Source: The Frontiers of Meaning: Three Informal Lectures on Music (1994), Ch. 3 : Explaining the Obvious
Quotes about difficulty
page 5

As quoted in Toole, Betty Alexandra (1998), Ada, the Enchantress of Numbers: Prophet of the Computer Age, Strawberry Press, ISBN 0912647183. p. 99

Quoted in "Germany plots with the Kremlin" - Page 49 - by Tete Harens Tetens - 1953

“It is surmounting difficulties that makes heroes.”
Lajos Kossuth (also known as Louis Kossoth), as quoted in Human Development in Action (1942) by University of California, and The Forbes Book of Business Quotations (1997) by Edward C. Goodman and Ted Goodman.
Misattributed
Hamming cites Forsythe, G.E., "What to do until the computer scientist comes", Am. Math. Monthly 75 (5), May 1968, p. 454-461.
One Man's View of Computer Science (1969)

Advertisement, p.3
The Differential and Integral Calculus (1836)

Small Houses: Their Economic Design and Construction (1922)

What sympathy is demanded of the viewer! He is asked to 'see' the future links
1961 - 1980, ARTnews Annual', October 1966

p. 184. Detailing the salvaging of U.S.S. S-51.

Interview with William Warren Bartley, cited in — [Bartley, William Warren, w:William Warren Bartley, Werner Erhard: the Transformation of a Man: the Founding of est, Clarkson N. Potter, Inc., 1978, New York, 157, 0-517-53502-5]

Source: Rodin : the man and his art, with leaves from his notebook, 1917, p. 281; About the sculpture The Gates of Hell

American Middle East Expert Daniel Pipes Clashes with Lebanese Journalist Khodhor 'Awarki on whether Israel Should Be a Jewish State, MEMRI, December 4, 2007 http://www.memritv.org/clip_transcript/en/1626.htm,.

“You are successful and creative only when you see an opportunity in every difficulty.”
Quotations from Gurudev’s teachings, Chinmya Mission Chicago

Quote in a letter to his friend Peiresc, 18 Dec. 1634; as cited by Simon Schrama, in Rembrandt's eyes, Alfred A. Knopf - Borzoi Books, New York 1999, p. 402
1625 - 1640

Resignation letter to Gladstone (12 July 1882), quoted in G. M. Trevelyan, The Life of John Bright (London: Constable, 1913), p. 433.
1880s

Speech http://hansard.millbanksystems.com/commons/1979/may/22/the-economy-pay-and-prices#S5CV0967P0_19790522_HOC_260 in the House of Commons (22 May 1979)
1970s
Source: 1960s - 1970s, The Systems Approach and Its Enemies (1979), p. 145; cited in C. WEST CHURCHMAN: CHAMPION OF THE SYSTEMS APPROACH http://filer.case.edu/nxb41/churchman.html, 2004-2007 Case Western Reserve University

As quoted by Alexander Macfarlane, Lectures on Ten British Physicists of the Nineteenth Century (1916) p. 95, https://books.google.com/books?id=43SBAAAAIAAJ&pg=PA95 "Henry John Stephen Smith (1826-1883) A Lecture delivered March 15, 1902"

1920s, Ways to Peace (1926)

Lecture I, , R. Manheim, trans. (1967), p. 2
Lectures on the Essence of Religion http://www.marxists.org/reference/archive/feuerbach/works/lectures/index.htm (1851)

This appeared in Banneker's Almanac in 1794, and is commonly attributed to him, but originates earlier in "Reflections on different Subjects of Morality, by Stanisław Leszczyński, King of Poland, Duke of Lorrain and Bar" in The Universal Magazine (1765), p. 119
Misattributed

1930s, Address at the dedication of the memorial on the Gettysburg battlefield (1938)

To Leon Goldensohn, April 9, 1946, from "The Nuremberg Interviews" by Leon Goldensohn, Robert Gellately - History - 2004

Source: The Man With the Iron Heart (2008), p. 11

Source: 1940s and later, Otto Neurath Economic Writings. Selections 1904-1945 (2004), p. 269

"Reflections on different Subjects of Morality, in The Universal Magazine (1765), p. 119.

