1. Aristotle, On the Heavens, Book ii, chapters 13–14.
2. Galileo Galilei, Dialogue Concerning the Two Chief World Systems (trans. Stillman Drake), Berkeley and Los Angeles, 1962.
3. I. Newton, Principia, Scholium to Definition viii.
4. Leibniz—Clarke Correspondence (originally published 1717) (ed. H. G. Alexander), Manchester, 1956.
5. G. Berkeley, De Motu (originally published 1721), §§ 52–66.