Kant, Immanuel. (1785) 2002. Groundwork for the Metaphysics of Morals. Edited and translated by Allen W. Wood. New Haven: Yale University Press.
For a more accessible translation, see Jonathan Bennett’s translation at earlymoderntexts.
Ancient Greek philosophy was divided into three branches of knowledge: physics, ethics and logic. Kant explains the “necessary subdivisions” which is schematised below (see image).
The realm of ethics is in part determined by our experiences — what Kant calls ‘practical anthropology’ — but it also has a rational part determined a priori through pure reason — this part is the ‘metaphysics of morals’. Practical reason/moral philosophy/ethics should be grounded on the rational part. Only those laws derived from it find universal application.
“Thus a metaphysics of morals is indispensably necessary not merely from a motive of speculation, in order to investigate the source of the practical principles lying a priori in our reason, but also because morals themselves remain subject to all sorts of corruption as long as that guiding thread and supreme norm of their correct judgment is lacking. For as to what is to be morally good, it is not enough that it conform to the moral law, but it must also happen for the sake of this law; otherwise, that conformity is only contingent and precarious, because the unmoral ground will now and then produce lawful actions but more often actions contrary to the law.”
Kant’s whole project is thus to develop a metaphysics of morals — which he does with the Critique of (Pure) Practical Reason, published in 1788, three years after the Groundwork. But before he does that, he wants to lay the foundation by establishing the supreme principle of morality. The Groundwork lays that, well, groundwork.