Powerful quotes

As quoted by LIFE magazine (2 May 1955)
1950s
Variant: Strive not to be a success, but rather to be of value.

Attributed on the internet but not found in print prior to an attribution in Aero Digest, Vols. 58–59, 1949, p. 115 https://books.google.com/books?id=q2ofAQAAMAAJ&dq=%22Life+is+simple%22+but+we+insist+on+making+it+complicated&focus=searchwithinvolume&q=%22Life+is+simple%22+
Misattributed, Not Chinese

Georges Duhamel in THE HEART'S DOMAIN (1919). As it was composed in French, the wording in English may vary in translation. Theodore Geisel / Dr. Seuss was born in 1904, and would have been about 15 years old at the time that it was published. The full text can be found at the link below: We do not know the true value of our moments until they have undergone the test of memory. Like the images the photographer plunges into a golden bath, our sentiments take on color; and only then, after that recoil and that trans-figuration, do we understand their real meaning and enjoy them in all their tranquil splendor.
Misattributed

Variant: When I was 5 years old, my mother always told me that happiness was the key to life. When I went to school, they asked me what I wanted to be when I grew up. I wrote down ‘happy’. They told me I didn’t understand the assignment, and I told them they didn’t understand life.

1960s, The Trumpet of Conscience (1967)
Variant: In the end, we will remember not the words of our enemies, but the silence of our friends.

The Cancer Journals, Special Edition, Aunt Lute Books, San Francisco, CA, 1997, p. 13.

Reported in Yolanda Brooks, Do Animals Have Rights? (2008), p. 23

Quiet Strength (2000)

Often attributed to Stalin, there is not a single source which show that Stalin said this at any given time. There is only one source outside the blogosphere which attributes the quote to Stalin, but does not provide any evidence for the attribution. That source is the book Quotations for Public Speakers : A Historical, Literary, and Political Anthology (2001), p. 121 by the former US senator Robert Torricelli.
Misattributed

This is actually a quotation http://books.google.com/books?id=FUIHmRHf8SUC&lpg=PA130&dq=%22not%20on%20fighting%20the%20old%20but%20on%20building%20the%20new%22&pg=PA130#v=onepage&q=%22not%20on%20fighting%20the%20old%20but%20on%20building%20the%20new%22&f=false from a character named Socrates in Way of the Peaceful Warrior: A Book that Changes Lives http://en.wikipedia.org/wiki/Way_of_the_Peaceful_Warrior, by Dan Millman.
Misattributed

UN speech, June 2013
Context: So let us wage a glorious struggle against illiteracy, poverty and terrorism, let us pick up our books and our pens, they are the most powerful weapons. One child, one teacher, one book and one pen can change the world. Education is the only solution.

Shared on her Facebook page https://www.facebook.com/MayaAngelou/posts/10150251846629796, July 4, 2011

Variant: Be a rainbow in somebody else's cloud.
Source: Letter to My Daughter

The Note Book of Elbert Hubbard (1927)
Variant: The greatest mistake you can make in life is to be continually fearing you will make one.

Letter to his son Eduard (5 February 1930), as quoted in Walter Isaacson, Einstein: His Life and Universe (2007), p. 367
1930s

Start where you stand, and work with whatever tools you may have at your command, and better tools will be found as you go along.
Source: Think and Grow Rich (1938), p. 127

Variant: The best and most beautiful things in the world cannot be seen or even touched. They must be felt with the heart.
Variant: The best and most beautiful things in the world cannot be seen, nor touched... but are felt in the heart.

Variant: A friend is one soul abiding in two bodies.
Variant: Love is composed of a single soul inhabiting two bodies.
Source: The Lives and Opinions of Eminent Philosophers, p. 188; also reported in various sources as:
Friendship is a single soul dwelling in two bodies.
A true friend is one soul in two bodies.
Love is composed of a single soul inhabiting two bodies.
What is a friend? A single soul dwelling in two bodies.

Attributed quotes

Variant: Do not wait: the time will never be 'just right'. Start where you stand, and work whatever tools you may have at your command and better tools will be found as you go along.
Source: Think and Grow Rich (1938), p. 127
Context: Do not wait; the time will never be "just right." Start where you stand, and work with whatever tools you may have at your command, and better tools will be found as you go along.

Hawthorne and His Mosses (1850)
Context: It is better to fail in originality, than to succeed in imitation. He who has never failed somewhere, that man can not be great. Failure is the true test of greatness.
Context: It is better to fail in originality, than to succeed in imitation. He who has never failed somewhere, that man can not be great. Failure is the true test of greatness. And if it be said, that continual success is a proof that a man wisely knows his powers, — it is only to be added, that, in that case, he knows them to be small. Let us believe it, then, once for all, that there is no hope for us in these smooth pleasing writers that know their powers.

"The Power of One", Time Magazine (26 August 2002)

As quoted in Investing with Impact: Why Finance is a Force for Good (2016) by Jeremy Balkin

Frequently attributed to Nin, but without cited source in her work (possibly due to a quotation in Living on Purpose: Straight Answers to Universal Questions (2000) by Dan Millman that attributed the quote to Nin without source).
In March 2013, a former Director of Public Relations at John F. Kennedy University in Orinda, Elizabeth Appell, claimed she had authored the quote in 1979 for an inspirational header on a class schedule: http://anaisninblog.skybluepress.com/2013/03/who-wrote-risk-is-the-mystery-solved/
Disputed
Variant: The day came when the risk to remain tight in a bud was more painful than the risk it took to blossom.

