“He who offends others, does not secure himself.”
The Notebooks of Leonardo da Vinci (1883), X Studies and Sketches for Pictures and Decorations
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Leonardo Da Vinci 363
Italian Renaissance polymath 1452–1519Related quotes

Source: Aphorisms and Reflections (1901), p. 270

dies slowly…
Muere lentamente quien no viaja, quien no lee,
quien no oye música,
quien no encuentra gracia en sí mismo.
Muere lentamente
quien destruye su amor propio,
quien no se deja ayudar...
Poem "Muere lentamente" (Dying Slowly), wrongly attributed to Pablo Neruda. See "Fake Pablo Neruda Poem Spreads on Internet" http://www.laht.com/article.asp?ArticleId=325275&CategoryId=14094 by Ana Mendoza, Latin American Herald Tribune (12 January 2009).
Misattributed
Source: Selected Poems

“Who does not in some sort live to others, does not live much to himself.”
Book III, Ch. 10
Attributed

“He is an atheist who does not believe in himself.”
The old religions said that he was an atheist who did not believe in God. The new religion says that he is an atheist who does not believe in himself.
Call to the Nation

“He who wishes to secure the good of others, has already secured his own.”

“He who does not reflect his life back to God in gratitude does not know himself.”
Source: Reverence for Life: The Words of Albert Schweitzer

“He who knows others is wise. He who knows himself is enlightened.”

Undated manuscript, quoted in Ian Bradley, The Optimists: Themes and Personalities in Victorian Liberalism (1980), p. 76