The soul of man is divided into three parts, intelligence, reason, and passion. Intelligence and passion are possessed by other animals, but reason by man alone.

Socrates. Quoted in The Lives of Eminent Philosophers, written by Diogenes Laërtius, translated by Pamela Mensch, edited by James Miller. Oxford University Press, 2018, bk. 2, ch. 27.

Luther, Martin. The Table Talk of Martin Luther, edited by Thomas S. Kepler. Dover Press, 2005.

Homer. The Odyssey, edited by Bernard Knox, translated by Robert Fagles. Penguin Classics, 2006, bk. 18, l. 130.

Thales. Quoted in The Lives of Eminent Philosophers, written by Diogenes Laërtius, translated by Pamela Mensch, edited by James Miller. Oxford University Press, 2018, bk. 1, ch. 1.

Aristotle. Politics, translated by R. F. Stalley and Sir Ernest Barker. Oxford University Press, 2009, bk. 4.