“The difficulties of economics are mainly the difficulties of conceiving clearly and fully the conditions of utility.”

Knox to George Washington on the difficulties of taking to Boston. Reported in David McCullough, 1776 (2005), p. 83.

Quotes, Concession speech (2000)
Context: I know that many of my supporters are disappointed. I am too. But our disappointment must be overcome by our love of country.
And I say to our fellow members of the world community, let no one see this contest as a sign of American weakness. The strength of American democracy is shown most clearly through the difficulties it can overcome.

Source: The Geological Evidences of the Antiquity of Man (1863), Ch.21, p. 417
Context: To many, this doctrine of Natural Selection, or 'the preservation of favoured races in the struggle for life,' seems so simple, when once clearly stated, and so consonant with known facts and received principles, that they have difficulty in conceiving how it can constitute a great step in the progress of science. Such is often the case with important discoveries, but in order to assure ourselves that the doctrine was by no means obvious, we have only to refer back to the writings of skilful naturalists who attempted in the earlier part of the nineteenth century, to theorise on this subject, before the invention of this new method of explaining how certain forms are supplanted by new ones, and in what manner these last are selected out of innumerable varieties, and rendered permanent.

Journal entry (8 March 1915) p. 40
1910s, Notebooks 1914-1916

Josh Billings, as quoted in Mac's Giant Book of Quips and Quotes (1983) by E. C. McKenzie
Misattributed

Peace and the Public Mind (1935)

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.

Dr Reilly
Murder in Mesopotamia (1936)
Context: Believe me, nurse, the difficulty of beginning will be nothing to the difficulty of knowing how to stop. At least that's the way it is with me when I have to make a speech. Someone's got to catch hold of my coat-tails and pull me down by main force.

The Essays Or Counsels, Civil And Moral, Of Francis Ld. Verulam Viscount St. Albans (1857), Of Empire