“Humor is the most honest of emotions. Applause for a speech can be insincere, but with humor, if the audience doesn't like it there's no faking it.”

Quoted in New York Post (29 February 1960)
Letters and interviews

BALIW
Source: The Manila Tribune. April 19, 1928.

The Humanist interview (2012)
Context: There were never that many women stand-up comics in the past because the power to make people laugh is also a power that gets people upset. But the ones who were performing were making jokes on themselves usually and now that’s changed. So there are no rules exactly but I think if you see a whole group of people only being self-deprecating, it’s a problem.
But I have always employed humor, and I think it’s absolutely crucial that we do because, among other things, humor is the only free emotion. I mean, you can compel fear, as we know. You can compel love, actually, if somebody is isolated and dependent — it’s like the Stockholm syndrome. But you can’t compel laughter. It happens when two things come together and make a third unexpectedly. It happens when you learn something, too. I think it was Einstein who said he had to be careful when he shaved because if he thought of something suddenly, he’d laugh and cut himself.
So I think laughter is crucial. Some of the original cultures, like the Dalit and the Native American, don’t separate laughter and seriousness. There’s none of this kind of false Episcopalian solemnity.

Source: The Hippopotamus Pool

Letter http://books.google.com/books?id=R8ksAAAAIAAJ&q=%22I+own+any+form+of+humor+shows+fear+and+inferiority+Irony+is+simply+a+kind+of+guardedness+So+is+a+twinkle+It+keeps+the+reader+from+criticism%22+%22Humor+is+the+most+engaging+cowardice%22&pg=PA166#v=onepage to Louis Untermeyer (10 March 1924)
General sources
Context: I own any form of humor shows fear and inferiority. Irony is simply a kind of guardedness. So is a twinkle. It keeps the reader from criticism. Whittier, when he shows any style at all is probably a greater person than Longfellow as he is lifted priestlike above consideration of the scornful. Belief is better than anything else, and it is best when rapt, above paying its respects to anybody's doubt whatsoever. At bottom the world isn't a joke. We only joke about it to avoid an issue with someone to let someone know that we know he's there with his questions: to disarm him by seeming to have heard and done justice to this side of the standing argument. Humor is the most engaging cowardice.

Unjust Judgments (1874)
Miscellany
Context: Humor is, however, nearer right than any emotion we have. Humor is the atmosphere in which grace most flourishes.

Part II, Chapter II (p. 130)
The Privilege of the Sword (2006)

[NewsBank, E1, That science guy is back, in 'Eyes of Nye', Seattle Post-Intelligencer, Seattle, Washington, April 5, 2005, Tom Paulson]