Where it comes from
This is from the same talk as “the imagination of nature”: “The Value of Science”, which Feynman gave at the National Academy of Sciences’ autumn meeting on the Caltech campus in November 1955. Engineering and Science printed it that December, and we opened Caltech’s scan of the printed pages.
The magazine’s subtitle sums up the whole talk: “Of all its many values, the greatest must be the freedom to doubt.” The sentence we’re checking is on page 14, where Feynman turns to what he calls the third value of science.
The ladder of not knowing
He builds up to it in three steps, and they’re worth quoting because they’re the argument:
“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 in some doubt.”
Then: “We have found it of paramount importance that in order to progress we must recognize the ignorance and leave room for doubt. Scientific knowledge is a body of statements of varying degrees of certainty—some most unsure, some nearly sure, none absolutely certain.”
So the popular version is right. The only differences from the 1955 print are small: many copies add “but” before “none”, and some say “our ignorance” where the magazine says “the ignorance”.
Why he cared
The point wasn’t modesty for its own sake. Feynman goes straight on to say that scientists take it for granted “that it is possible to live and not know”, and that he doesn’t know “whether everyone realizes that this is true.” Then: “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, that’s all—not to be sure.”
He ends the talk by calling it a scientist’s responsibility “to teach how doubt is not to be feared but welcomed and discussed”. Twenty-six years later, in a BBC interview, he was still saying it: see our check of “questions that can’t be answered”.
Our take
Real. If you quote it, cite “The Value of Science” (1955), and drop the “but” if you want the original wording. The “freedom to doubt” lines that follow are also his, from the same page.
