“Why is doubt the one thing we're never skeptical of? We question other peoples' beliefs, and the more sure they are the more we doubt them. But it never occurs to us to doubt our own doubt. Question our own questions. We think our questions are answers.”

Source: Speaker for the Dead

Assorted Themes, On the Meaning of Life

The Value of Science (1955)
Context: The scientist has a lot of experience with ignorance and doubt and uncertainty, and this experience is of very great importance, I think. When a scientist doesn’t know the answer to a problem, he is ignorant. When he has a hunch as to what the result is, he is uncertain. And when he is pretty darn sure of what the result is going to be, he is still in some doubt. We have found it of paramount importance that in order to progress we must recognize our ignorance and leave room for doubt. Scientific knowledge is a body of statements of varying degrees of certainty — some most unsure, some nearly sure, but none absolutely certain. Now, we scientists are used to this, and we take it for granted that it is perfectly consistent to be unsure, that it is possible to live and not know. But I don’t know whether everyone realizes this is true. Our freedom to doubt was born out of a struggle against authority in the early days of science. It was a very deep and strong struggle: permit us to question — to doubt — to not be sure. I think that it is important that we do not forget this struggle and thus perhaps lose what we have gained.

Variant: The key to wisdom is this - constant and frequent questioning, for by doubting we are led to question and by questioning we arrive at the truth.

It is not odd at all. You only think you know, as a matter of fact. And most of your actions are based on incomplete knowledge and you really don't know what it is all about, or what the purpose of the world is, or know a great deal of other things. It is possible to live and not know.
from lecture "What is and What Should be the Role of Scientific Culture in Modern Society", given at the Galileo Symposium in Italy (1964)
The Pleasure of Finding Things Out (1999)

As quoted in Peter Godspeed, 'It is my duty' http://www.rezapahlavi.org/details_article.php?article=462&page=2, Canada National Post, September 24, 2010.
Interviews, 2010

6.51
1920s, Tractatus Logico-Philosophicus (1922)

1961, Inaugural Address
Context: Finally, to those nations who would make themselves our adversary, we offer not a pledge but a request: that both sides begin anew the quest for peace, before the dark powers of destruction unleashed by science engulf all humanity in planned or accidental self-destruction. We dare not tempt them with weakness. For only when our arms are sufficient beyond doubt can we be certain beyond doubt that they will never be employed.

साधुको माहात्म्य