“The most costly of all follies is to believe passionately in the palpably not true. It is the chief occupation of mankind.”
1940s–present, A Mencken Chrestomathy (1949)
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H.L. Mencken 281
American journalist and writer 1880–1956Related quotes

Graduation Exercises at the United States Military Academy at West Point, New York, U.S. (3 June 1947) http://www.eisenhower.archives.gov/all_about_ike/quotes.html
1940s
Context: War is mankind's most tragic and stupid folly; to seek or advise its deliberate provocation is a black crime against all men. Though you follow the trade of the warrior, you do so in the spirit of Washington — not of Genghis Khan. For Americans, only threat to our way of life justifies resort to conflict.

“To Mankind
And the hope that the war against folly may someday be won, after all.”
Dedication, p. 5; this refers to the quotation of Friedrich Schiller from which Asimov derived the title of this novel: "Against stupidity the gods themselves contend in vain."
The Gods Themselves (1972)

“To believe all men honest is folly. To believe none is something worse.”

Regarding slavery (1838), as quoted in Brother Against Brother: The War Begins, (The Civil War series) vol. 1, William C. Davis, New York, NY, Time-Life Books, (1983) p. 40
1830s

Original: (de) "Glaubt mir, des Menschen wahrster Wahn
wird ihm im Traume aufgetan:
all' Dichtkunst und Poeterei
ist nichts als Wahrtraumdeuterei."
Source: Quotes from his operas, Die Meistersinger von Nürnberg, Hans Sachs, Act 3, Scene 2

Dorothy Parker: Complete Broadway, 1918–1923 (2014) https://openlibrary.org/books/OL25758762M/Dorothy_Parker_Complete_Broadway_1918-1923, Chapter 3: 1920

Source: My Share Of The Task (2013), p. 393-394
Context: All leaders are human. They get tired, angry, and jealous and carry the same range of emotions and frailties common to mankind. Most leaders periodically display them. The leaders I most admired were totally human but constantly strove to be the best humans they could be. Leaders make mistakes, and they are often costly. The first reflex is normally to deny the failure to themselves; the second is to hide it from others, because most leaders covet a reputation for infallibility. But it's a fool's dream and inherently dishonest. There are few secrets to leadership. It is mostly just hard work. More than anything else it requires self-discipline. Colorful, charismatic characters often fascinate people, even soldiers. But over time, effectiveness is what counts. Those who lead most successfully do so while looking out for their followers' welfare.