Source: Pygmalion

Source: The Autobiography of Eleanor Roosevelt

Frequently misattributed to Marilyn Monroe or Kurt Cobain.
Source: https://books.google.com/books?id=xUtdDnEhkMMC&pg=PT12&lpg=PT12#v=onepage&q&f=false
Source: Autumn Leaves, Philosophical eLibrary, 2012, (Feuillets d'automne, 1941, trans. Jeanine Parisier Plottel)

From a 1982 interview with Boston Globe journalist Marian Christy. Christy, Marian. "Winning according to Schwarzenegger." https://secure.pqarchiver.com/boston/doc/294151457.html Boston Globe: Boston, MA. 9 May 1982: p 51. Accessed 25 Jun 2016.
1980s
Context: Strength does not come from winning. Your struggles develop your strengths. When you go through hardships and decide not to surrender, that is strength. When you make an impasse passable, that is strength. But you must have ego, the kind of ego which makes you think of yourself in terms of superlatives. You must want to be the greatest. We are all starved for compliments. So we do things that get positive feedback.

Source: Journals of Ralph Waldo Emerson, with Annotations - 1841-1844

Variant: Walking with a friend in the dark is better than walking alone in the light.

Laozi in the Tao Te Ching, Chapter 64
Misattributed, Chinese

Variant: You build on failure. You use it as a stepping sone. Close the door on the past. You don't try to forget the mistakes, but you don't dwell on it. You don't let it have any of your energy, or any of your time, or any of your space.

My Own Story (1914), p. 129, Hearst's International Library.

Il y a assurément un autre monde, mais il est dans celui-ci...
Œuvres complètes, vol. 1, Gallimard, 1968.

Source: Thank You and You're Welcome (2009), p.22

Letter 3
Letters

Statement of advice on being presented the Radcliffe Medal, as quoted in "Honoring Ruth Bader Ginsburg" by Colleen Walsh, in The Harvard Gazette (29 May 2015) https://news.harvard.edu/gazette/story/2015/05/honoring-ruth-bader-ginsburg/
2010s

Variant: You must expect great things of yourself before you can do them.

Source: The Diary of Anne Frank

Attributed to Kierkegaard in a number of books, the earliest located on Google Books being the 1976 book Jack Kerouac: Prophet of the New Romanticism by Robert A. Hipkiss, p. 83 http://books.google.com/books?id=g_JaAAAAMAAJ&q=%22problem+to+be+solved%22#search_anchor. In the 1948 The Hibbert Journal: Volumes 46-47 the quote is referred to as "the famous Kierkegaardian slogan" on p. 237 http://books.google.com/books?id=UuDRAAAAMAAJ&q=%22the+famous+Kierkegaardian+slogan+life+is+not+a+problem+to+be+solved%22#search_anchor, which may be intended to suggest the phrase is Kierkegaard-esque rather than being something written by Kierkegaard. In reality this seems to be a slightly altered version of the quote "The mystery of life is not a problem to be solved; it is a reality to be experienced" which appeared in the 1928 book The Conquest of Illusion by Jacobus Johannes Leeuw, p. 9 http://books.google.com/books?id=OFdVAAAAMAAJ&q=%22not+a+problem+to+be+solved%22#search_anchor.
Misattributed

Attributed to Winston Churchill in The Prodigal Project : Book I : Genesis (2003) by Ken Abraham and Daniel Hart, p. 224 and other places, though no source attribution is given. It actually derives from an advertising campaign for Budweiser beer in the late 1930s.
Misattributed
Variant: Success is not final, failure is not fatal: it is the courage to continue that counts.
Source: http://quoteinvestigator.com/2013/09/03/success-final/

Source: Tuesdays with Morrie

"Security" (1951); excerpted in Outlaw Journalist: The Life & Times of Hunter S. Thompson (2008), page 15
1950s

Source: The Alchemist

Variant: Too many of us are not living our dreams because we are living out fears.

Source: I Can Read With My Eyes Shut! (1978)

This has become attributed to both Walt Whitman and Helen Keller, but has not been found in either of their published works, and variations of the quote are listed as a proverb commonly used in both the US and Canada in A Dictionary of American Proverbs (1992), edited by Wolfgang Mieder, Kelsie B. Harder and Stewart A. Kingsbury.
Misattributed

This quotation's origin is actually unknown, however it is not found in the Dao De Jing.
生命是一连串的自发的自然变化。逆流而动只会徒增伤悲。接受现实，万物自然循着规律发展。
Misattributed
Variant: Life is a series of natural and spontaneous changes. Don't resist them — that only creates sorrow. Let reality be reality. Let things flow naturally forward in whatever way they like.

Statement after his speech before the League of Nations (30 June 1936), as quoted in " "The Lion is Freed" in TIME magazine (8 September 1975) http://www.time.com/time/magazine/article/0,9171,917777,00.html?iid=chix-sphere

Press conference (June 1972),also quoted in Elvis Culture : Fans, Faith, & Image (1999) by Erika Lee Doss, p. 218

Source: Kabir, Hajara Muhammad (2010). Northern women development. [Nigeria]. ISBN 978-978-906-469-4. OCLC 890820657 note: 1940s, Male and Female (1949)

Actually said by Napoleon Hill, Stone later added the line "...with P.M.A." (Positive Mental Attitude) to the end of this quote.
Misattributed
Variant: Whatever the mind of man can conceive and believe, it can achieve.