“The power to do good is also the power to do harm; those who control the power today may not tomorrow; and, more important, what one man regards as good, another may regard as harm.”
Introduction, p. 3
Capitalism and Freedom (1962)
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Milton Friedman 158
American economist, statistician, and writer 1912–2006Related quotes

Acceptance Speech http://www.washingtonpost.com/wp-srv/politics/daily/may98/goldwaterspeech.htm as the Republican Presidential candidate, San Francisco (July 1964)
Unsourced variant: Now those who seek absolute power, even though they seek it to do what they regard as good, are simply demanding the right to enforce their own version of heaven on earth, and let me remind you they are the very ones who always create the most hellish tyranny.
Context: Those who seek absolute power, even though they seek it to do what they regard as good, are simply demanding the right to enforce their own version of heaven on earth. And let me remind you, they are the very ones who always create the most hellish tyrannies. Absolute power does corrupt, and those who seek it must be suspect and must be opposed. Their mistaken course stems from false notions of equality, ladies and gentlemen. Equality, rightly understood, as our founding fathers understood it, leads to liberty and to the emancipation of creative differences. Wrongly understood, as it has been so tragically in our time, it leads first to conformity and then to despotism.

“No people do so much harm as those who go about doing good.”
Quoted by Louise Creighton in Life and Letters of Mandell Creighton, vol. 2 http://books.google.com/books?id=XFrIeWud0_wC&q=%22no+people+do+so+much+harm+as+those+who+go+about+doing+good%22&pg=PA501#v=onepage. (1905)

Quoted in "Tony Abbott says climate change is 'probably doing good'" https://www.theguardian.com/australia-news/2017/oct/10/tony-abbott-says-climate-change-is-probably-doing-good, The Guardian, October 10, 2017
2017

Knowledge is Power
The Note-Books of Samuel Butler (1912), Part VII - On the Making of Music, Pictures, and Books

The Plague (1947)
Context: The evil that is in the world always comes of ignorance, and good intentions may do as much harm as malevolence, if they lack understanding. On the whole men are more good than bad; that, however, isn't the real point. But they are more or less ignorant, and it is this that we call vice or virtue; the most incorrigible vice being that of an ignorance which fancies it knows everything and therefore claims for itself the right to kill. There can be no true goodness, nor true love, without the utmost clear-sightedness.

Homeopathy and Its Kindred Delusions (1842)