If you want to improve,
be content to be thought foolish and stupid.
Epictetus

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The man who never alters his opinion is like standing water,
and breeds reptiles of the mind.
William Blake

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I write entirely to find out what I’m thinking, what I’m looking at, what I see and what it means. What I want and what I fear.
Joan Didion

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To know that you do not know is the best.
To think you know when you do not is a disease.
Lao Tzu

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Most people are other people. Their thoughts are someone else’s opinions, their lives a mimicry, their passions a quotation.
Oscar Wilde

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Thinking is the hardest work there is, which is
probably the reason why so few engage in it.
Henry Ford

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There is a kind of work which any man can do, but from
which many men shrink, generally because it is very hard work.
It is called thinking.
G.K. Chesterton

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No problem can withstand the assault of sustained thinking.
Voltaire

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The conventional view serves to protect us
from the painful job of thinking.
John Kenneth Galbraith

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There is no expedient to which a man will not go
to avoid the labor of thinking.
Thomas A. Edison.

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Those who know how to think need no teachers.
Mahatma Gandhi

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Just think. Just be quiet and think.
It’d make all the difference in the world.
Fred Rogers

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Think left and think right and think low and think high.
Oh, the thinks you can think up if only you try!
Dr. Seuss

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The average man never really thinks from end to end of his life.  The mental activity of such people is only a mouthing of clichés.
H.L. Mencken

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There are no dangerous thoughts; thinking itself is dangerous.
Hannah Arendt

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Most of one’s life is one prolonged
effort to prevent oneself thinking.
Aldous Huxley

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A great many people think they are thinking
when they are only rearranging their prejudices.
William James

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A thought, even a possibility, can shatter and transform us.
Friedrich Wilhelm Nietzsche

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Men fear thought as they fear nothing else on earth — more than ruin, more even than death. Thought is subversive and revolutionary, destructive and terrible, thought is merciless to privilege, established institutions, and comfortable habits; thought is anarchic and lawless, indifferent to authority, careless of the well-tried wisdom of the ages. Thought looks into the pit of hell and is not afraid. Thought is great and swift and free, the light of the world, and the chief glory of man. 

Bertrand Russell

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People mistakenly assume that their thinking is done by their head; it is actually done by the heart which first dictates the conclusion, then commands the head to provide the reasoning that will defend it.
Anthony de Mello

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All thought is immoral.
Its very essence is destruction.
If you think of anything, you kill it.
Nothing survives being thought of.
Oscar Wilde