“It is not only by one's impulses that one achieves greatness, but also by patiently filing away the steel wall that separates what one feels from what one is capable of doing.”

Martí : Thoughts/Pensamientos (1994)
Context: To busy oneself with what is futile when one can do something useful, to attend to what is simple when one has the mettle to attempt what is difficult, is to strip talent of its dignity. It is a sin not to do what one is capable of doing.

Cambridge 1995, p. 237
The Ego and Its Own (1845)

Nobel lecture (8 December 1980)

As quoted in Goal Mapping : How to Turn Your Dreams into Realities (2006) by Brian Mayne, p. 84

Brown : The Last Discovery of America (2003)
Context: Books should confuse. Literature abhors the typical. Literature flows to the particular, the mundane, the greasiness of paper, the taste of warm beer, the smell of onion or quince. Auden has a line: "Ports have names they call the sea." Just so will literature describe life familiarly, regionally, in terms life is accustomed to use — high or low matters not. Literature cannot by this impulse betray the grandeur of its subject — there is only one subject: What it feels like to be alive. Nothing is irrelevant. Nothing is typical.

Query 2
Opticks (1704)

Source: Address to the American Association to Promote the Teaching of Speech to the Deaf at Mt. Airy, Philadelphia, Pennsylvania (8 July 1896) http://www.afb.org/mylife/book.asp?ch=P3Ch4, quoted in supplement to The Story of My Life