The thing for us to do is just to do our duty, and not worry about whether anybody sees us do it or not.
- Adventures of Huckleberry Finn

Do something every day that you don't want to do; this is the golden rule for acquiring the habit of doing your duty without pain.
- Following the Equator, Pudd'nhead Wilson's New Calendar

Do your duty and repent tomorrow.
- More Maxims of Mark, Merle Johnson, 1927

...a man's first duty is to his own conscience and honor; the party and country come second to that, and never first.
- Mark Twain, a Biography

No man ever does a duty for duty's sake but only for the sake of the satisfaction he personally gets out of doing the duty, or for the sake of avoiding the personal discomfort he would have to endure if he shirked that duty; also I indicated that there is no such thing as free will and no such thing as self-sacrifice.
- Autobiographical dictation, 4 September 1907. Published in Autobiography of Mark Twain, Vol. 3 (University of California Press, 2015)

I said I was so habituated to shirking my duty that I was able now to shirk it fifty times a day without a pang; that is, that I could shirk fifty duties a day without a pang if the opportunity to do it were furnished me; that I did not get fifty opportunities a day, but that I got an average of about that many a week, and that I had noticed a peculiarity, a quite interesting peculiarity, of these opportunities -- to wit, that the opportunity to do a duty was always furnished me by an outsider, it seldom originated with me; it was always furnished by some person who knew more about my duties toward the public than I did. I said I believed that if I should become the champion of every cause that was brought to my attention and shown by argument that it was my duty to take hold of it and champion it, I shouldn't ever have any time left to punch up the China missionaries or revel in any of the other duties that were of my own invention and that were occupying all the spare room in my heart.
- Autobiographical dictation, 13 January 1908. Published in Autobiography of Mark Twain, Vol. 3 (University of California Press, 2015)