In her "Translator's Introduction" to Kant's later work The Metaphysics of Morals (1797) – "MM" for short – Mary Gregor usefully summarizes what she describes as the "three main tasks of a moral philosopher" he outlines in the Preface to the Groundwork: 1) to clarify "the supreme principle of morality", the principle on which a rational agent is thought to act insofar as his action is morally good. 2) to justify this principle, that is, to show that it actually holds for or is binding upon imperfectly rational agents such as human beings; and 3) to apply this principle in a "metaphysics of morals", so as to obtain "the whole system" of human duties. (1991, p. 1)