“So it is more useful to watch a man in times of peril, and in adversity to discern what kind of man he is; for then at last words of truth are drawn from the depths of his heart, and the mask is torn off, reality remains.”
Book III, lines 55–58 (reported in Bartlett's Familiar Quotations)
De Rerum Natura (On the Nature of Things)
Original
Quo magis in dubiis hominem spectare periclis convenit adversisque in rebus noscere qui sit; nam verae voces tum demum pectore ab imo eliciuntur et eripitur persona, manet res.
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Lucretius 45
Roman poet and philosopher -94–-55 BCRelated quotes

“Man should know from this rule that he is cut off from truth.”
Source Book in Ancient Philosophy (1907), The Fragments

Freeman (1948), p. 150
May 1, 2007 http://littlegreenfootballs.com/weblog/?entry=25321_Kos_on_Truth&only

“A man begins cutting his wisdom teeth the first time he bites off more than he can chew.”
Editors of the Reader's Digest. Quotable Quotes, page 144. http://books.google.com/books?id=YdYPgwWFFR0C&pg=PT144 Penguin, 1997 ISBN 1606525956
Attributed

The Fantastic Imagination (1893)
Context: "But a man may then imagine in your work what he pleases, what you never meant!"
Not what he pleases, but what he can. If he be not a true man, he will draw evil out of the best; we need not mind how he treats any work of art! If he be a true man, he will imagine true things: what matter whether I meant them or not? They are there none the less that I cannot claim putting them there! One difference between God's work and man's is, that, while God's work cannot mean more than he meant, man's must mean more than he meant. For in everything that God has made, there is layer upon layer of ascending significance; also he expresses the same thought in higher and higher kinds of that thought: it is God's things, his embodied thoughts, which alone a man has to use, modified and adapted to his own purposes, for the expression of his thoughts; therefore he cannot help his words and figures falling into such combinations in the mind of another as he had himself not foreseen, so many are the thoughts allied to every other thought, so many are the relations involved in every figure, so many the facts hinted in every symbol. A man may well himself discover truth in what he wrote; for he was dealing all the time with things that came from thoughts beyond his own.
Source: True Grit (1968), Chapter 2, p. 20 : thoughts of 'Mattie Ross'

Conclusion
The Ethics of Ambiguity (1947)
Context: A conquest of this kind is never finished; the contingency remains, and, so that he may assert his will, man is even obliged to stir up in the world the outrage he does not want. But this element of failure is a very condition of his life; one can never dream of eliminating it without immediately dreaming of death. This does not mean that one should consent to failure, but rather one must consent to struggle against it without respite.

“Man keeps looking for a truth to fit his reality. Given our reality, the truth doesn't fit.”
[Adelaide Bry, 1976, est, 60 Hours that Transform Your Life, New York, Avon, 17]
Attributed