Related quotes
“If someone offers to furnish a sure test, ask what the test was which made the sure test sure.”
Source: Meditations in Wall Street (1940), p. 120

“Of all the paths you take in life, make sure a few of them are dirt.”
I searched for keywords of that text in his books online and on an electronic copy of Muir of the Mountains, and was unable to find it. On rare occasions we find something in Muir's unpublished journals that is new and which can be verified. So I also did a search of the John Muir Papers at the University of the Pacific, and once again came up empty:
Misattributed

Source: A Path with Heart: A Guide Through the Perils and Promises of Spiritual Life
Quoted in Craig Modderno, "Newman remains animated at 81," Reuters (2006-06-12)