“Mucius put his hand into the fire. It is painful to be burned; but how much more painful to inflict such suffering upon oneself!”
Epistulae Morales ad Lucilium (Moral Letters to Lucilius), Letter XXIV: On despising death
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Seneca the Younger 225
Roman Stoic philosopher, statesman, and dramatist -4–65 BCRelated quotes

“Vegan Diet Damages Baby's Brain—Sensationalism!,” in VegNews (March–April 2003), p. 10; as quoted in Will Tuttle, The World Peace Diet (Lantern Books, 2005), p. 66 https://books.google.it/books?id=BTqLjAOwsSMC&pg=PA66.

“One fire burns out another's burning,
One pain is lessen'd by another's anguish.”
Source: Romeo and Juliet

“All pain is a punishment, and every punishment is inflicted for love as much as for justice.”
"Fifth Dialogue," p. 149
St. Petersburg Dialogues (1821)
“Societies which inflict pain and discomfort upon their infants tend to neglect them as well.”
"Body Pleasure and the Origins of Violence" (1975)

Misattributed
Source: Robert McAfee Brown https://en.wikipedia.org/wiki/Robert_McAfee_Brown. Preface for the 25th anniversary edition of Night https://en.wikipedia.org/wiki/Night_%28book%29. Page v, Bantam Books paperback; 1982 reissue edition.

"Villanelle" (1928), line 1; cited from John Haffenden (ed.) The Complete Poems (London: Allen Lane, 2000) p. 33.
The Complete Poems