Nature was a form of religion for naturalist, essayist, and early environmentalist Henry David Thoreau (1817–62). In communing with the natural world, he wished to live deliberately, to front only the essential facts of life, and learn what it had to teach. Toward that end Thoreau built a cabin in the spring of 1845 on the shores of Walden Pond, on land owned by Ralph Waldo Emerson, outside Concord, Massachusetts. There he observed nature, farmed,...