“Whether we believe the Greek poet, "it is sometimes even pleasant to be mad", or Plato, "he who is master of himself has knocked in vain at the doors of poetry"; or Aristotle, "no great genius was without a mixture of insanity"; the mind cannot express anything lofty and above the ordinary unless inspired. When it despises the common and the customary, and with sacred inspiration rises higher, then at length it sings something grander than that which can come from mortal lips. It cannot attain anything sublime and lofty so long as it is sane: it must depart from the customary, swing itself aloft, take the bit in its teeth, carry away its rider and bear him to a height whither he would have feared to ascend alone.”
In Latin, nullum magnum ingenium sine mixtura dementiae fuit (There is no great genius without some touch of madness). This passage by Seneca is the source most often cited in crediting Aristotle with this thought, but in Problemata xxx. 1, Aristotle says: 'Why is it that all those who have become eminent in philosophy or politics or poetry or the arts are clearly melancholic?' The quote by Plato is from the Dialogue Phaedrus (245a).
On Tranquility of the Mind
Help us to complete the source, original and additional information
Seneca the Younger 225
Roman Stoic philosopher, statesman, and dramatist -4–65 BCRelated quotes

“He who has a contempt for poetry, cannot have much respect for himself, or for anything else.”
Lectures on the English Poets http://www.gutenberg.org/files/16209/16209.txt (1818), Lecture I, "On Poetry in General"
Context: Poetry is the universal language which the heart holds with nature and itself. He who has a contempt for poetry, cannot have much respect for himself, or for anything else.

A Defence of Poetry http://www.bartleby.com/27/23.html (1821)

Writings of the Young Marx on Philosophy and Society, L. Easton, trans. (1967), p. 36
Reflections of a Youth on Choosing an Occupation (1835)

Confusion of Feelings or Confusion: The Private Papers of Privy Councillor R. Von D (1927)

1950s, Conquering Self-centeredness (1957)
Context: I look at my little daughter every day and she wants certain things and when she wants them, she wants them. And she almost cries out, “I want what I want when I want it.” She is not concerned about what I think about it or what Mrs. King thinks about it. She wants it. She’s a child and that’s very natural and normal for a child. She is inevitably self-centered because she’s a child. But when one matures, when one rises above the early years of childhood, he begins to love people for their own sake. He turns himself to higher loyalties. He gives himself to something outside of himself. He gives himself to causes that he lives for and sometimes will even die for. He comes to the point that now he can rise above his individualistic concerns, and he understands then what Jesus meant when he says, “He who finds his life shall lose it; he who loses his life for my sake, shall find it.”’ In other words, he who finds his ego shall lose his ego, but he who loseth his ego for my sake, shall find it. And so you see people who are apparently selfish; it isn’t merely an ethical issue but it is a psychological issue. They are the victims of arrested development, and they are still children. They haven’t grown up. And like a modern novelist says about one of his characters, “Edith is a little country, bounded on the east and the west, on the north and the south, by Edith.” And so many people are little countries, bounded all around by themselves and they never quite get out of themselves. And these are the persons who are victimized with arrested development.