“No history of the world can be complete which does not mention Mary Helen Keller… whose overcoming of her blindness and deafness were arguably victories more important than those of Alexander the Great, because they have implications still for every living person.”

— Theodore Zeldin

Theodore Zeldin in An Intimate History of Humanity (1994) This quote seems to obviously refer to Helen Adams Keller, but why she is referred to as "Mary Helen Keller" is not clear.
An Intimate History of Humanity (1994)

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Theodore Zeldin 8

English academic 1933

Peter de Noronha (1897–1970) Indian businessman

The Pageant of Life (1964), On Suffering

“The problems of deafness are deeper and more complex, if not more important, than those of blindness.”

Helen Keller (1880–1968) American author and political activist

Letter to Dr. James Kerr Love (1910), published in Helen Keller in Scotland: a personal record written by herself (1933), edited by James Kerr Love. Paraphrasing of this statement may have been the origin of a similar one which has become attributed to her:
Context: The problems of deafness are deeper and more complex, if not more important, than those of blindness. Deafness is a much worse misfortune. For it means the loss of the most vital stimulus — the sound of the voice that brings language, sets thoughts astir and keeps us in the intellectual company of man.

“Pierrette, like all those who suffer more than they have strength to bear, kept silence. Silence is the only weapon by which such victims can conquer; it baffles the Cossack charges of envy, the savage skirmishings of suspicion; it does at times give victory, crushing and complete, — for what is more complete than silence? it is absolute; it is one of the attributes of infinity.”

Honoré de Balzac book Pierrette

Pierrette fit comme les gens qui souffrent au delà de leurs forces, elle garda le silence.Ce silence est, pour tous les êtres attaqués, le seul moyen de triompher: il lasse les charges cosaques des envieux, les sauvages escarmouches des ennemis; il donne une victoire écrasante et complète. Quoi de plus complet que le silence?Il est absolu, n'est-ce pas une des manières d'être de l'infini?
Source: Pierrette (1840), Ch. VI: An Old Maid's Jealousy

“The person who is dedicated to nonviolence is more active than anyone, because he does not only want to overcome indifference and hatred, weariness and egoism within himself, but he wants to overcome anything which divides and hurts all people and therefore the person dedicated to nonviolence does not accept society as it is.”

Aldo Capitini (1899–1968) Italian philosopher and political activist

Hymn

“Kindness is the language which the deaf can hear and the blind can see.”

Mark Twain (1835–1910) American author and humorist

Unsourced in The Philosophy of Mark Twain: The Wit and Wisdom of a Literary Genius (2014) by David Graham
Disputed

“This man more than superseded my every expectation I had…if you live with a man under duress (this is before the victory…) he is I think…he will rank in history with some of the greats.”

Errol Flynn (1909–1959) Australian actor

Source: Statement made by Errol Flynn about Fidel Castro in a TV interview on the Canadian TV program Front Page held in 1959. http://www.youtube.com/watch?v=filBYa1AJEA