"On Tranquility of the Mind". Seneca the Younger's letter to Serenus as translated in William Bell Langsdorf "Tranquillity of Mind and Providence", 1900.
John Muir (2015). “THE YOSEMITE COLLECTION of John Muir (Illustrated): The Yosemite, Our National Parks, Features of the Proposed Yosemite National Park, A Rival of the Yosemite, The Treasures of the Yosemite, Yosemite Glaciers, Yosemite in Winter & Yosemite in Spring”, p.190, e-artnow