From Citizendium, the Citizens' Compendium
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- Frederick Copleston, S.J. 'A History of Philosophy', Vol. I: Greece and Rome - from the pre-Socratics to Plotinus
- 'The Cambridge Dictionary of Philosophy', ed. by Robert Audi
- Bertrand Russell: 'A History of Western Philosophy And Its Connection with Political and Social Circumstances from the Earliest Times to the Present Day'; Book I: Ancient Philosophy
- Encyclopædia Britannica 15th Edition, Volume 25: 'Western Philosophical Schools and Doctrines'