Doubt is not a pleasant condition, but certainty is absurd.

The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge.

It is better to remain silent and be thought a fool than to speak and remove all doubt.

Extraordinary claims require extraordinary evidence.

What can be asserted without evidence can also be dismissed without evidence.

The trouble with the world is that the stupid are cocksure and the intelligent are full of doubt.

Keep an open mind, but not so open that your brain falls out.

For every complex problem there is an answer that is clear, simple, and wrong.

Belief is the death of intelligence. As soon as one believes a doctrine of any sort, one stops thinking about that aspect of existence.

The essence of the independent mind lies not in what it thinks, but in how it thinks.

Convictions are more dangerous enemies of truth than lies.

Science is organized skepticism. It's the willingness to abandon currently accepted beliefs when better evidence comes along.

Doubt is the beginning, not the end, of wisdom.

The most difficult subjects can be explained to the most slow-witted man if he has not formed any idea of them already; but the simplest thing cannot be made clear to the most intelligent man if he is firmly persuaded that he knows already.

It is what you learn after you know it all that counts.

The skeptic does not mean he who doubts, but he who investigates or researches, as opposed to he who asserts and thinks that he has found.

All claims to knowledge should be tentative and subject to revision in the light of new evidence.

We are all entitled to our own opinions, but not to our own facts.

The true skeptic questions the reliability of his or her own beliefs as much as those of others.

Ignorance more frequently begets confidence than does knowledge.

Healthy skepticism is the first step toward truth. Blind faith is the last refuge of those afraid to think.

The most common of all follies is to believe passionately in the palpably not true. It is the chief occupation of mankind.

When you want to believe in something, you also have to believe in everything that's necessary for believing in it.

The art of being wise is the art of knowing what to overlook, and what to question relentlessly.