1. M. T. Cicero, De Res Publica, trans. C. W. Keyes, (Cambridge: Loeb Classical Library, 1928).
2. Dikaion nomikon (positive law) and dikaion physikon (natural law). See Francis de Zulueta, The Institutes of Gaius (Oxford: Clarendon Press, 1976), Vol. 2, p. 13.
3. A History of Christian Spirituality;J Leclercq,1968
4. T. Aquinas, Summa Theologica, I–II, Q. 93.
5. H. Tristram Engelhardt, Jr., Bioethics and Secular Humanism: The Search for a Common Morality (London: SCM Press, 1991).