Galileo Galilei (1967). “Dialogue Concerning the Two Chief World Systems, Ptolemaic and Copernican, Second Revised Edition”, p.59, Univ of California Press
David Hume (1854). “The Philosophical Works of David Hume: Including All the Essays, and Exhibiting the More Important Alterations and Corrections in the Successive Editions Pub. by the Author”, p.9