Mark Twain (2014). “Mark Twain on Common Sense: Timeless Advice and Words of Wisdom from America's Most-Revered Humorist”, p.33, Skyhorse Publishing, Inc.
John Adams, Abigail Adams, Thomas Jefferson (1988). “The Adams-Jefferson Letters: The Complete Correspondence Between Thomas Jefferson and Abigail and John Adams”
"The Anthropic Principle: Laws and Environments". Article in "The Anthropic Principle: Proceedings of the Second Venice Conference on Cosmology and Philosophy", book edited by F. Bertola and U. Curi, Cambridge University Press, p. 30, 1988.