“Though love repine, and reason chafe,
There came a voice without reply, —
"'Tis man's perdition to be safe,
When for the truth he ought to die."”

The Tragic Sense of Life (1913), VI : In the Depths of the Abyss
Context: To all this, someone is sure to object that life ought to subject itself to reason, to which we will reply that nobody ought to do what he is unable to do, and life cannot subject itself to reason. "Ought, therefore can," some Kantian will retort. To which we shall demur: "Cannot, therefore ought not." And life cannot submit itself to reason, because the end of life is living and not understanding.

Source: Lover Unleashed

Source: When Beauty Tamed the Beast

The Virtues of a Religious Superior

As quoted in World Unity, Vol. IX, 3rd edition (1931), p. 190
1930s

Act III, scene iv
Manfred (1817)

Introductio ad prudentiam: Part II (1727), Gnomologia (1732)

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Source: As quoted in Lasker's Chess Magazine https://en.wikisource.org/wiki/Lasker%27s_Chess_Magazine/Volume_1

Book IV, Ch. 19 : Of Enthusiasm (Chapter added in the fourth edition).
Variant paraphrase, sometimes cited as a direct quote: One unerring mark of the love of truth is not entertaining any proposition with greater assurance than the proofs it is built upon will warrant.
As paraphrased in Peter's Quotations : Ideas for our Time (1979) by Laurence J. Peter, p. 500; also in The Demon-Haunted World : Science as a Candle in the Dark (1994) by Carl Sagan, p. 64
An Essay Concerning Human Understanding (1689)
Context: He that would seriously set upon the search of truth, ought in the first place to prepare his mind with a love of it. For he that loves it not, will not take much pains to get it; nor be much concerned when he misses it. There is nobody in the commonwealth of learning who does not profess himself a lover of truth: and there is not a rational creature that would not take it amiss to be thought otherwise of. And yet, for all this, one may truly say, that there are very few lovers of truth, for truth's sake, even amongst those who persuade themselves that they are so. How a man may know whether he be so in earnest, is worth inquiry: and I think there is one unerring mark of it, viz. The not entertaining any proposition with greater assurance than the proofs it is built upon will warrant. Whoever goes beyond this measure of assent, it is plain receives not the truth in the love of it; loves not truth for truth's sake, but for some other bye-end.