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
John Muir (1997). “Nature Writings: The Story of My Boyhood and Youth, My First Summer in the Sierra, the Mountains of California, Stickeen, Selected Essays”, p.814, Library of America
Albert Einstein (2015). “Bite-Size Einstein: Quotations on Just About Everything from the Greatest Mind of the Twentieth Century”, p.15, St. Martin's Press