The Philosophical Emperor, a Political Experiment, or, The Progress of a False Position: (1841)
Context: Man soon finds what he wants to find. If he cannot find it otherwise, he creates it for his special enjoyment: for instance, if a man wants to see a ghost, he need only promulge his wish some night around a decaying fire, with a few alarmed and shocked listeners. Then let him ascend in the dark to a remote chamber, carefully looking over his shoulder every few moments; and if he will not see a ghost, he will feel as if he saw one, and that will be tantamount thereto.
“If a man cannot tell what he wants to do, then he must find out what he ought to do. If desire has become complicated, then hold fast to duty.”
Source: Arthur & George
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Julian Barnes 77
English writer 1946Related quotes

“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.”

A New Kind of Man
Song lyrics, A Sense of Wonder (1985)

Source: Dictionary of Burning Words of Brilliant Writers (1895), p. 616

“What one has, one ought to use; and whatever he does, he should do with all his might.”

As quoted in Existentialism from Dostoevsky to Sartre, p. 155
History as a System (1962)

Studs Terkel (2006). And They All Sang: The Great Musicians of the 20th Century Talk about Their Music. p. 287