“Boldness will be at a disadvantage only in an encounter with deliberate caution, which may be considered bold in its own right, and is certainly just as powerful and effective; but such cases are rare.”
On War (1832), Book 3
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Carl von Clausewitz 68
German-Prussian soldier and military theorist 1780–1831Related quotes

Book XXXIV, sec. 12 http://books.google.com/books?id=5f08AAAAYAAJ&q="For+he+considered+that+in+many+cases+but+especially+in+war+mere+appearances+have+had+all+the+effect+of+realities+and+that+a+person+under+a+firm+persuasion+that+he+can+command+resources+virtually+has+them+that+very+prospect+inspiring+him+with+hope+and+boldness+in+his+exertions"&pg=PA443#v=onepage
History of Rome

Both above from a speech regarding the Anglo-Irish Treaty (1921) given on 21 December 1921 at University College Dublin. Cited in "Great Irish Speeches" by Michael McLoughlin, Poolbeg, London (1997), pp. 103-107.
Literary Years and War (1900-1918), Last Years: Ireland (1919-1922)

2010s, Update on Investigations in Ferguson (2015)

“Whatever you do or dream you can do—begin it. Boldness has genius and power and magic in it.”
Source: Deep Blue

Attributed to Goethe by popular British novelist Marie Corelli in her essay "The Spirit of Work" as published in The Queen's Christmas carol : an anthology of poems, stories, essays, drawings and music / by British authors, artists and composers in 1905 by The Daily Mail of London.
Attributed to Goethe by William Hutchinson Murray, in his book The Scottish Himalayan Expedition (1951), this has been shown to be a misattribution at "German Myth 12: The Famous 'Goethe' Quotation", Answer.com http://german.about.com/library/blgermyth12.htm and "Popular Quotes: Commitment", Goethe Society of North America http://www.goethesociety.org/pages/quotescom.html
Misattributed
Variant: Whatever you can do, or dream you can, begin it. Boldness has genius, power, and magic in it. Begin it now.

Source: The Characteristics of the Present Age (1806), p. 83