Source: My Years with General Motors, 1963, p. 48-49

The Bill Of Rights (1958), p. 72.
Extra-judicial writings

Broadcast from London (25 September 1933), quoted in This Torch of Freedom (1935), p. 13.
1933

“You will get your difficulties with the point electron.”
as quoted by Hendrik Casimir in an interview http://www.aip.org/history/ohilist/4550_1.html by Thomas Kuhn, Léon Rosenfeld, Aage Bohr and Erik Ruedinger at the Institute for Theoretical Physics, Copenhagen, 5 and 6 July, 1963.
Attributed

Source: 1840s, Sermon Preached at Trinitatis Kirke, 1844, P. 162

As quoted in Carl Anderson. Some notes about his life and work at Caltech. The first of a series of biographical sketches of Caltech faculty members. Engineering and Science, Vol. 15:1 (October 1951) http://resolver.caltech.edu/CaltechES:15.1.0

Vol. 4. Part 2. Translated by W.P. Dickson
The History of Rome - Volume 4: Part 2

after 2000, Gerhard Richter: An Artist Beyond Isms' (2002)
Source: Endymion (1996), Chapter 30 (p. 262)

"Freedom National, Slavery Sectional," speech in the Senate (July 27, 1852).

Source: Aphorisms and Reflections (1901), p. 202

The Queen v. Justices of County of London, &c. (1893), L. R. 2 Q. B. 492.

Source: Don't Start the Revolution Without Me! (2008), Ch. 14 (p. 270)

Closing lines
The Trials of Life (1990)
Knowing Yourself: The True in the False (1996)

This would be impossible. A part cannot properly function separately from the whole. This is the natural order of the universe.
Source: Books, Spiritual Warrior, Volume I: Uncovering Spiritual Truths in Psychic Phenomena (Hari-Nama Press, 1996), Chapter 4: Fire and Brimstone, Horns and Tail, p. 66-67

An Old Man's Thoughts on Many Things, Of Education I
Source: The Mechanism of Economic Systems (1953), p. 2

As quoted in Ann Coulter: The blonde assassin" in The Independent 16 August 2004).
2004

Source: The Cotton Manufacture of Great Britain, 1836, p. 234

This group said in substance that "We will go on in spite of...," that "We will not allow anything to stop us," that "We will move on amid the difficulties, amid the trials, amid the tribulations."
1960s, Keep Moving From This Mountain (1965)

with the meekness and gentleness of Christ.
Reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895), p. 129.

As quoted by B. A. Bernstein, Bulletin of the American Mathematical Society 38 (1932), pp. 611-612.
The Foundations of Mathematics (1925)

Opening to Ch 14. Translation from: What Is Art and Essays on Art (Oxford University Press, 1930, trans. Aylmer Maude)
As quoted by physicist Joseph Ford in Chaotic Dynamics and Fractals (1985) edited by Michael Fielding Barnsley and Stephen G. Demko
What is Art? (1897)
Variant: I know that most men, including those at ease with problems of the greatest complexity, can seldom accept even the simplest and most obvious truth if it be such as would oblige them to admit the falsity of conclusions which they have delighted in explaining to colleagues, which they have proudly taught to others, and which they have woven, thread by thread, into the fabric of their lives.

1870s, Speech in the House of Representatives (1871)

Full transcript of bin Ladin's speech http://www.aljazeera.com/archive/2004/11/200849163336457223.html Aljazeera, (01 Nov 2004)
2000s, 2004

Quote of Camille Pissarro, Eragny, 26 April 1888, in a letter to his son Lucien; from Camille Pissarro - Letters to His Son Lucien ed. John Rewald, with assistance of Lucien Pissarro; from the unpublished French letters; transl. Lionel Abel; Pantheon Books Inc. New York, second edition, 1943, p. 124
Theo van Gogh was working in the Paris' art-gallery Goupil & Cie and selling Impressionist artists
1880's

Last words, as reported in Dictionary of Basic Tesuji by Fujisawa Shuko, trans. Steven Bretherick (Slate & Shell, 2007), Vol. IV, p. 23
" Libertarians Can Make a Difference by Being Different http://www.libertyforall.net/?p=7323," Liberty For All (8 February 2012, retrieved 25 February 2012).
Republished http://www.independentpoliticalreport.com/2012/02/lee-wrights-libertarians-can-make-a-difference-by-being-different/ by Independent Political Report (18 February 2012).
2012

Braque is referring in this quote to the early common start of Cubism ca. 1907 -1910, in Paris with Picasso
Source: 1946 - 1963, In conversation with Dora Vallier' (1954), p. 265
Case of Edmonds and others (1821), 1 St. Tr. (N. S.) 899.
On the advantage of facing doubt when one has discovered something new. Speech at the Nobel Banquet, December 10, 1986 http://nobelprize.org/nobel_prizes/physics/laureates/1986/ruska-speech.html.

