Book I, Ch. 8 http://www.gutenberg.org/files/3187/3187-h/3187-h.htm#link2HCH0008
Christian Science (1907)
Mark Twain Quotes
Book I, Ch. 1 http://www.gutenberg.org/files/3187/3187-h/3187-h.htm#link2H_4_0002
Christian Science (1907)
“A lie can travel half way around the world while the truth is putting on its shoes.”
Variant: A lie can travel halfway around the world while the truth is putting on its shoes.
“Reader, suppose you were an idiot. And suppose you were a member of Congress. But I repeat myself.”
Draft manuscript (c.1881), quoted by Albert Bigelow Paine in Mark Twain: A Biography (1912), p. 724 http://books.google.com/books?id=2UYLAAAAIAAJ&pg=PA724#v=onepage&q&f=false
Variant: Suppose you were an idiot, and suppose you were a member of Congress; but I repeat myself.
“I did not attend his funeral, but I sent a nice letter saying I approved of it.”
Variant: I didn’t attend the funeral, but I sent a nice letter saying that I approved of it.
Letter to George Bainton, 15 October 1888, solicited for and printed in George Bainton, The Art of Authorship: Literary Reminiscences, Methods of Work, and Advice to Young Beginners (1890), pp. 87–88 http://books.google.com/books?id=XjBjzRN71_IC&pg=PA87.
Twain repeated the lightning bug/lightning comparison in several contexts, and credited Josh Billings for the idea:
Josh Billings defined the difference between humor and wit as that between the lightning bug and the lightning.
Speech at the 145th annual dinner of St. Andrew's Society, New York, 30 November 1901, Mark Twain Speaking (1976), ed. Paul Fatout, p. 424
Billings' original wording was characteristically affected:
Don't mistake vivacity for wit, thare iz about az mutch difference az thare iz between lightning and a lightning bug.
Josh Billings' Old Farmer's Allminax, "January 1871" http://books.google.com/books?id=sUI1AAAAMAAJ&pg=PT30. Also in Everybody's Friend, or; Josh Billing's Encyclopedia and Proverbial Philosophy of Wit and Humor (1874), p. 304 http://books.google.com/books?id=7rA8AAAAYAAJ&pg=PA304
Source: The Wit and Wisdom of Mark Twain
Pudd'nhead Wilson's New Calendar, Ch. XV
Misquoted as "Why shouldn’t truth be stranger than fiction? Fiction, after all, has to make sense." by Laurence J. Peter in "Peter’s Quotations: Ideas for Our Time", among many others.
Following the Equator (1897)
Source: Following the Equator: A Journey Around the World
Source: Following the Equator (1897), Ch. LXII
“Clothes make the man. Naked people have little or no influence on society.”
More Maxims of Mark (1927) edited by Merle Johnson
Variant: Clothes make the man. Naked people have little or no influence on society.
“You can't depend on your eyes when your imagination is out of focus.”
Ch. 43 http://www.literature.org/authors/twain-mark/connecticut/chapter-43.html
Source: A Connecticut Yankee in King Arthur's Court (1889)
“Always do what is right. It will gratify half of mankind and astound the other.”
To the Young People's Society, Greenpoint Presbyterian Church, Brooklyn (February 16, 1901).
Variant: Always do right. This will gratify some people, and astonish the rest.
“Kindness is the language which the deaf can hear and the blind can see.”
Unsourced in The Philosophy of Mark Twain: The Wit and Wisdom of a Literary Genius (2014) by David Graham
Disputed
marginal note in Moncure D. Conway's Sacred Anthology
quoted by Albert Bigelow Paine in Mark Twain: A Biography (1912)
“All you need in this life is ignorance and confidence, and then Success is sure.”
Mark Twain's Notebook, 1887
Letter to Cordelia Welsh Foote (Cincinnati), 2 December 1887. Letter reprinted http://www.twainquotes.com/Success.html in Benjamin De Casseres's When Huck Finn Went Highbrow https://www.worldcat.org/title/when-huck-finn-went-highbrow/oclc/2514292 (1934)
“History doesn't repeat itself, but it does rhyme.”
Origins unclear. Earliest known match in print comes from 1970, in a collection called “Neo Poems” by Canadian artist John Robert Colombo, who recalled reading it sometime in the 1960s. Twain did say "History never repeats itself, but the Kaleidoscopic combinations of the pictured present often seem to be constructed out of the broken fragments of antique legends." in the 1874 edition of “The Gilded Age: A Tale of To-Day”. A thematic precursor, "History May Not Repeat, But It Looks Alike", appears in a 1941 article by Chicago Tribune in Illinois. (Source: Quote Investigator https://quoteinvestigator.com/2014/01/12/history-rhymes/)
Misattributed
“Get your facts first, and then you can distort them as much as you please.”
As quoted in "An Interview with Mark Twain" http://ebooks.adelaide.edu.au/k/kipling/rudyard/seatosea/chapter37.html, From Sea to Sea: Letters of Travel (1899) by Rudyard Kipling, Ch. 37, p. 180
Commonly paraphrased as: "First get your facts, then you can distort them at your leisure."

Source: Tom Sawyer Abroad
“I must have a prodigious quantity of mind; it takes me as much as a week sometimes to make it up.”
Source: The Innocents Abroad (1869), Ch. 7
“It is curious that physical courage should be so common in the world, and moral courage so rare.”
Mark Twain in Eruption: Hitherto Unpublished Pages About Men and Events (1940) edited by Bernard DeVoto
More Maxims of Mark (1927) edited by Merle Johnson
“If voting made any difference they wouldn't let us do it.”
Variant: If voting made any difference they wouldn't let us do it.
“I've had a lot of worries in my life, most of which never happened.”
Variant: I've lived through some terrible things in my life, some of which actually happened.
“Education consists mainly in what we have unlearned.”
Source: Notebook
Source: Mark Twain's Notebook (1935), p. 346
Source: Following the Equator: A Journey Around the World