“My whole life is woven of threads which are in blatant contrast to my principles. … I love self-chosen poverty, and live among rich people; I avoid all honours, and yet some have come to me. … I believe that illusions are necessary to man, yet live without illusion; I believe that the passions are more profitable than reason, and yet no longer know what passion is.”

"Queen of the Black Coast" (1934)
Source: Conan the Barbarian Omnibus -The Original Stories
Context: He shrugged his shoulders. "I have known many gods. He who denies them is as blind as he who trusts them too deeply. I seek not beyond death. It may be the blackness averred by the Nemedian skeptics, or Crom's realm of ice and cloud, or the snowy plains and vaulted halls of the Nordheimer's Valhalla. I know not, nor do I care. Let me live deep while I live; let me know the rich juices of red meat and stinging wine on my palate, the hot embrace of white arms, the mad exultation of battle when the blue blades flame and crimson, and I am content. Let teachers and priests and philosophers brood over questions of reality and illusion. I know this: if life is illusion, then I am no less an illusion, and being thus, the illusion is real to me. I live, I burn with life, I love, I slay, and am content."

Variant transcription from "Death of a Genius" in Life Magazine: "Certainly there are things worth believing. I believe in the brotherhood of man and the uniqueness of the individual. But if you ask me to prove what I believe, I can't. You know them to be true but you could spend a whole lifetime without being able to prove them. The mind can proceed only so far upon what it knows and can prove. There comes a point where the mind takes a leap—call it intuition or what you will—and comes out upon a higher plane of knowledge, but can never prove how it got there. All great discoveries have involved such a leap."
Unsourced variant: "The intellect has little to do on the road to discovery. There comes a leap in consciousness, call it intuition or what you will, and the solution comes to you and you do not know how or why. All great discoveries are made in this way." The earliest published version of this variant appears to be The Human Side of Scientists by Ralph Edward Oesper (1975), p. 58 http://books.google.com/books?id=-J0cAQAAIAAJ&q=%22solution+comes+to+you+and+you+do+not+know%22&dq=%22solution+comes+to+you+and+you+do+not+know%22&hl=en, but no source is provided, and the similarity to the "Life Magazine" quote above suggests it's likely a misquote.
Source: Attributed in posthumous publications, Einstein and the Poet (1983), p. 136

I am
Poems Chiefly from Manuscript

Source: Frankenstein; Or, the Modern Prometheus

Voces (1943)

Quote from John Constable's letter to C.R. Leslie (March 1833), from The Letters of John Constable, R.A. to C. R. Leslie, R.A. 1826-1837 (Constable & Co., 1931), p. 104
1830s

Fry on the reason for the popularity of alternative medicine. "Last Chance to Think" Interview (2010) by Kylie Sturgess in Skeptical Inquirer. Vol 34 (1)
2000s

Letter to the Bailli de Ploën, as quoted in Recollections of the Private Life of General Lafayette (1835) by Jules Germain Cloquet, Vol. I, p. 24
Context: An irresistible passion that would induce me to believe in innate ideas, and the truth of prophecy, has decided my career. I have always loved liberty with the enthusiasm which actuates the religious man with the passion of a lover, and with the conviction of a geometrician. On leaving college, where nothing had displeased me more than a state of dependance, I viewed the greatness and the littleness of the court with contempt, the frivolities of society with pity, the minute pedantry of the army with disgust, and oppression of every sort with indignation. The attraction of the American revolution transported me suddenly to my place. I felt myself tranquil only when sailing between the continent whose powers I had braved, and that where, although our arrival and our ultimate success were problematical, I could, at the age of nineteen, take refuge in the alternative of conquering or perishing in the cause to which I had devoted myself.

Source: A Day In the Life of Brunello Cucinelli https://www.harpersbazaar.com/fashion/designers/a17874/brunello-cucinelli-profile/ Harper's Bazaar, Lauren McCarthy, 15 September 2016