“Oh, threats of Hell and Hopes of Paradise!
One thing at least is certain — This Life flies;
One thing is certain and the rest is Lies;
The Flower that once has blown for ever dies.”
Oh, come with old Khayyam, and leave the Wise
To talk; one thing is certain, that Life flies;
One thing is certain, and the Rest is Lies;
The Flower that once has blown for ever dies.
FitzGerald's first edition (1859).
The Rubaiyat (1120)
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Omar Khayyám 94
Persian poet, philosopher, mathematician, and astronomer 1048–1131Related quotes

“Certain success evicts one from the paradise of winning against the odds.”
In Memory Yet Green (1979), p. 420
General sources

Source: The Summing Up (1938), Ch. 5, p. 12 http://books.google.com/books?id=2hNbAAAAMAAJ&q=%22There+is+only+one+thing+about+which+I+am+certain+and+this+is%22&pg=PA12#v=onepage- 13 http://books.google.com/books?id=2hNbAAAAMAAJ&q=%22that+there+is+very+little+about+which+one+can+be+certain%22&pg=PA13#v=onepage

“Remember, Hope is a good thing, maybe the best of things, and no good thing ever dies.”
Sometimes quoted as "Nothing good ever dies."
Different Seasons (1982), Rita Hayworth and Shawshank Redemption
Source: The Shawshank Redemption

“There's only one thing that's certain
And that's everybody, everybody's hurting”
"Everybody's Hurting"
Women + Country (2010)

“Not to be absolutely certain is, I think, one of the essential things in rationality.”
"Don't Be Too Certain!"
1940s, Am I An Atheist Or An Agnostic? http://www.positiveatheism.org/hist/russell8.htm (1947)
Source: Am I an Atheist or an Agnostic?

“For whatever the future holds, one thing is certain… It just won't be the same.”
Source: V for Vendetta, Vol. I of X

"Childhood Is the Kingdom Where Nobody Dies," lines 1-4, from Wine from These Grapes (1934)

“One thing is certain in business. You and everyone around you will make mistakes.”
From Richard Branson's blog ‘Learning from mistakes’ on the Virgin Website