“All the measurements in the world do not balance one theorem by which the science of eternal truths is actually advanced.”

Source: Mathematics: Queen and Servant of Science (1938), p. 291
Context: Science makes no pretension to eternal truth or absolute truth; some of its rivals do. That science is in some respects inhuman may be the secret of its success in alleviating human misery and mitigating human stupidity.

Letter to http://memory.loc.gov/cgi-bin/query/r?ammem/mtj:@field(DOCID+@lit(tj110158)) Thomas Leiper (12 June 1815). Published in The Works of Thomas Jefferson in Twelve Volumes http://oll.libertyfund.org/ToC/0054.php, Federal Edition, Paul Leicester Ford, ed., New York: G. P. Putnam's Sons, 1904, Vol. 11 http://oll.libertyfund.org/Texts/Jefferson0136/Works/0054-11_Bk.pdf, pp. 477–478.
The sentence "I hope our wisdom will grow with our power, and teach us, that the less we use our power, the greater it will be." was used by US-President Barack Obama in his A New Beginning Speech.
1810s
Context: We concur in considering the government of England as totally without morality, insolent beyond bearing, inflated with vanity and ambition, aiming at the exclusive dominion of the sea, lost in corruption, of deep-rooted hatred towards us, hostile to liberty wherever it endeavors to show its head, and the eternal disturber of the peace of the world. In our estimate of Bonaparte, I suspect we differ. [... ] Our form of government is odious to him, as a standing contrast between republican and despotic rule; and as much from that hatred, as from ignorance in political economy, he had excluded intercourse between us and his people, by prohibiting the only articles they wanted from us, that is, cotton and tobacco. Whether the war we have had with England, and the achievements of that war, and the hope that we may become his instruments and partisans against that enemy, may induce him, in future, to tolerate our commercial intercourse with his people, is still to be seen. For my part, I wish that all nations may recover and retain their independence; that those which are overgrown may not advance beyond safe measures of power, that a salutary balance may be ever maintained among nations, and that our peace, commerce, and friendship, may be sought and cultivated by all. It is our business to manufacture for ourselves whatever we can, to keep our markets open for what we can spare or want; and the less we have to do with the amities or enmities of Europe, the better. Not in our day, but at no distant one, we may shake a rod over the heads of all, which may make the stoutest of them tremble. But I hope our wisdom will grow with our power, and teach us, that the less we use our power, the greater it will be.

also attributed to Lord Kelvin

Source: Truth and Truthfulness (2002), p. 2

But philosophy has always been, and will always be, a fight with and a conquest of self-evident truths; philosophy is not looking for any "natural necessity", it sees in naturalness and in necessity alike an evil magic, which, if one cannot quite shake it off (for in this no mortal has ever yet succeeded), yet one must at least call by its right name; and even this is an important step! p. 342
Source: In Job's Balances: on the sources of the eternal truths, Words That Are Swallowed Up - Plotinus's Ecstasies

Review of Selected Essays by Simone Weil, The New York Review of Books (1 February 1963)
Context: The need for truth is not constant; no more than is the need for repose. An idea which is a distortion may have a greater intellectual thrust than the truth; it may better serve the needs of the spirit, which vary. The truth is balance, but the opposite of truth, which is unbalance, may not be a lie.

Banquet speech http://nobelprize.org/nobel_prizes/physics/laureates/1927/compton-speech.html for his Nobel Prize, 1927.