“There's nothing clever that hasn't been thought of before — you've just got to try to think it all over again.”

Life and Habit http://www.gutenberg.org/dirs/etext04/lfhb10h.htm, ch. 5 (1877)
Context: "Words, words, words," he writes, "are the stumbling-blocks in the way of truth. Until you think of things as they are, and not of the words that misrepresent them, you cannot think rightly. Words produce the appearance of hard and fast lines where there are none. Words divide; thus we call this a man, that an ape, that a monkey, while they are all only differentiations of the same thing. To think of a thing they must be got rid of: they are the clothes that thoughts wear—only the clothes. I say this over and over again, for there is nothing of more importance. Other men's words will stop you at the beginning of an investigation. A man may play with words all his life, arranging them and rearranging them like dominoes. If I could think to you without words you would understand me better."

Alles Gescheite ist schon gedacht worden.
Man muss nur versuchen, es noch einmal zu denken.
Bk. II, Observations in the Mindset of the Wanderer: Art, Ethics, Nature
Wilhelm Meister's Wanderjahre (Journeyman Years) (1821–1829)
Variant: All truly wise thoughts have been thoughts already thousands of times; but to make them truly ours, we must think them over again honestly, till they take root in our personal experience.

Variant: You have been criticizing yourself for years and it hasn't worked. Try approving of yourself and see what happens.
Source: You Can Heal Your Life

Source: Practical Magic

David R. Brower, in an interview with Terkel, but much of it is sometimes misattributed to Terkel himself.
Misattributed

Website

Ian Smith, as quoted in Heidi Holland, Dinner with Mugabe, Penguin Books; Reprint edition (5 Feb 2009), ISBN 0143026186.