“Five exclamation marks, the sure sign of an insane mind.”
Variant: Multiple exclamation marks,' he went on, shaking his head, 'are a sure sign of a diseased mind.
Source: Reaper Man
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Terry Pratchett 796
English author 1948–2015Related quotes

“A kiss can be a comma, a question mark, or an exclamation point.”
Quoted in Theatre Arts Magazine, December 1955 http://books.google.com/books?id=jkNNAAAAYAAJ&q=%22A+kiss+can+be+a+comma+a+question+mark+or+an+exclamation+point%22&pg=PA8#v=onepage

“What sign is there more plain
Than self-destruction, of a mind insane?”
Quale è di pazzia segno più espresso
Che, per altri voler, perder se stesso?
Canto XXIV, stanza 1 (tr. B. Reynolds)
Orlando Furioso (1532)

a remark made in the House of Commons responding to a Laborite speech; reported as unverified in Respectfully Quoted: A Dictionary of Quotations (1989).
Disputed
“Being able to embrace contradictions is a sign of intelligence.
Or insanity.”
Source: Butcher Bird

“Compassion isn’t a sign of weakness, but a mark of civilization.”
" Where is the Love? http://www.nytimes.com/2013/11/28/opinion/kristof-where-is-the-love.html?src=recg", New York Times, 27 November 2013
“Nineteen forty-five marked the nadir of Western Civilization.”
Quoted by Paul Fussell. The Boys' Crusade. Random House, New York, 2003. pg xv. ISBN: 0-679-64088-6