“Every optimist moves along with progress and hastens it, while every pessimist would keep the world at a standstill. The consequence of pessimism in the life of a nation is the same as in the life of the individual. Pessimism kills the instinct that urges men to struggle against poverty, ignorance and crime, and dries up all the fountains of joy in the world.”
Optimism (1903)
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Helen Keller 156
American author and political activist 1880–1968Related quotes

If I regarded my life from the point of view of the pessimist, I should be undone. I should seek in vain for the light that does not visit my eyes and the music that does not ring in my ears. I should beg night and day and never be satisfied. I should sit apart in awful solitude, a prey to fear and despair. But since I consider it a duty to myself and to others to be happy, I escape a misery worse than any physical deprivation.
Optimism (1903)

Source: An Essay on Aristocratic Radicalism (1889), p. 21

Book II, ch. 6 (trans. Constance Garnett)
Pyotr Miusov, summarizing an argument made by Ivan at a social gathering
The Brothers Karamazov (1879–1880)

This quote is commonly attributed to Churchill, but appears in the "Red Herrings: False Attributions" appendix of Churchill by Himself : The Definitive Collection of Quotations (2008) by Richard Langworth, without citation as to where it originates.
In American Character, a 1905 address by Brander Matthews, a similar quotation is attributed to L. P. Jacks ( link http://hdl.handle.net/2027/mdp.39015059451156?urlappend=%3Bseq=238).
""Our civilization is a perilous adventure for an uncertain prize... Human society is not a constructed thing but a human organization... We are adopting a false method of reform when we begin by operations that weaken society, either morally or materially, by lower its vitality, by plunging it into gloom and despair about itself, by inducing the atmosphere of the sick-room, and then when its courage and resources are at a low ebb, expecting it to perform some mighty feat of self-reformation... Social despair or bitterness does not get us anywhere... Low spirits are an intellectual luxury. An optimist is one who sees an opportunity in every difficulty. A pessimist is one who sees a difficulty in every opportunity... The conquest of great difficulties is the glory of human nature." L. P. Jacks, quoted in American character, by Brander Matthews, 1906
Misattributed
Variant: A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty.

Source: On having a pessimistic nature in “Jesmyn Ward: ‘So much of life is pain and sorrow and wilful ignorance’” https://www.theguardian.com/books/2017/nov/12/jesmyn-ward-sing-unburied-sing-interview-meet-author in The Guardian (2017 Nov 12)

“The Optimist thinks this is the best of all possible worlds, the Pessimist fears it is true.”
This is derived from a statement of James Branch Cabell, in The Silver Stallion (1926) : The optimist proclaims that we live in the best of all possible worlds; and the pessimist fears this is true.
Misattributed
Variant: The optimist thinks this is the best of all possible worlds. The pessimist fears it is true.

Source: A Neglected Argument for the Reality of God (1908), V