Marcus Tullius Cicero (1872). “The Academic Questions: Treatise De Finibus and Tusculan Disputations of M. R. Cicero, with a Sketch of the Greek Philosophers Mentioned by Cicero”, p.369
John Adams, Abigail Adams, Thomas Jefferson (1959). “The Adams-Jefferson Letters: The Complete Correspondence Between Thomas Jefferson and Abigail and John Adams. 1812-1826”