“The object of oratory alone is not truth, but persuasion.”

Speaking as the Director of USIA, in testimony before a Congressional Committee (May 1963) http://pdaa.publicdiplomacy.org/?page_id=6
Context: American traditions and the American ethic require us to be truthful, but the most important reason is that truth is the best propaganda and lies are the worst. To be persuasive we must be believable; to be believable we must be credible; to be credible we must be truthful. It is as simple as that.

Volume III, chapter II, section 99.
The Stones of Venice (1853)
Source: The Stones of Venice: Volume I. The Foundations

Commencement address, Yale University, New Haven, Connecticut (11 June 1962) http://millercenter.org/scripps/archive/speeches/detail/3370
1962
Context: The great enemy of the truth is very often not the lie — deliberate, contrived and dishonest — but the myth — persistent, persuasive, and unrealistic. Too often we hold fast to the cliches of our forebears. We subject all facts to a prefabricated set of interpretations. We enjoy the comfort of opinion without the discomfort of thought.

Frag B 1.28-30, quoted by Sextus Empiricus, Against the Mathematicians, vii. 3; Simplicius, Commentary on the Heavens, 557-8; Proclus, Commentary on the Timaeus I, 345

Light (1919), Ch. XXII - Light
Context: It is not enough to speak; you must know words. When you have said, "I am in pain," or when you have said, "I am right," you have said nothing in reality, you have only spoken to yourself. The real presence of truth is not in every word of truth, because of the wear and tear of words, and the fleeting multiplicity of arguments. One must have the gift of persuasion, of leaving to truth its speaking simplicity, its solemn unfoldings. It is not I who will be able to speak from the depths of myself. The attention of men dazzles me when it rises before me. The very nakedness of paper frightens me and drowns my looks. Not I shall embellish that whiteness with writing like light. I understand of what a great tribune's sorrow is made; and I can only dream of him who, visibly summarizing the immense crisis of human necessity in a work which forgets nothing, which seems to forget nothing, without the blot even of a misplaced comma, will proclaim our Charter to the epochs of the times in which we are, and will let us see it. Blessed be that simplifier, from whatever country he may come, — but all the same, I should prefer him, at the bottom of my heart, to speak French.

Speaking as the Director of USIA, in testimony before a Congressional Committee (May 1963) http://pdaa.publicdiplomacy.org/?page_id=6

Source: The Meaning of God in Human Experience (1912), Ch. XIV : The Need of an Absolute, p. 192.
Context: As in reply to the skeptic or agnostic, who asserts in despair that there is no absolute truth. The dialectician retorts: Then at least your own assertion must be absolutely true. There must be some absolute truth, for you cannot assert that there is none without self-contradiction. As in Descartes' case, the doubter is reminded of himself. There, in his own assertion, is a certainty from which he cannot escape.
This turn of thought which reminds the enquirer of himself, we shall call the reflexive turn. It reappears in all discoveries of the Absolute. It is clinching--but is likely to disappoint, even as Descartes' result disappoints. For the skeptic finds that he also was in search of objective truth: and that the absolute truth of his statement is irrelevant to his quest. Whence his skepticism toward objective truth remains unanswered.

Frag. B 2.2-6, quoted by Proclus, Commentary on the Timaeus I, 345

1990s, Radical Thought (1994)