Galileo Galilei (1967). “Dialogue Concerning the Two Chief World Systems, Ptolemaic and Copernican, Second Revised Edition”, p.58, Univ of California Press
David Ricardo, John Ramsay McCulloch (1846). “The Works of David Ricardo, Esq., M.P.: With a Notice of the Life and Writings of the Author”, p.31, London, J. Murray
"The development of intelligence in children" by Alfred Binet, Théodore Simon, translated by Elizabeth S. Kite, Reprinted 1973, New York: Arno Press, (pp. 42-43), 1983.