Source: The Phoenix and the Mirror (1969), Chapter 1
Quotes about doubt
page 15
Hindu Temples – What Happened to Them, Volume II (1993)
Source: Systems Engineering Tools, (1965), Systems Engineering Methods (1967), p. 24; First paragraph of Ch. 2. System Organization, Scheduling, and Record-Keeping

"It's Raining In Love"
The Pill Versus the Springhill Mining Disaster

1963, Speech at Amherst College
Context: The men who create power make an indispensable contribution to the Nation's greatness, but the men who question power make a contribution just as indispensable, especially when that questioning is disinterested, for they determine whether we use power or power uses us.

Jede Äußerung menschlichen Geisteslebens kann als eine Art der Sprache aufgefaßt werden, und diese Auffassung erschließt nach Art einer wahrhaften Methode überall neue Fragestellungen.
"On Language as Such and on the Language of Man" (1916), translated by E. Jephcott, in Walter Benjamin: Selected Writings, Vol. 1 (1996), p. 62

2000s, The Real Abraham Lincoln: A Debate (2002), Q&A

Homecoming saga, The Memory Of Earth (1992)

“What expertise can theologians bring to deep cosmological questions that scientists cannot?”
Source: The God Delusion (2006), p. 79

"British and American warships standing by", The Times, 25 October 1983, p. 4.
Answering a question on Grenada in the House of Commons, 24 October 1983. The United States invaded that night.

Letter to his mother from Cuzco, Peru (22 August 1953); as quoted in "Making of a Marxist" in The Guardian (16 June 2001) http://books.guardian.co.uk/extracts/story/0,,507694,00.html

Speech to the People's Party Congress (11 October 1924), quoted in W. M. Knight-Patterson, Germany. From Defeat to Conquest 1913-1933 (London: George Allen and Unwin, 1945), p. 352
1920s

Source: Why Men Are the Way They Are (1988), p. 310.
Authority and persuasion in philosophy (1985)

"On the Roof of the World", Liberty Bell magazine (December 1987)
1970s, 1980s

Quoted in DNA India (04 March 2015) http://www.dnaindia.com/india/interview-maharashtra-s-beef-ban-is-not-merely-communal-it-is-theocratic-kancha-ilaiah-2066223.

The Other World (1657)

" Josh Homme - Guitar Moves - Episode 3 http://www.youtube.com/watch?v=AJDUHq2mJx0" Noisey (May 29, 2013)

Without Me, You're Nothing: The Essential Guide to Home Computers (1981), co-written with Max Barnard
General sources

“To ask the hard question is simple,
The simple act of the confused will.”
To Ask the Hard Question is Simple, first published in book form in Poems (1930)

Page 105
2000s, (2008)

“He had nowhere to look for answers, and he was tired of asking questions.”
Source: Gardens of the Moon (1999), Chapter 11 (p. 341)

Christine
O'Donnell
Opposite Attraction; Pitching Abstinence to the Young and the Restless at the HFStival
1997-06-15
The Washington Post
C1
2010-09-15
Remembering Christine O'Donnell: Praising Helms, Missing Lenny and Squiggy, and Worries of Rampant Satanism
Kyle
Right Wing Watch
http://www.rightwingwatch.org/content/remembering-christine-odonnell-praising-helms-missing-lenny-and-squiggy-and-worries-rampant-
2010-10-20

Source: New Options for America (1991), Chapter 19, "Twenty-eight Ways of Looking at Terrorism," pp. 167–68.

Accusations that USADA fabricated evidence http://inrng.com/2012/08/can-liggett-save-armstrong/ (30 August 2012)

2000s, God Bless America (2008), Slavery and the American Cause
Source: The Mind Of The Strategist, 1982, p. 57
Source: Facets of Liberty: A Libertarian Primer, (1985), pp. 139-140. (Chapter 17: “Who’s Afraid of No Government?”)

"Human Rights, Rationality, and Sentimentality." Truth and Progress: Philosophical Papers, Volume 3 (1998).

First Lecture, The Definition of Probability, p. 26
Probability, Statistics And Truth - Second Revised English Edition - (1957)

The Discover Interview: Lisa Randall (July 2006)

Pearl, Judea (2008) "Causal Inference," in: Pearl, Judea. The science and ethics of causal modeling. (2010).

Speech of Jefferson Davis before the Mississippi Legislature http://www.confederatepastpresent.org/index.php?option=com_content&view=article&id=117:speech-of-jefferson-davis-before-the-mississippi-legislature-nov-16-1858q-where-he-advocates-secession-if-an-abolitionist-is-elected-president-&catid=41:the-gathering-storm (16 November 1858)
1850s
Source: "The Conduct of Inquiry", p. 43.

“The people you hate, well, this is the question about such people: why do you hate them?”
Chance Meetings (1978)
Source: The Repossession Mambo (2009), Chapter 2 (p. 16)
Source: Fiction Sets You Free: Literature, Liberty and Western Culture (2007), p. 19.

