“Experience, that excellent master.”
Letter 20, 12.
Letters, Book I
Original
Usus, magister egregius.
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Pliny the Younger 50
Roman writer 61–113Related quotes

“Money is in some respects like fire; it is a very excellent servant but a terrible master.”
Ch. 3: "Avoid Debt" http://www.fourmilab.ch/etexts/www/barnum/moneygetting/moneygetting_chap4.html
Art of Money Getting (1880)

“Sometimes important meetings happen by chance. Learn to appreciate time, excellent master of life.”
Original: (it) A volte incontri importanti avvengono casualmente. Impara ad apprezzare il tempo, eccellente maestro di vita.
Source: prevale.net

Letter to George Washington (26 April 1779)

Letter to David Hartley (2 July 1787) https://founders.archives.gov/documents/Jefferson/01-11-02-0441
1780s
Context: I have no fear that the result of our experiment will be that men may be trusted to govern themselves without a master. Could the contrary of this be proved, I should conclude either that there is no god, or that he is a malevolent being.
Source: Modern thinkers and present problems, (1923), p. 37: Chapter 2. Benedict de Spinoza, 1632-1677
“If not excellence, what? If not excellence now, when?”
Source: The Little Big Things: 163 Ways To Pursue Excellence (2010), p. 9.

William S. Burroughs, Helnwein's Work http://www.helnwein.com/texte/selected_authors/artikel_103.html, Lawrence, Kansas, 1990