Knowing Yourself: The True in the False (1996)
“Get it straight. Your boy you lose. Love you lose. Honor has been gone for a long time. Duty you do.
Sure and what's your duty? What I said I'd do. And all the other things you said you'd do?”
Pt. 2: Cuba (a few paragraphs from the end). The 'boy' is Thomas Hudson's last surviving son, Tom, a fighter pilot who was killed in action.
Islands in the Stream (1970)
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Ernest Hemingway 501
American author and journalist 1899–1961Related quotes

“In life you always fight for what you love to do. Defending your art is your duty.”
Original: (it) Nella vita combatti sempre per ciò che ami fare. Difendere la tua arte è il tuo dovere.
Source: prevale.net

“Always do sober what you said you'd do drunk. That will teach you to keep your mouth shut!”
From a set of "rules for life" sent to publisher Charles Scribner IV; quoted in Scribner's memoir In the Company of Writers (New York: Scribner, 1991), p. 64 https://books.google.com/books?id=yYdHGtlgIsYC&pg=PA64&dq=hemingway+%22rules+for+life%22&hl=en&sa=X&ved=0ahUKEwj-zvyfgNDMAhUJ_mMKHU6zDrYQ6AEIIzAB#v=onepage&q=%20%22rules%20for%20life%22&f=false
Source: The Monuments Men: Allied Heroes, Nazi Thieves, and the Greatest Treasure Hunt in History

Letter purportedly written to his son, G. W. Custis Lee (5 April 1852); published in The New York Sun (26 November 1864). Although the “Duty Letter” was presumed authentic for many decades and included in many biographies of Lee, it was repudiated in December 1864 by “a source entitled to know.” This repudiation was rediscovered by University of Virginia law professor Charles A. Graves who verified that the letter was inconsistent with Lee's biographical facts and letter-writing style. Lee's son also wrote to Graves that he did not recall ever receiving such a letter. “The Forged Letter of General Robert E. Lee”, Proceedings of the 26th annual meeting of the Virginia State Bar Association 17:176 http://books.google.com/books?id=EMkDAQAAIAAJ&pg=PA176 (1914)
Misattributed

Source: Morals and Dogma of the Ancient and Accepted Scottish Rite of Freemasonry (1871), Ch. XI : Sublime Elect of the Twelve, or Prince Ameth, p. 176
Context: The duties of a Prince Ameth are, to be earnest, true, reliable, And sincere; to protect the people against illegal impositions and exactions; to contend for their political rights, and to see, as far as he may or can, that those bear the burdens who reap the benefits of the Government.
You are to be true unto all men.
You are to be frank and sincere in all things.
You are to be earnest in doing whatever it is your duty to do.
And no man must repent that he has relied upon your resolve, your profession, or your word.
The great distinguishing characteristic of a Mason is sympathy with his kind. He recognizes in the human race one great family, all connected with himself by those invisible links, and that mighty net-work of circumstance, forged and woven by God.