From Thoreau's Journal (Sept 12, 1853):

It occurred to me when I awoke this morning feeling regret for intemperance of the day before in eating fruit, which had dulled my sensibilities, that man was to be treated as a musical instrument, and if any viol was to be made of sound timber and kept well tuned always, it was he, so that when the bow of events is drawn across him, he may vibrate and resound in perfect harmony. A sensitive soul will be continually trying its strings to see if they are in tune. A man's body must be rasped down exactly to a shaving. It is of far more importance than the wood of a Cremona violin.