Knowing Yourself: The True in the False (1996)
“Duty is the sublimest word in our language. Do your duty in all things. You cannot do more. You should never wish to do less.”
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
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Robert E. Lee 55
Confederate general in the Civil War 1807–1870Related quotes

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.

Rookwood's Case (1696), 13 How. St. Tr. 154.

“And now for all of us to do our duty!”
The Canton, Ohio Speech, Anti-War Speech (1918)
Context: And now for all of us to do our duty! The clarion call is ringing in our ears and we cannot falter without being convicted of treason to ourselves and to our great cause.
Do not worry over the charge of treason to your masters, but be concerned about the treason that involves yourselves. Be true to yourself and you cannot be a traitor to any good cause on earth.
Yes, in good time we are going to sweep into power in this nation and throughout the world. We are going to destroy all enslaving and degrading capitalist institutions and re-create them as free and humanizing institutions. The world is daily changing before our eyes. The sun of capitalism is setting; the sun of socialism is rising. It is our duty to build the new nation and the free republic.

“Do not be duped by little duties. Do not be a chore man all your days.”

“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