John Muir (2015). “STEEP TRAILS: California - Utah - Nevada - Washington - Oregon - The Grand Canyon: Adventure Memoirs, Travel Sketches, Nature Essays and Wilderness Studies from the author of The Yosemite, Our National Parks, A Thousand-mile Walk to the Gulf & Picturesque California”, p.67, e-artnow
Soren Kierkegaard's letter to Henriette Kierkegaard (1847), as quoted in Soren Kierkegaard "Letters and Documents" (translated by Henrik Rosenmeier), www.huffingtonpost.com. 1978.