“No man knows what he can do until he tries.”
Source: The Mis-Education of the Negro
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Carter G. Woodson 15
African-American historian and writer 1875–1950Related quotes
“No one knows what he can do till he tries.”
Maxim 786
Sentences, The Moral Sayings of Publius Syrus, a Roman Slave

“A man never knows what a fool he is until he hears himself imitated by one.”
Quoted by Max Beerbohm in Hebert Beerbohm Tree: Some Memories of Him and of His Art Collected by Max Beerbohm http://books.google.com/books?id=wM08AAAAIAAJ&q="A+man+never+knows+what+a+fool+he+is+until+he+hears+himself+imitated+by+one"&pg=PA312#v=onepage (1920).

Source: The Idea of History (1946), p. 10

“Man can do what he wills but he cannot will what he wills.”
Der Mensch kann tun was er will; er kann aber nicht wollen was er will.
Einstein paraphrasing Schopenhauer. Reportedly from On The Freedom Of The Will (1839), as translated in The Philosophy of American History: The Historical Field Theory (1945) by Morris Zucker, p. 531
Variant translations:
Man can do what he wants but he cannot want what he wants.
As quoted in The Motivated Brain: A Neurophysiological Analysis of Human Behavior (1991) by Pavel Vasilʹevich Simonov, p. 198
We can do what we wish, but we can only wish what we must.
As quoted by Einstein in "What Life Means to Einstein: An Interview by George Sylvester Viereck" The Saturday Evening Post (26 October 1929) p. 17. A scan of the article is available online here http://www.saturdayeveningpost.com/wp-content/uploads/satevepost/what_life_means_to_einstein.pdf (see p. 114).
Attributed
Source: Essays and Aphorisms

“When a man has put a limit on what he will do, he has put a limit on what he can do.”

Conversation with Jean Martet (1 June 1928), Ch. 30
Clemenceau, The Events of His Life (1930)

Song lyrics, Highway 61 Revisited (1965), Positively 4th Street