Harry Truman used to have a sign on his desk, which I’ve liked since I first heard of it, quoting Mark Twain:
“Always do right. This will gratify some people and astonish the rest.”
A while ago (might it have been used by Al Gore in An Inconvenient Truth?) I came across another Mark Twain quote (or more exactly, a quote attributed to Mark Twain) which I have been using from time to time in workshops to remind people of the dangers of certainty:
“It ain’t what you don’t know that gets you into trouble. It’s what you know for sure that just ain’t so.”

November 13, 2008 at 12:19 am
Thanks for the link to my blog post about the “Twain” quote. I was wondering what accounted for the thundering herds coming to my blog.
Yours in certainty,
Bob Kalsey
December 1, 2008 at 10:08 pm
I found a great post about Mart Twain on Peterman’s Eye that I thought I’d share…
http://www.petermanseye.com/curiosities/notables-gossip/381-the-first-american-writer
Cheers!
April 22, 2010 at 4:08 am
Appreciate the link to Truman, and the great quote.