Book I, Ch. 1 http://www.gutenberg.org/files/3187/3187-h/3187-h.htm#link2H_4_0002
Christian Science (1907)
Mark Twain: Trending quotes (page 2)
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A Tramp Abroad (1880)
Context: You may say a cat uses good grammar. Well, a cat does -- but you let a cat get excited once; you let a cat get to pulling fur with another cat on a shed, nights, and you'll hear grammar that will give you the lockjaw. Ignorant people think it's the noise which fighting cats make that is so aggravating, but it ain't so; it's the sickening grammar they use.
A Tramp Abroad (1880)
Variant: Never argue with an idiot. They will drag you down to their level and beat you with experience.
“The two most important days in your life are the day you were born and the day you find out why.”
This appears on the opening placard of the film The Equalizer, attributing it to Twain, but there is no evidence that Twain wrote it. A precursor is found in Taylor Hartman's self-help book The Character Code (first published 1991), where it is not attributed to Twain: "The three most significant days in your life are: 1. The day you were born. 2. The day you find out why you were born. 3. The day you discover how to contribute the gift you were born to give" ( Google Books link https://books.google.com/books?id=gIKCxWxNmeMC&pg=PA147&dq=%22day+you+find+out+why%22&hl=en&sa=X&ved=0ahUKEwijrJzc84vLAhUJzGMKHajvADEQ6AEIIjAB#v=onepage&q=%22day%20you%20find%20out%20why%22&f=false)
Disputed
“If you tell the truth you don't have to remember anything.”
Notebook entry, January or February 1894, Mark Twain's Notebook, ed. Albert Bigelow Paine (1935), p. 240 http://books.google.com/books?id=DjBVlb7cBSIC&pg=PA240
Variant: If you tell the truth you do not need a good memory!
Source: The Adventures of Huckleberry Finn
Vol. II, p. 7
Mark Twain's Autobiography (1924)
Source: Autobiographical dictation, 2 December 1906. Published in Autobiography of Mark Twain, Volume 2 (University of California Press, 2013)
“Don't believe the world owes you a living. The world owes you nothing. It was here first.”
Misattributed
Source: Often attributed to Twain, but sourced to Robert J. Burdette, Quote Investigator http://quoteinvestigator.com/2014/06/06/world-owes/
“The only reason why God created man is because he was disappointed with the monkey.”
Autobiographical Dictation (1906)
It is the choicest compliment that has ever been paid us, and the most gratifying to our feelings. It is simple, direct, gracefully phrased: it always sounds well — In God We Trust. I don't believe it would sound any better if it were true. And in a measure it is true — half the nation trusts in Him. That half has decided it.
Source: Mark Twain's Notebook (1935), p. 394