Micko

Variant: I would like to paint the way a bird sings.
Source: Monet By Himself

Source: Island

Meistens belehrt uns erst der Verlust über den Wert der Dinge.
Source: Parerga and Paralipomena (1851), Aphorisms on the Wisdom of Life

Though this statement and a few other variants of it have been widely attributed to Herman Melville, it is actually a paraphrase of one found in a sermon of Henry Melvill, "Partaking in Other Men's Sins", St. Margaret's Church, Lothbury, England (12 June 1855), printed in Golden Lectures (1855) :
: There is not one of you whose actions do not operate on the actions of others—operate, we mean, in the way of example. He would be insignificant who could only destroy his own soul; but you are all, alas! of importance enough to help also to destroy the souls of others. ...Ye cannot live for yourselves; a thousand fibres connect you with your fellow-men, and along those fibres, as along sympathetic threads, run your actions as causes, and return to you as effects.
Misattributed

Other

Source: A Tale of Love and Darkness

Der eitle, schwache Mensch sieht in Jedem einen Richter, der stolze, starke hat keinen Richter als sich selbst.
Source: Aphorisms (1880/1893), p. 34.

Misattributed
First recorded appearance: Germaine de Staël's On Germany (1813). ". . . sometimes even in the habitual course of life, the reality of this world disappears all at once, and we feel ourselves in the middle of its interests as we should at a ball, where we did not hear the music; the dancing that we saw there would appear insane." There are several other pre-Nietzsche examples, indicating that the phrase was widespread in the nineteenth-century; it was referred to in 1927 as an "old proverb".

" Alone http://gothlupin.tripod.com/valone.html", l. 1-8 (written 1829, published 1875).
Context: From childhood's hour I have not been
As others were — I have not seen
As others saw — I could not bring
My passions from a common spring —
From the same source I have not taken
My sorrow — I could not awaken
My heart to joy at the same tone —
And all I lov'd — I lov'd alone

Letter http://www.eapoe.org/works/letters/p4801040.htm to George W. Eveleth, Jan. 4, 1848.

Variant: What's done, is done
Source: Macbeth

Alternate (also Twain's): Whenever you find that you are on the side of the majority, it is time to pause and reflect.
Source: Mark Twain's Notebook (1935), p. 393

Letter to writer André Gide, as quoted in The Tell-Tale Heart: The Life and Works of Edgar Allan Poe (1978) by Julian Symons, Pt. 1, Epilogue