“All of humanity is in peril of extinction if each one of us does not dare, now and henceforth, always to tell only the truth, and all the truth, and to do so promptly — right now.”

Source: The Dream of a Ridiculous Man (1877), II
Context: Yes, I dreamed a dream, my dream of the third of November. They tease me now, telling me it was only a dream. But does it matter whether it was a dream or reality, if the dream made known to me the truth? If once one has recognized the truth and seen it, you know that it is the truth and that there is no other and there cannot be, whether you are asleep or awake. Let it be a dream, so be it, but that real life of which you make so much I had meant to extinguish by suicide, and my dream, my dream — oh, it revealed to me a different life, renewed, grand and full of power!

The Dignity of Human Nature (1754)

James Burgh, in The Dignity of Human Nature (1754)
Misattributed

Source: 1910s, Mysticism and Logic and Other Essays http://archive.org/stream/mysticism00russuoft/mysticism00russuoft_djvu.txt (1918), Ch. 3: A Free Man's Worship
Context: Such... but even more purposeless, more void of meaning, is the world which Science presents for our belief. Amid such a world, if anywhere, our ideals henceforward must find a home. That Man is the product of causes which had no prevision of the end they were achieving; that his origin, his growth, his hopes and fears, his loves and his beliefs, are but the outcome of accidental collocations of atoms; that no fire, no heroism, no intensity of thought and feeling, can preserve an individual life beyond the grave; that all the labours of the ages, all the devotion, all the inspiration, all the noonday brightness of human genius, are destined to extinction in the vast death of the solar system, and that the whole temple of Man's achievement must inevitably be buried beneath the débris of a universe in ruins—all these things, if not quite beyond dispute, are yet so nearly certain, that no philosophy which rejects them can hope to stand. Only within the scaffolding of these truths, only on the firm foundation of unyielding despair, can the soul's habitation henceforth be safely built.
Context: That Man is the product of causes that had no prevision of the end they were achieving; that his origin, his growth, his hopes and fears, his loves and his beliefs, are but the outcome of accidental collocations of atoms; that no fire, no heroism, no intensity of thought and feeling, can preserve individual life beyond the grave; that all the labors of the ages, all the devotion, all the inspiration, all the noonday brightness of human genius, are destined to extinction in the vast death of the solar system, and that the whole temple of Man's achievement must inevitably be buried beneath the debris of a universe in ruins – all these things, if not quite beyond dispute, are yet so nearly certain that no philosophy which rejects them can hope to stand. Only within the scaffolding of these truths, only on the firm foundation of unyielding despair, can the soul's habitation henceforth be safely built.

"Quotes", The Educated Imagination (1963), Talk 6: The Vocation of Eloquence

Interview with Glenn Greenwald, 6 June 2013, Part 1

Source: Wise Blood

Autobiographical sketch (23 July 1920), published in Portrait of a Marriage : Vita Sackville-West and Harold Nicolson (1998), p. 3
Context: Of course I have no right whatsoever to write down the truth about my life involving as it naturally does the lives of so many other people, but I do so urged by a necessity of truth-telling, because there is no living soul who knows the complete truth; here, may be one who knows a section; and there, one who knows another section: but to the whole picture not one is initiated.