Blank v. Footman & Co. (1888), 57 L. J. (N. S.) C. D. 914.

endorsing Barack Obama, Telegraph Column, October 21, 2008
2000s, 2008

Original French: J’ajoute que les Français connaissent et aiment le Maroc. Entre nos deux pays, il y a une capillarité culturelle, sociale et humaine qui transcende les difficultés de la conjoncture. Mais il y a aussi, en France, un réflexe sécuritaire parce qu’on fait l’amalgame entre le Maroc et d’autres pays de la rive sud de la Méditerranée.
Interview with Le Figaro–September 2001 http://www.maroc.ma/fr/discours-royaux/interview-accord%C3%A9e-par-sa-majest%C3%A9-le-roi-mohammed-vi-au-quotidien-fran%C3%A7ais-%C2%AB-le
[John M. Ziman, The Force of Knowledge: The Scientific Dimension of Society, Cambridge University Press, 1976, 0-521-09917-X, 119]

Simone Campbell, interviewed by Al Sharpton, " Nun Responds To Hannity's 'Communist' Comparison: 'Name Calling Is About All That Exists On That Side' http://www.mediamatters.org/video/2014/04/21/nun-responds-to-hannitys-communist-comparison-n/198961," Media Matters for America video, 4:12, April 21, 2014.

Speech to the twelfth congress of the Confederation of Socialist Parties of the EEC in Paris (12 November 1982), quoted in The Times (13 November 1982), p. 3
1980s

1860s, Reply to Charles Kingsley (1860)

Review http://www.rogerebert.com/reviews/latter-days-2004 of Latter Days (13 February 2004)
Reviews, Two-and-a-half star reviews

Variant: The man of virtue makes the difficulty to be overcome his first business, and success only a subsequent consideration: this may be called perfect virtue.
Source: The Analects, Other chapters
Source: Grass (1989), Chapter 2 (p. 7)

On the Correct Handling of Contradictions Among the People
Source: (zh-CN) 任何新生事物的成长都是要经过艰难曲折的。在社会主义事业中,要想不经过艰难曲折,不付出极大努力,总是一帆风顺,容易得到成功,这种想法,只是幻想。

But generally the positivistic scheme taken from mathematical logic is too narrow in a description of nature which necessarily uses words and concepts that are only vaguely defined.
Physics and Philosophy (1958)

Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 432.
Sultãn Mahmûd BegDhã of Gujarat (AD 1458-1511) Dwarka (Gujarat)
Tabqãt-i-Akharî

Source: Dictionary of Burning Words of Brilliant Writers (1895), p. 107

Speech at the reception for Booker T. Washington held in Essex Hall, Strand, London (3 July 1899), quoted in The Times (4 July 1899), p. 13.
1890s

Nelson Mandela on challenges, Letter to Winnie Mandela (1 February 1975), written on Robben Island. Source: From Nelson Mandela By Himself: The Authorised Book of Quotations © 2010 by Nelson R. Mandela and The Nelson Mandela Foundation http://www.nelsonmandela.org/content/mini-site/selected-quotes
1970s

IN THE WORLD OF SCREEN & STAGE The Oakland Tribune, Oakland, California https://beladraculalugosi.wordpress.com/1930-2/ (July 6, 1930)

Speech http://hansard.millbanksystems.com/commons/1937/may/05/supply in the House of Commons (5 May 1937).
1937

An Apology for Idlers.
Virginibus Puerisque and Other Papers (1881)

“I hold the imitation of colour to be the greatest difficulty of art.”
Quote from notes of El Greco, in one of his commentaries; as cited by Fernando Marías and Agustín Bustamante García in Las Ideas Artísticas de el Greco (Cátedra, 1981), p. 80; taken from Wikipedia/El Greco: in 'Technique and Style'
Original: una es la imitación de los <span class="plainlinks"> colores https://es.wikiquote.org/wiki/Colores</span> que yo tengo por la mayor <span class="plainlinks"> dificultad https://es.wikiquote.org/wiki/Dificultad</span>