“There are many things that seem impossible only so long as one does not attempt them.”
Si le grain ne meurt [If It Die] (1924), ch. III
Source: Autumn Leaves
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André Gide 74
French novelist and essayist 1869–1951Related quotes
“So many things are possible just as long as you don't know they're impossible.”
Variant: So many things are possible as long as you don't know they're impossible.
Source: The Phantom Tollbooth
“Only the impossible is worth attempting. In everything else one is sure to fail.”
The Decline and Fall of Science (1976)

“Things always seem impossible until people do them.”
2016, Interview with Bill Kristol (2016)

Speech at the Democratic National Convention (26 August 1996) http://www.chrisreevehomepage.com/sp-dnc1996.html

“So much easier is it to do many things than to do one thing for a long time continuously.”
Adeo facilius est multa facere quam diu.
Book I, Chapter XII, 7; translation by H. E. Butler
De Institutione Oratoria (c. 95 AD)

Big Lies in Politics http://townhall.com/columnists/thomassowell/2012/05/22/big_lies_in_politics/page/full, 22 May 2012.
2010s