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.163
Marcus Tullius Cicero, Pliny (2010). “Letters of Marcus Tullius Cicero with His Treatises on Friendship and Old Age: Letters of Pliny the Younger”, p.13, Cosimo, Inc.
Socrates, Plato, Aristotle (1967). “Wit and Wisdom of Socrates, Plato, Aristotle: Being a Treasury of Thousands of Glorious, Inspiring and Imperishable Thoughts, Views and Observations of the Three Great Greek Philosophers, Classified Under about Four Hundred Subjects for Comparative Study”