“The passion for seeking the truth for truth's sake…can be kept alive only if we continue to seek the truth for truth's sake.”

Mas, sea verdad o sueño,
obrar bien es lo que importa.
Si fuere verdad, por serlo;
si no, por ganar amigos
para cuando despertemos.
Segismundo, Act III, l. 236.
La vida es sueño (Life is a Dream)

Quoted in Meenakshi Jain, "Flawed Narratives – History in the old NCERT Textbooks" http://hindureview.com/2001/02/22/flawed-narratives-history-old-ncert-textbooks/, And Quoted in R.C. Majumdar, The History and Culture of the Indian People, Vol. 7, Bharatiya Vidya Bhavan, Bombay, 1984, pp. xiii (quoted from a Presidential speech given at a historical conference in Bengal, 1915)

Speech in the U.S. Senate (2018)

The Philosophy of Atheism

Vol. VI, p 5, "First Talk in Rajahmundry (20 November 1949) http://www.jkrishnamurti.com/krishnamurti-teachings/view-text.php?tid=335&chid=4655&w=%22You+cannot+find+truth+through+anybody+else%22, J.Krishnamurti Online, JKO Serial No. 491120
Posthumous publications, The Collected Works
Context: You cannot find truth through anybody else. How can you? Surely, truth is not something static; it has no fixed abode; it is not an end, a goal. On the contrary, it is living, dynamic, alert, alive. How can it be an end? If truth is a fixed point, it is no longer truth; it is then a mere opinion. Sir, truth is the unknown, and a mind that is seeking truth will never find it. For mind is made up of the known; it is the result of the past, the outcome of time — which you can observe for yourself. Mind is the instrument of the known; hence it cannot find the unknown; it can only move from the known to the known. When the mind seeks truth, the truth it has read about in books, that "truth" is self-projected, for then the mind is merely in pursuit of the known, a more satisfactory known than the previous one. When the mind seeks truth, it is seeking its own self-projection, not truth. After all, an ideal is self-projected; it is fictitious, unreal. What is real is what is, not the opposite. But a mind that is seeking reality, seeking God, is seeking the known. When you think of God, your God is the projection of your own thought, the result of social influences. You can think only of the known; you cannot think of the unknown, you cannot concentrate on truth. The moment you think of the unknown, it is merely the self-projected known. So, God or truth cannot be thought about. If you think about it, it is not truth. Truth cannot be sought; it comes to you. You can go after only what is known. When the mind is not tortured by the known, by the effects of the known, then only can truth reveal itself. Truth is in every leaf, every tear; it is to be known from moment to moment. No one can lead you to truth; and if anyone leads you, it can only be to the known.

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.

Source: The Eye of the Heron (1978), Chapter 5 (p. 77)

Variant: The secrets of this earth are not for all men to see, but only for those who will seek them (pg. 52).
Source: Anthem