Charles Sanders Peirce, Nathan Houser, Christian J.W. J. W. Kloesel (1992). “The Essential Peirce, Volume 1: Selected Philosophical Writings? (1867–1893)”, p.149, Indiana University Press
John Muir (2015). “A Thousand-Mile Walk to the Gulf (With Original Drawings & Photographs): Adventure Memoirs, Travel Sketches & Wilderness Studies”, p.43, e-artnow
Charles Sanders Peirce, Nathan Houser, Christian J.W. J. W. Kloesel (1992). “The Essential Peirce, Volume 1: Selected Philosophical Writings? (1867–1893)”, p.149, Indiana University Press
John Muir (2015). “A Thousand-Mile Walk to the Gulf (With Original Drawings & Photographs): Adventure Memoirs, Travel Sketches & Wilderness Studies”, p.43, e-artnow