“Just as we suffer from excess in all things, so we suffer from excess in literature; thus we learn our lessons, not for life, but for the lecture room.”
Alternate translation: Not for life, but for school do we learn. (translator unknown)
Alternate translation: We are taught for the schoolroom, not for life. (translator unknown).
Source: Epistulae Morales ad Lucilium (Moral Letters to Lucilius), Letter CVI: On the corporeality of virtue, Line 12
Original
Quemadmodum omnium rerum, sic litterarum quoque intemperantia laboramus: non vitae sed scholae discimus.
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Seneca the Younger 225
Roman Stoic philosopher, statesman, and dramatist -4–65 BCRelated quotes

“To suffer without complaint is the only lesson we have to learn in this life”
Source: The Letters of Vincent van Gogh

Attributed in Einstein: The Life and Times by Ronald W. Clark (1971), p. 737. The only source given in the end notes is "personal information". Einstein is said to have made this comment when a box of candy was being passed around after dinner, and he said that his doctor wouldn't let him eat it. The book also says that 'A friend asked him why it was the devil and not God who had imposed the penalty. "What's the difference?" he answered. "One has a plus in front, the other a minus."'.
Attributed in posthumous publications

“…we learn resignation not by our own suffering, but by the suffering of others.”
Source: The Summing Up (1938), p. 64

“We suffer primarily not from our vices or our weaknesses, but from our illusions.”
Preface
The Image: A Guide to Pseudo-Events in America (1961)
Context: We suffer primarily not from our vices or our weaknesses, but from our illusions. We are haunted, not by reality, but by those images we have put in their place.