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Fehr, Ernst, and Urs Fischbacher. “The Nature of Human Altruism.” Nature 425, no. 6960 (2003): 785–91.
Wireless Philosophy. “PHILOSOPHY—Religion: The Problem of Evil.” May 28, 2013. YouTube.
Plato. The Allegory of the Cave (PDF). Translated by Thomas Sheehan. CreateSpace Independent Publishing Platform, 2010. ISBN: 9781452800882.
Nails, Debra. “Socrates.” Stanford Encyclopedia of Philosophy. Edited by Edward N. Zalta, March 2014.
D’Ancona, Cristina. “Greek Sources in Arabic and Islamic Philosophy.” Stanford Encyclopedia of Philosophy. Edited by Edward N. Zalta, October 2013.
El-Abbadi, Mostafa. “Library of Alexandria.” Encyclopædia Britannica, May 2016.
Derveni papyrus—information about the oldest Greek papyrus.