“He began with an exordium, containing an apology for his resignation of the office of Advocate-General in the Court of Admiralty; and for his appearance in that cause, in opposition to the Crown, and in favor of the town of Boston, and the merchants of Boston and Salem.”

Letter to Pamela Clemens Moffet, 9 November 1869, in Albert Bigelow Paine, Mark Twain's Letters: Arranged with Comment (1917), Vol. 1, p. 168 http://books.google.com/books?id=Ia8hAAAAMAAJ&pg=PA168

p, 125
1860s, A Short Autobiography (1860)

Attributed to a 2007 Senate speech by Kathy Kiely, "Kennedy 'fashioned the modern day legal system of immigration' " http://www.courier-journal.com/article/20090826/NEWS01/908260380/Kennedy++fashioned+the+modern+day+legal+system+of+immigration+, USA Today, 26 August 2009
Attributed

The Analects, The Doctrine of the Mean
Context: The superior man does what is proper to the station in which he is; he does not desire to go beyond this. In a position of wealth and honor, he does what is proper to a position of wealth and honor. In a poor and low position, he does what is proper to a poor and low position. Situated among barbarous tribes, he does what is proper to a situation among barbarous tribes. In a position of sorrow and difficulty, he does what is proper to a position of sorrow and difficulty. The superior man can find himself in no situation in which he is not himself. In a high situation, he does not treat with contempt his inferiors. In a low situation, he does not court the favor of his superiors. He rectifies himself, and seeks for nothing from others, so that he has no dissatisfactions. He does not murmur against Heaven, nor grumble against men. Thus it is that the superior man is quiet and calm, waiting for the appointments of Heaven, while the mean man walks in dangerous paths, looking for lucky occurrences.

Source: Catch-22

Letter to George Robertson (15 August 1855)
1850s

Facebook post, 11-17-2017 PM

The Rickover Effect (1992)
Context: As a guide to engineering ethics, I should like to commend to you a liberal adaptation of the injunction contained in the oath of Hippocrates that the professional man do nothing that will harm his client. Since engineering is a profession which affects the material basis of everyone’s life, there is almost always an unconsulted third party involved in any contact between the engineer and those who employ him — and that is the country, the people as a whole. These, too, are the engineer’s clients, albeit involuntarily. Engineering ethics ought therefore to safeguard their interests most carefully. Knowing more about the public effects his work will have, the engineer ought to consider himself an “officer of the court” and keep the general interest always in mind.

Letter to William Beckford MP (15 October 1761), quoted in W. S. Taylor and J. H. Pringle (eds.), Correspondence of William Pitt, Earl of Chatham: Vol. II (London: John Murray, 1838), p. 158
1760s