1. Plato. 1928. Protagoras. In The works of Plato, ed. I. Edman, trans. B. Jowett, 193–260. New York: Modern Library.
2. Kant, Immanuel. 1949. Fundamental principles of the metaphysic of morals. Trans. T.K. Abbott. Indianapolis: Bobbs-Merrill.
3. Donagan, Alan. 1977. The theory of morality. Chicago: University of Chicago Press.
4. Gert, Bernard. 1973. The moral rules: A new rational foundation for morality. New York: Harper Torchbooks.
5. Gert, Bernard. 2005. Morality: Its nature and justification. New York: Oxford University Press.