“A great truth is a truth whose opposite is also a truth.”
Essay on Freud (16 May 1929)
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Thomas Mann 159
German novelist, and 1929 Nobel Prize laureate 1875–1955Related quotes

As quoted by his son Hans Bohr in "My Father", published in Niels Bohr: His Life and Work (1967), p. 328
Unsourced variant: The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth.
As quoted in Max Delbrück, Mind from Matter: An Essay on Evolutionary Epistemology, (1986) p. 167. It is the hallmark of any deep truth that its negation is also a deep truth

Il y a aussi deux sortes de vérités, celles de Raisonnement et celle de Fait. Les vérités de Raisonnement sont nécessaires et leur opposé est impossible, et celles de Fait sont contingentes et leur opposé est possible.
La monadologie (33).
The Monadology (1714)

“The truth is balance, but the opposite of truth, which is unbalance, may not be a lie.”
Review of Selected Essays by Simone Weil, The New York Review of Books (1 February 1963)
Context: The need for truth is not constant; no more than is the need for repose. An idea which is a distortion may have a greater intellectual thrust than the truth; it may better serve the needs of the spirit, which vary. The truth is balance, but the opposite of truth, which is unbalance, may not be a lie.

Speech http://hansard.millbanksystems.com/commons/1851/feb/11/agricultural-distress in the House of Commons (2 February 1851).
1850s

“It is not truth that makes man great, but man that makes truth great.”
As quoted in The Importance of Living (1937) by Lin Yutang, p. v
Attributed

“Poets are all who love, who feel great truths,
And tell them; and the truth of truths is love.”
Scene XVI, The Hesperian Sphere
Festus (1839)