“A little sincerity is a dangerous thing, and a great deal of it is absolutely fatal.”
Source: The Critic as Artist (1891), Part II
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Oscar Wilde 812
Irish writer and poet 1854–1900Related quotes

“A little knowledge is a dangerous thing. I regret that this isn't fatal.”
Re: unibyte http://groups.google.com/group/gnu.emacs.help/msg/d767a45084444a5a (Usenet article).
Usenet articles, Miscellaneous

“No man can produce great things who is not thoroughly sincere in dealing with himself.”
Literary Essays, vol. II (1870–1890), Rousseau and the Sentimentalists

Address at the National Archives dedicating a shrine for the Declaration of Independence, the Constitution, and the Bill of Rights (15 December 1952) https://trumanlibrary.org/calendar/viewpapers.php?pid=2102
Context: Of course, there are dangers in religious freedom and freedom of opinion. But to deny these rights is worse than dangerous, it is absolutely fatal to liberty. The external threat to liberty should not drive us into suppressing liberty at home. Those who want the Government to regulate matters of the mind and spirit are like men who are so afraid of being murdered that they commit suicide to avoid assassination.
All freedom-loving nations, not the United States alone, are facing a stern challenge from the Communist tyranny. In the circumstances, alarm is justified. The man who isn't alarmed simply doesn't understand the situation — or he is crazy. But alarm is one thing, and hysteria is another. Hysteria impels people to destroy the very thing they are struggling to preserve.
Invasion and conquest by Communist armies would be a horror beyond our capacity to imagine. But invasion and conquest by Communist ideas of right and wrong would be just as bad.
For us to embrace the methods and morals of communism in order to defeat Communist aggression would be a moral disaster worse than any physical catastrophe. If that should come to pass, then the Constitution and the Declaration would be utterly dead and what we are doing today would be the gloomiest burial in the history of the world.

“It is always a silly thing to give advice, but to give good advice is absolutely fatal.”
The Portrait of Mr. W. H. (1889), p. 5
Source: Lady Windermere's Fan

On the Ergodic Behavior of Dynamical Systems (LA-2055, May 10, 1955) in [Stanisław Marcin Ulam, Analogies between Analogies, The Mathematical Reports of S.M. Ulam and His Los Alamos Collaborators, University of California Press, 1990, http://ark.cdlib.org/ark:/13030/ft9g50091s/]

1840s, Heroes and Hero-Worship (1840), The Hero As King

“It takes a great deal of history to produce a little literature.”
Hawthorne http://www.ibiblio.org/eldritch/hjj/nhhj1.html, (1879) ch. I: The Early Years.