Related quotes

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.

Sancho to Don Quixote, in Ch. 9, Peter Anthony Motteux translation (1701).
Don Quixote de la Mancha (1605–1615), Part I, Book III
Context: To withdraw is not to run away, and to stay is no wise action when there is more reason to fear than to hope. 'Tis the part of a wise man to keep himself today for tomorrow, and not venture all his eggs in one basket. And though I am but a clown, or a bumpkin, as you may say, yet I would have you to know I know what is what, and have always taken care of the main chance...

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

“What the wise do in the beginning, fools do in the end.”

“He who has begun has half done. Dare to be wise; begin!”
Dimidium facti qui coepit habet; sapere aude;
incipe!
Book I, epistle ii, lines 40–41
Epistles (c. 20 BC and 14 BC)