And their confidence was seductive!
John Oliver: Terrifying Times (2008)
Adam Przeworski and Fernando Limongi, The Journal of Economic Perspectives (Summer, 1993)

“Half a lifetime is spent with the unspoken question: Will it happen will it not?”
Sketchbook 1946-1949

A: "Because he's a fake. I find it sort of fascinating how when people are running for office they're really fiscally conservative. When they're in office, they vote for something different, and then when they explain themselves they say "Oh! I want to repeal that." So the Senator voted for No Child Left Behind. He voted for it, but now he's running on the effort to get rid of it!.....This loses credibility." https://www.youtube.com/watch?v=YZPexbzJUU4
2012

“Great question to ask, to relax: What should I consider right now, decide what, and let go?”
2 July 2012 https://twitter.com/gtdguy/status/220039630240747521
Official Twitter profile (@gtdguy) https://twitter.com/gtdguy
2000s

“Hamm: Ah, the old questions, the old answers, there's nothing like them!”
Endgame (1957)

Mainstream human rights into trade agreements and WTO practice – UN expert urges in new report http://www.ohchr.org/EN/NewsEvents/Pages/DisplayNews.aspx?NewsID=20473&LangID=E#sthash.bn9VjkJJ.dpuf.
2016, Mainstream human rights into trade agreements and WTO practice – UN expert urges in new report

Letter to John Bright (14 September 1854), quoted in John Morley, The Life of Richard Cobden (London: T. Fisher Unwin, 1905), p. 626.
1850s

Source: The Intelligent Investor: The Classic Text on Value Investing (1949), Chapter III, The Investor and His Advisers, p. 51

Michael Halliday in: G. Thompson (1998) " Interview with M. A. K. Halliday, Cardiff, July 1998 http://www.scielo.br/pdf/delta/v17n1/a06v17n1.pdf". Answer to the question, how he saw his own work as fitting into the development of linguistics.
1970s and later

We should take them back. Not until then can we have a constructive debate.
Nils Funcke (Swedish journalist and expert on freedom of expression) in interview with Sanna Trygg, October 2010. http://blogs.lse.ac.uk/polis/files/2012/01/IsCommentFree_PolisLSETrygg.pdf http://blogs.lse.ac.uk/polis/2012/01/19/is-comment-free-new-polis-research-report-on-the-moderation-of-online-news/

Source: New Testament and Mythology and Other Basic Writings (1941), p. 10

A Conversation with Ward Cunningham (2003), Exploring with Wiki
Rudolf Carnap (1937) cited in: Irving J. Lee (1967) The Language of Wisdom and Folly: Background Readings in Semantics. International Society for General Semantics, p. 44

Source: The Anti-Post-Modern Post-Modernist http://errolmorris.com/content/lecture/theantipost.html
Source: Liberty Before Liberalism (1998), p. 112

1850s, Latter-Day Pamphlets (1850), The Present Time (February 1, 1850)
"The Practical Wisdom of Ikujiro Nonaka," 2008
Source: Mathematics and the Physical World (1959), p. 148

Newsweek September 2006 http://www.msnbc.msn.com/id/14870541/site/newsweek/?page=6
Robert L. Flood (1999, p. 252-253) as cited in: Michael H. G. Hoffmann (2007) Searching for Common Ground on Hamas Through Logical Argument Mapping. p. 5.

Interview with John Humphrys on The Today Program (23 December 2006)

Variant transcription from "Death of a Genius" in Life Magazine: "Then do not stop to think about the reasons for what you are doing, about why you are questioning. The important thing is not to stop questioning. Curiosity has its own reasons for existence. One cannot help but be in awe when he contemplates the mysteries of eternity, of life, of the marvelous structure of reality. It is enough if one tries merely to comprehend a little of this mystery each day. Never lose a holy curiosity."
Source: Attributed in posthumous publications, Einstein and the Poet (1983), p. 138
Source: Speaking the Truth: Ecumenism, Liberation, and Black Theology (1986), pp. 7-8

Source: The Cambridge Encyclopedia of the English Language, 1987, p. 10

Brown : The Last Discovery of America (2003)

George Boole, " Solution of a Question in the Theory of Probabilities http://books.google.nl/books?id=9xtDAQAAIAAJ&pg=PA32" (30 November 1853) published in The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science (January 1854), p. 32
1850s
Source: The Look of Maps (1952), p. 17; as cited in: Kirk Patrick Goldsberry (2007) Real-time Traffic Maps. p. 23-24
Source: Leisure, the Basis of Culture (1948), The Philosophical Act, pp. 101–102

Source: 1890s, The Principles of Psychology (1890), Ch. 21

Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 114.

Source: Russia Under The Bolshevik Regime (1994), p. 241

Debate, House of Commons, Ottawa, Ontario, April 3, 1939.

Video History interview (1995)

Letter to F. Cobden (5 October 1838), quoted in John Morley, The Life of Richard Cobden (London: T. Fisher Unwin, 1905), p. 126.
1830s

Source: "Presidential Address British Association for the Advancement of Science," 1890, p. 467 : On the theory of numbers

Source: Sayings of Sri Ramakrishna (1960), p. 153

On the world's lack of knowledge about her native island, Barbados. Allure magazine, January 2008.

Concepts
“Liberalism and its Discontents,” pp. 24-25.
Outside Ethics (2005)