Knowing Yourself: The True in the False (1996)
“He does not confound it with probability; he takes for true what is true, for false what is false, for doubtful what is doubtful, and probable what is only probable. He does more, and here you have a great perfection of the philosopher: when he has no reason by which to judge, he knows how to live in suspension of judgment…
The philosophical spirit is, then, a spirit of observation and exactness, which relates everything to true principles…”
Article on Philosophy, Vol. 25, p. 667, as quoted in Main Currents of Western Thought : Readings in Western European Intellectual History from the Middle Ages to the Present (1978) by Franklin Le Van Baumer
Variant translation: Reason is to the philosopher what grace is to the Christian. Grace moves the Christian to act, reason moves the philosopher. Other men walk in darkness; the philosopher, who has the same passions, acts only after reflection; he walks through the night, but it is preceded by a torch. The philosopher forms his principles on an infinity of particular observations. … He does not confuse truth with plausibility; he takes for truth what is true, for forgery what is false, for doubtful what is doubtful, and probable what is probable. … The philosophical spirit is thus a spirit of observation and accuracy.
L'Encyclopédie (1751-1766)
Context: Reason is to the philosopher what grace is to the Christian.
Grace causes the Christian to act, reason the philosopher. Other men are carried away by their passions, their actions not being preceded by reflection: these are the men who walk in darkness. On the other hand, the philosopher, even in his passions, acts only after reflection; he walks in the dark, but by a torch.
The philosopher forms his principles from an infinity of particular observations. Most people adopt principles without thinking of the observations that have produced them, they believe the maxims exist, so to speak, by themselves. But the philosopher takes maxims from their source; he examines their origin; he knows their proper value, and he makes use of them only in so far as they suit him.
Truth is not for the philosopher a mistress who corrupts his imagination and whom he believes to be found everywhere; he contents himself with being able to unravel it where he can perceive it. He does not confound it with probability; he takes for true what is true, for false what is false, for doubtful what is doubtful, and probable what is only probable. He does more, and here you have a great perfection of the philosopher: when he has no reason by which to judge, he knows how to live in suspension of judgment...
The philosophical spirit is, then, a spirit of observation and exactness, which relates everything to true principles...
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Denis Diderot 106
French Enlightenment philosopher and encyclopædist 1713–1784Related quotes
Who could have the conceit, the self-confidence to believe that that is what we should do throughout all the rest of human history?
Letter to Charles Humboldt (mid-1962), p. 64
The Selected Letters of George Oppen (1990)

This quote is itself quoting Floyd 'Red Crow' Westerman in the film Grey Owl (1999)
Examples of self-translation (c. 2004), Quotes - Zitate - Citations - Citazioni

As quoted in Novelists in Interview (1985) edited by John Haffenden

Source: The Roving Mind (1983), Ch. 25
Context: How often people speak of art and science as though they were two entirely different things, with no interconnection. An artist is emotional, they think, and uses only his intuition; he sees all at once and has no need of reason. A scientist is cold, they think, and uses only his reason; he argues carefully step by step, and needs no imagination. That is all wrong. The true artist is quite rational as well as imaginative and knows what he is doing; if he does not, his art suffers. The true scientist is quite imaginative as well as rational, and sometimes leaps to solutions where reason can follow only slowly; if he does not, his science suffers.

Oration delivered at Daniel O'Connell celebration, Boston (6 August 1870), published in Wendell Phillips: The Agitator (1890) by William Carlos Martyn, p. 563
1870s

“The true test of a man’s character is what he does when no one is watching.”

The Decline and Fall of Practically Everybody (1950), Part II: Ancient Greeks and Worse, Hannibal
Source: The Wheel of Time: Shamans of Ancient Mexico, Their Thoughts About Life, Death and the Universe], (1998), Quotations from "Journey to Ixtlan" (Chapter 8)