“But oppression by your Mock-Superiors well shaken off, the grand problem yet remains to solve: That of finding government by your Real-Superiors! Alas, how shall we ever learn the solution of that, benighted, bewildered, sniffing, sneering, godforgetting unfortunates as we are? It is a work for centuries; to be taught us by tribulations, confusions, insurrections, obstructions; who knows if not by conflagration and despair! It is a lesson inclusive of all other lessons; the hardest of all lessons to learn.”
1840s, Past and Present (1843)
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Thomas Carlyle 481
Scottish philosopher, satirical writer, essayist, historian… 1795–1881Related quotes

1910s, The Progressives, Past and Present (1910)
“The hardest lesson that I learned is that “rejection is protection.””
Rejection never feels good, but as artists I think we tend to take rejection so personally. It can cause us to doubt our work or talent. However, rejection isn’t always someone saying we don’t like your work or you’re not talented. Sometimes it’s someone else recognizing that they can’t give you what you need to fly. It’s a venue saying this is not quite the right fit for you right now. That doesn’t mean that you won’t find home for your work. That doesn’t mean that venue won’t come looking for you one day. It means you have to keep working hard until you find the perfect fit and when the time is right it will work itself out.
On learning to take rejection in “Q&A Session with Award-Winning Author, Sheri Booker” https://www.huffpost.com/entry/interview-with-award-winning-author-sheri-booker_b_5684760 in HuffPost (2014 Aug 19)

“Lesson one: If you're ever in a beautiful cathedral, take your hat off!”
From "The Diary of Billy Talent":

As quoted by Nadezhda Popova, Reza Pahalavi, Son Of The Last Shah Of Iran: “No War Is Needed – We Will Topple This Regime Ourselves” http://www.rezapahlavi.org/details_article.php?article=22&page=6, Izvetsiya, May 29, 2006.
Interviews, 2006
Source: Sartre: Romantic Rationalist (1953), Ch. 10, p. 148 (the concluding sentence of the book)

“The hardest lesson for humans to learn: that organic complexity will entail organic time.”
Out of Control: The New Biology of Machines, Social Systems and the Economic World (1995)
“All the lessons learned, unlearned”
"Fall of a City"
Selected Poems (1941)
Context: All the lessons learned, unlearned;
The young, who learned to read, now blind
Their eyes with an archaic film;
The peasant relapses to a stumbling tune
Following the donkey`s bray;
These only remember to forget. But somewhere some word presses
On the high door of a skull and in some corner
Of an irrefrangible eye
Some old man memory jumps to a child
— Spark from the days of energy.
And the child hoards it like a bitter toy.

“Some lessons can't be taught, they simply have to be learned.”
Source: Vanishing Acts