This was more difficult to find than I expected. He wrote it in one of his journals on August 19, 1851. http://en.wikiquote.org/wiki/Henry_David_Thoreau If you scroll down about a quarter of the page and reference by date, you'll see it. ~~~~~~~ ErikaJ - Check out the customer review written by, ironically. Erika Mitchell, at http://www.amazon.com/Writings-Thoreau-Bradford-1850-September-Thoreaus/dp/B000HNI22I The following is an excerpt from her review of "The Writings of Henry David Thoreau Journal Edited by Bradford Torrey II 1850-September 15, 1851 (Thoreau's Writings, 6)" (Hardcover) "This book contains Thoreau's personal journals from 1850 until September 15, 1851. In addition to the journal entries, it also includes black-and-white photographic plates taken by H.W. Gleason around the Concord area from 1899-1906 (first snow, Fair Haven Pond from the cliffs, November woods, midwinter, Town Brook, Plymouth). Thoreau also commented on his own writing process and the writing of others in several journal entries. He quips, "The intellect of most men is barren....It is the marriage of the soul with Nature that makes the intellect fruitful, that gives birth to imagination." (Aug. 20, 1851) "How vain it is to sit down to write when you have not stood up to live! Methinks that the moment my legs begin to move, my thoughts begin to flow....The writing which consists with habitual sitting is mechanical, wooden, dull to read." (Aug. 19, 1851)" I was concerned that the quote originally appeared in something besides a journal, but apparently, that's not the case.

The line was written in his journal, which he kept from 1837 to 1861. Specifically, he wrote it in his journal on August 19, 1851. Thoreau's journals have been published in various volumes. This particular quote can be found in "A Year in Thoreau's Journal: 1851" published by Penguin Classics, and should also be in either "Journal, Volume 3: 1848-1851" or "Journal, Volume 4: 1851-1852" published by Princeton University Press (I don't own the Princeton volumes, so I don't know on which date in 1851 the break occurs between volumes 3 & 4).