1. Aquinas, S.T. 1988 [1265–1275]. On Politics and Ethics, trans. P.E. Sigmund. New York: W. W. Norton.
2. Aristotle. 1985 [340 BCE]. Nichomachean Ethics, trans. T. Irwin. Indianapolis, IN: Hackett.
3. Attfield, R. 2003. Environmental Ethics. Cambridge, UK: Polity Press.
4. Beauchamp, T.L., and J.F. Childress. 2012. Principles of Biomedical Ethics, 7th ed. New York, NY: Oxford University Press.
5. Billings, J.A. 2011. Double Effect: A Useful Rule That Alone Cannot Justify Hastening Death. Journal of Medical Ethics 37 (7): 437–440.