Kant's Metaphysics of Morals is a complex and important work, reflecting Kant's mature legal and ethical thought. It contains the system of duties Kant takes to hold for human beings as such, as well as his accounts of will, right, obligation, virtue, and other fundamental moral concepts.
The Project and its Evolution
The Metaphysics of Morals is likely far different from what Kant originally envisaged. Letters indicate that Kant had planned a work roughly like The Metaphysics of Morals from at least as far back as the mid-1760s (see 10:56); such aspirations were expressed periodically until The Metaphysics of Morals finally emerged. Kant's moral thought had changed significantly by the late 1790s. During the mid-1760s, it was heavily influenced by British moral sense theorists and, increasingly, Rousseau. Kant had yet to develop transcendental idealism, with its profound implications for freedom and morality.
Yet The Metaphysics of Morals differs also from what Kant's Critique of Pure Reason (1781, 1787) and Groundwork of the Metaphysics of Morals (1785) might lead us to expect. The first Critique and the Groundwork describe a metaphysics of morals as a system of pure moral philosophy, containing a priori concepts and principles. For concepts and principles to be a priori as opposed to a posteriori is for them to be inherent in reason and revealed through its operation rather than derived from experience or observation. For a moral philosophy to be pure, it must be based only on a priori principles (see 4:388), “completely cleansed of everything that may be only empirical and that belongs to anthropology” (4:389).
The first Critique says, “the metaphysics of morals is really the pure morality, which is not grounded on any anthropology (no empirical condition)” (A 841/B 869). That a metaphysics of morals is a species of “pure philosophy” – and as such the principal part of moral philosophy – is explicit in the Groundwork too, where Kant explains that only a pure philosophy can reveal “the moral law in its purity and genuineness” (4:390; cf. 410f.). Kant also says there, “a metaphysics of morals has to examine the idea and the principles of a possible pure will and not the actions and conditions of human volition generally, which for the most part are drawn from psychology” (4:390f.).