¹ The original version can be found in Plato, Euthyphro, 10a. Modern translations include the one in Plato, The Last Days of Socrates, published by Penguin.

² This can be found in David Hume, An Enquiry Concerning the Principles of Morals (originally published in 1751), especially Section 5, ‘Why Utility Pleases’. This is an easier text that Hume's earlier A Treatise of Human Nature.

³ Hume, David, An Enquiry concerning the Principles of Morals, ed. Beauchamp, Tom L. (Oxford, 1998), Section 5, Part 2, pp. 113–4.Google Scholar

⁴ Immanuel Kant's classic presentation of his moral theory is his Grundlegen zur Metaphysik der Sitten (originally published 1785), translated as Groundwork of the Metaphysics of Morals, or Foundations of the Metaphysics of Morals, or Fundamental Principles of the Metaphysics of Morals. There are various modern English translations.

⁵ Aristotle, Politics, Book I, Chapter 2, 1253 a 8-19.

⁶ I am leaving open here the question of whether we also have moral obligations to non-human animals. If we do, it will be because at least some animals matter to us in at least some of the ways that other humans matter to us. We would have to ask whether we can be motivated by the same kind of sympathetic concern for animals as for humans. We would have to ask whether, if we accept that there are certain ways in which we ought and ought not to treat human beings, rational consistency requires us to extend the same treatment to animals. And the question would then be whether there are morally relevant differences between humans and animals which would justify us in treating them differently.