“Tact is the knack of making a point without making an enemy.”
Actually a statement by American advertising executive and author Howard W. Newton (1903–1951); attributions to Isaac are relatively recent, those to Howard date at least to Sylva Vol. 1-3 (1945), p. 57 https://books.google.com/books?id=-QUcAQAAMAAJ&q=%22Tact+is+the+knack+of+making+a+point+without+making+an+enemy%22&dq=%22Tact+is+the+knack+of+making+a+point+without+making+an+enemy%22&hl=en&sa=X&ei=jtmwVJrZN43ksATPmID4BA&ved=0CNkBEOgBMCQ, where it is cited to an earlier publication in Redbook.
Misattributed
Variant: Tact is the art of making a point without making an enemy.
Variant: Tact is the knack of making a point without making an enemy.
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Isaac Newton 171
British physicist and mathematician and founder of modern c… 1643–1727Related quotes

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As quoted in Diogenes Laërtius, Lives and Opinions of Eminent Philosophers, "Pythagoras", Sect. 23, as translated in Dictionary of Quotations http://archive.org/details/dictionaryquota02harbgoog (1906) by Thomas Benfield Harbottle, p. 320
Last Act in Palmyra
Context: The new "humour", if you can call it that, is pure malicious gossip. Instead of making a genuine point, it's now good enough to repeat any ribald story without a thought for whether it's even true.

“A trifling debt makes a man your debtor; a large one makes him an enemy.”
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Wars I Have Seen (1945)