“nothing that is worth knowing can be taught”

Source: Assassin's Apprentice

Book VII, sec. 4.
Naturalis Historia

Compare Poor Richard's Almanack (1745) : An ounce of wit that is bought, Is worth a pound that is taught.
Introductio ad prudentiam: Part II (1727), Gnomologia (1732)

“The Obscurity of the Poet”, p. 17
Poetry and the Age (1953)
Context: Goethe said, “The author whom a lexicon can keep up with is worth nothing”; Somerset Maugham says that the finest compliment he ever received was a letter in which one of his readers said: “I read your novel without having to look up a single word in the dictionary.” These writers, plainly, lived in different worlds.

Source: Fighting Ruben Wolfe

1900s, A Square Deal (1903)
Context: Among ourselves we differ in many qualities of body, head, and heart; we are unequally developed, mentally as well as physically. But each of us has the right to ask that he shall be protected from wrong-doing as he does his work and carries his burden through life. No man needs sympathy because he has to work, because he has a burden to carry. Far and away the best prize that life offers is the chance to work hard at work worth doing; and this is a prize open to every man, for there can be no better worth doing than that done to keep in health and comfort and with reasonable advantages those immediately dependent upon the husband, the father, or the son. There is no room in our healthy American life for the mere idler, for the man or the woman whose object it is throughout life to shirk the duties which life ought to bring. Life can mean nothing worth meaning, unless its prime aim is the doing of duty, the achievement of results worth achieving.

Source: Living Dharma: Teachings of Twelve Buddhist Masters