1 2 3 4 (Inggeris) Mortimer J. Adler, Great Books of the Western World 11, London: Encyclopedia Britannica, Inc., 1968, Hal. 95
1 2 3 4 5 Christoper Rowe, Malcolm Schofield, Simon Harrison, and Melissa Lane., Sejarah Pemikiran Politik Yunani Romawi, Jakarta: PT. Grafindo Persada, 2001, hlmn. 718-723
↑ (Inggeris) Robert Audi., The Cambridge Dictionary of Philosophy, Edinburg: Cambridge University Press, 1995, Hal. 769
