Ralph Waldo Emerson, Alfred R. Ferguson (1965). “Journals and Miscellaneous Notebooks of Ralph Waldo Emerson, Volume V: 1835-1838”, p.334, Harvard University Press
Andrea Moro, Noam Chomsky, Ivano Caponigro, Daniel B. Kane (2015). “The Boundaries of Babel: The Brain and the Enigma of Impossible Languages”, p.37, MIT Press