Alfred Korzybski:

There are two ways to slide easily through life: to believe everything or to doubt everything; both ways save us from thinking.

Andre Gide:

Believe those who are seeking the truth; doubt those who find it.

Bertrand Russell:

When one admits that nothing is certain one must, I think, also add that some things are more nearly certain than others.

"Am I an Atheist or an Agnostic?"

Bertrand Russell:

Everything is vague to a degree you do not realize till you have tried to make it precise.

"The Philosophy of Logical Atomism"

Blaise Pascal:

To deny, to believe, and to doubt absolutely -- this is for man what running is for a horse.

Blaise Pascal:

We must learn our limits. We are all something, but none of us are everything.

Carl Sagan:

It is the tension between creativity and skepticism that has produced the stunning and unexpected findings of science.

Carl Sagan:

It seems to me what is called for is an exquisite balance between two conflicting needs: the most skeptical scrutiny of all hypotheses that are served up to us and at the same time a great openness to new ideas. If you are only skeptical, then no new ideas make it through to you. On the other hand, if you are open to the point of gullibility and have not an ounce of skeptical sense in you, then you cannot distinguish useful ideas from the worthless ones. (1987 lecture "The Burden of Skepticism")

David Hume:

When men are most sure and arrogant they are commonly most mistaken, giving views to passion without that proper deliberation which alone can secure them from the grossest absurdities.

Donald Rumsfeld:

As we know, there are known knowns. There are things we know we know. We also know there are known unknowns. That is to say we know there are some things we do not know. But there are also unknown unknowns, the ones we don't know we don't know.

Erich Fromm:

Creativity requires the courage to let go of certainties.

Frank Crane:

You may be deceived if you trust too much, but you will live in torment if you don't trust enough.

Franklin Delano Roosevelt:

The only limit to our realization of tomorrow will be our doubts of today.

G Gaia:

The common dogma [of fundamentalists] is fear of modern knowledge, inability to cope with the fast change in a scientific-technological society, and the real breakdown in apparent moral order in recent years.... That is why hate is the major fuel, fear is the cement of the movement, and superstitious ignorance is the best defense against the dangerous new knowledge. ... When you bring up arguments that cast serious doubts on their cherished beliefs you are not simply making a rhetorical point, you are threatening their whole Universe and their immortality. That provokes anger and quite frequently violence. ... Unfortunately you cannot reason with them and you even risk violence in confronting them. Their numbers will decline only when society stabilizes, and adapts to modernity.

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G. K. Chesterton:

It is not bigotry to be certain we are right; but it is bigotry to be unable to imagine how we might possibly have gone wrong.

Georg C. Lichtenberg:

One's first step in wisdom is to question everything - and one's last is to come to terms with everything.

Isaac Bashevis Singer:

Doubt is part of all religion. All the religious thinkers were doubters.

New York Times, December 3, 1978

James Baldwin:

Nobody is more dangerous than he who imagines himself pure in heart, for his purity, by definition, is unassailable.

James Joyce:

I've put in so many enigmas and puzzles that it will keep the professors busy for centuries arguing over what I meant, and that's the only way of insuring one's immortality.

Jane Addams:

Our doubts are traitors and make us lose the good we often might win, by fearing to attempt.

Johann Goethe:

If you must tell me your opinions, tell me what you believe in. I have plenty of douts of my own.

Johann Kaspar Lavater:

Trust him not with your secrets, who, when left alone in your room, turns over your papers.

John A. Hutchinson:

Unthinking faith is a curious offering to be made to the creator of the human mind.

John Wooden:

Do not let what you cannot do interfere with what you can do.

Mary Kay Ash:

Aerodynamically the bumblebee shouldn't be able to fly, but the bumblebee doesn't know that so it goes on flying anyway.

Oliver Wendell Holmes:

To have doubted one's own first principles is the mark of a civilized man.

Peter F. Drucker:

There are no creeds in mathematics.

Pierre Abelard:

The beginning of wisdom is found in doubting; by doubting we come to the question, and by seeking we may come upon the truth.

Princess Diana:

Only do what your heart tells you.

Ralph Waldo Emerson:

Don't waste life in doubts and fears; spend yourself on the work before you, well assured that the right performance of this hour's duties will be the best preparation for the hours and ages that will follow it.

Ralph Waldo Emerson:

Our distrust is very expensive.

Rollo May:

The relationship between commitment and doubt is by no means an antagonistic one. Commitment is healthiest when it is not without doubt but in spite of doubt.

Theodore Rubin:

There are two ways to slide easily through life: to believe everything or to doubt everything; both ways save us from thinking.

Thomas H. Huxley:

Sit down before fact as a little child, be prepared to give up every preconceived notion, follow humbly wherever or whatever abysses nature leads, or you will learn nothing.

Tom Peters:

If you're not confused, you're not paying attention.

Umberto Eco:

I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth.

Ursula K. LeGuin:

The only thing that makes life possible is permanent, intolerable uncertainty; not knowing what comes next.

Victor Frankl:

What man actually needs is not a tensionless state but rather the striving and struggling for some goal worthy of him. What he needs is not the discharge of tension at any cost, but the call of a potential meaning waiting to be fulfilled by him.

Vince Lombardi:

Winning is a habit. Unfortunately, so is losing.

Virginia Woolf:

It is worth mentioning, for future reference, that the creative power which bubbles so pleasantly in beginning a new book quiets down after a time, and one goes on more steadily. Doubts creep in. Then one becomes resigned. Determination not to give in, and the sense of an impending shape keep one at it more than anything.