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.”
Around the edges 
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
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!
Appreciate the link to Truman, and the great quote.