“You need a high degree of corruption or a very big heart to love absolutely everything”

Editorial in Analog Science Fiction/Fact magazine (1970)

Slightly edited version of text in Men and Women of the Corporation, Basic Books, 1977

Letter http://oll.libertyfund.org/index.php?option=com_content&task=view&id=1407&Itemid=283 to Mandell Creighton (5 April 1887), published in Historical Essays and Studies, by John Emerich Edward Dalberg-Acton (1907), edited by John Neville Figgis and Reginald Vere Laurence, Appendix, p. 504; also in Essays on Freedom and Power (1972)
Paraphrased variant: All power tends to corrupt and absolute power corrupts absolutely.
Context: I cannot accept your canon that we are to judge Pope and King unlike other men, with a favorable presumption that they did no wrong. If there is any presumption it is the other way against holders of power, increasing as the power increases. Historic responsibility has to make up for the want of legal responsibility. Power tends to corrupt and absolute power corrupts absolutely. Great men are almost always bad men, even when they exercise influence and not authority: still more when you superadd the tendency or the certainty of corruption by authority. There is no worse heresy than that the office sanctifies the holder of it. That is the point at which the negation of Catholicism and the negation of Liberalism meet and keep high festival, and the end learns to justify the means.

Commonly quoted on many websites, this quotation is actually from an address by President Gerald Ford to the US Congress (12 August 1974) http://www.bartleby.com/73/714.html
Misattributed

Interview in The Observer, 1990

Blessings (1998)

Source: There Will Be Time (1972), Chapter 10 (p. 107)