Having mastered epistemology and metaphysics, Kant believed that a rigorous application of the same methods of reasoning would yield an equal success in dealing with the problems of moral philosophy. Thus, in the Kritik der practischen Vernunft (Critique of Practical Reason) (1788), he proposed a "Table of the Categories of Freedom in Relation to the Concepts of Good and Evil," using the familiar logical distinctions as the basis for a catalog of synthetic a priori judgments that have bearing on the evaluation of human action, and declared that only two things inspire genuine awe: "der bestirnte Himmel über mir und das moralische Gesetz in mir" ("the starry sky above and the moral law within"). Kant used ordinary moral notions as the foundation ffor a derivation of this moral law in his Grundlegung zur Metaphysik der Sitten (Grounding for the Metaphysics of Morals) (1785).
From Good Will to Universal Law
We begin with the concept of that which can be conceived to be good without qualification, a good will. Other good features of human nature and the benefits of a good life, Kant pointed out, have value only under appropriate conditions, since they may be used either for good or for evil. But a good will is intrinsically good; its value is wholly self-contained and utterly independent of its external relations. Since our practical reason is better suited to the development and guidance of a good will than to the achievement of happiness, it follows that the value of a good will does not depend even on the results it manages to produce as the consequences of human action.
Kant's moral theory is, therefore, deontological: actions are morally right in virtue of their motives, which must derive more from duty than from inclination. The clearest examples of moral ...