Yes, even I am dishonest. Not in many ways, but in some. Forty-one, I think it is.
- Letter to Joseph Twichell, 14 March 1905

All my life I have been honest -- comparatively honest. I could never use money I had not made honestly -- I could only lend it.
- Speech, 22 December 1907

I would much prefer to suffer from the clean incision of an honest lancet than from a sweetened poison.
- Letter to Olivia Langdon Clemens, 29 December 1869

Every man is wholly honest to himself and to God, but not to any one else.
- More Maxims of Mark, Merle Johnson, ed. (1927)

No man is straitly honest to any but himself and God.
- More Maxims of Mark, Merle Johnson, ed. (1927)

I think we never become really and genuinely our entire and honest selves until we are dead -- and not then until we have been dead years and years. People ought to start dead, and they would be honest so much earlier.
- Autobiographical dictation, 31 July 1906. Published in Autobiography of Mark Twain, Vol. 2 (University of California Press, 2013)

The insincerity of man -- all men are liars, partial or hiders of facts, half tellers of truths, shirks, moral sneaks. When a merely honest man appears he is a comet -- his fame is eternal -- needs no genius, no talent -- mere honesty -- Luther, Christ, etc.
- Mark Twain's Notebook

....honest men are few when it comes to themselves.
- quoted in My Mark Twain, William Dean Howells

As a rule, we go about with masks, we go about looking honest, and we are able to conceal ourselves all through the day.
- Speech, 8 March 1902

Also see "Diogenes and His Lantern"