“Half Friendship is the bitterest Enmity…”
Frontiespiece, plate 1, line 8 (as it seen on the additional plate, Fitzwilliam Museum).
1800s, Jerusalem The Emanation of The Giant Albion (c. 1803–1820)
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William Blake 249
English Romantic poet and artist 1757–1827Related quotes

Letter to Lord Palmerston (9 October 1833), quoted in E. A. Smith, Lord Grey. 1764-1845 (Alan Sutton, 1996), p. 284.
1830s

“Ambition prompted many to become deceitful; to keep one thing concealed in the breast, and another ready on the tongue; to estimate friendships and enmities, not by their worth, but according to interest; and to carry rather a specious countenance than an honest heart.”
Ambitio multos mortales falsos fieri subegit, aliud clausum in pectore, aliud in lingua promptum habere, amicitias inimicitiasque non ex re, sed ex commodo aestimare, magisque vultum quam ingenium bonum habere.
Variant translation: It is the nature of ambition to make men liars and cheats, to hide the truth in their breasts, and show, like jugglers, another thing in their mouths, to cut all friendships and enmities to the measure of their own interest, and to make a good countenance without the help of good will.
Source: Bellum Catilinae (c. 44 BC), Chapter X, section 5
Sallust, Bellum Catilinae, X, 5. This particular translation of the original Latin is from the essay "On Liberty" by Abraham Cowley: "Sallust, therefore, who was well acquainted with them both and with many such-like gentlemen of his time, says, 'That it is the nature of ambition' (Ambitio multos mortales falsos fieri coegit, etc.) 'to make men liars and cheaters; to hide the truth in their breasts, and show, like jugglers, another thing in their mouths; to cut all friendships and enmities to the measure of their own interest, and to make a good countenance without the help of good will.'" http://ibiblio.org/pub/docs/books/gutenberg/etext02/cowes10.txt The Wikiquote page for Sallust has the quote and a different translation.
Misattributed

Magnus Bane to a monkey in 1791, p. 12.
Source: The Bane Chronicles, What Really Happened in Peru (2013)
Context: He paused and admired the bromeliads, huge iridescent flower-like bowls made out of petals, shimmering with color and water. There were frogs inside the jewel-bright recesses of the flowers.
Then he looked up into the round brown eyes of a monkey.
'Hello, companion,' said Magnus.
The monkey made a terrible sound, half snarl and half hiss.
'I begin to rather doubt the beauty of our friendship,' said Magnus.
Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 300.