimes, I know the best odds I can hope for are a thousand to one. You'll see what you see, what your life has conditioned you to see upon encountering that combination of words, not what I want or need you to see. Fiction writing is like making films for the blind.

Bell Telephone Talk (1901)

Source: Earthsea Books, The Tombs of Atuan (1971), Chapter 7, "The Great Treasure" (Arha)