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“His life was two lives now: the life he would have and the life he would forever wonder about.”
Source: Beautiful Ruins

“Rascals, would you live forever?”
To hesitant guards at the battle of Kolin https://en.wikipedia.org/wiki/Battle_of_Kol%C3%ADn#Battle, 18 June 1757
Attributed in E. M. Knowles, Oxford Dictionary Of Quotations (5th Edition) (Oxford University Press, 1999)
"Kerels, wollt ihr den euwig leben?" Frederick the Great, by David Fraser, Penguin 2000, p. 353 Other versions have it: "Hünde, wollt ihr euwig leben?" (Dogs, do you want to live for ever?).
Attributed

Said while making Fitzcarraldo
Herzog on Herzog (2002)

Mark Twain in eruption: hitherto unpublished pages about men and events, 1940, Mark Twain, Bernard Augustine De Voto, Harper & brothers. This appears to be the origin of the variant:
If you would have your work last forever, and by forever I mean fifty years, it must neither overtly preach nor overtly teach, but it must covertly preach and covertly teach.
Attributed to Twain by J. Michael Straczynski in The complete book of scriptwriting, 2002, Writer's Digest Books

“Cold Mountain Son
Forever not change
I live alone
Beyond life death”
Cold Mountain Transcendental Poetry

Interview, January 1994 http://bleacherreport.com/articles/20702-ayrton-senna-fourteen-years-later