After studying the moral theories of Aristotle and Hume one can argue that both suffer from cultural relativism. The upshot of cultural relativism is that what is deemed moral is contingent upon the society in which it is found. Kant held that the source of the contingency found in previous moral theories was the attempt to ground these theories in the empirical, or natural, realm. In his Groundwork of the Metaphysic of Morals, Kant argues that the principles of morality must be stripped of all contingent elements in order to be consistent with our intuitions about morality. Absolute moral principles must be arrived at independent of empirical considerations. These moral principles are the subject of what Kant terms formal philosophy, for formal philosophy is a discipline not grounded in the empirical realm. The Groundwork is Kant's attempt at establishing an a priori or rational foundation for morality. The supreme principle of morality which he concludes with is what he defines as the categorical imperative.
Kant begins by distinguishing between three branches of philosophy: logic, physics, and ethics. He considers logic an a priori science, for it is a purely formal discipline-- its laws are proven independent of empirical evidence. Both physics and ethics have empirical and rational elements. Physics is the study of natural objects governed by natural laws. Physical laws govern determinate objects and as such are descriptive statements (or "is"'s) about the world. Ethics is concerned with free objects which are governed by moral laws. Moral laws are prescriptive statements (or "ought"'s). Although physics and ethics study objects in the changing, empirical universe, Kant argues that physical and moral laws must be unchanging if they are to be considered laws(57). Laws cannot be considered absolute if they are grounded only in experience due to the contingency of the material realm. These principles are thus the a priori elements of the material sciences. Kant focuses upon the a priori foundation of ethics in an attempt to elucidate the supreme principle of morality.
Kant begins by addressing the concept of a good will, arguing that only the good will is that which is good in itself. Certain qualities thought to be good without qualification-- such as moderation and prudence-- are good only to the extent in which they are aids to a good will. As Kant points out, "the very coolness of a scoundrel makes him... immediately more abominable in our eyes"(62). The will is good independent of its effects, laying the indicator of value solely upon intention. Kant's moral theory is thus non-consequentialist in nature, setting it apart from such theories as Aristotle's and Hobbes'.
The preconditions of morality are free will, voluntary action, and the idea of goodness. These preconditions are met by the faculty of reason, the function of which is to make the will good in itself. Kant refutes Aristotle's belief that the function of reason is the attainment of eudaemonia, pointing out that instinct would have been more expedient than reason in attaining this end(63). Kant also counters Hume's notion that reason is impotent in making moral judgements and merely illumines the most appropriate means given an end. Hume emphasized the role of sentiment in moral decision-making, whereas Kant argues that reason, not sentiment, is the final arbiter of moral action. That component of reason which binds rational agents to act morally is the concept of duty.
Paton outlines Kant's formal principle of duty as follows: "An action done from duty has its moral worth, not from the results it attains or seeks to attain, but from a moral principle or maxim- the principle of doing one's duty whatever that duty may be"(20). As we have seen in our discussion of the good will, consequences are not the arbiters of moral goodness. Moral goodness stems from acting in accord with duty, which entails acting in accord with a moral principle or maxim. Maxims are principles upon which we act(20), and Kant distinguishes between material maxims and formal maxims. Material maxims are merely principles abstracted from individual actions. Kant uses the example of stealing to clarify his meaning: if I decide to steal food when I cannot afford it I seem to be acting upon the material maxim "I will steal food whenever I cannot afford it". Material maxims have a generalising function-- they create principles from isolated acts and their intended results. However, Kant has already argued that moral goodness is judged independent of results; thus, moral goodness cannot be derived from material maxims.
Kant contends that moral goodness stems from doing one's duty whatever that particular duty may be. This formal maxim relies solely upon the concept of duty for its meaning. Duty has morally binding force, as it implies oughtness; for if one has a duty to perform X then independent of all other considerations one ought to perform X. According to Kant, goodness is a result of acting for the sake of duty, and duty is the obligation to act out of reverence for the law(68). This point can be illustrated by drawing a line between acting in accord with duty and acting for the sake of duty. If I act for the sake of duty, I act in such a way that inclinations are not an issue in my decision to act. In other words, acting for the sake of duty I act solely out of reverence for the law. But if I act in accord with duty, it is possible that my motivation stems from something other than reverence for the law but just happens to coincide with what duty would otherwise oblige me to do. Kant argues that acts which are motivated by inclination rather than duty are not, properly speaking, moral acts.
As we have stated earlier, moral laws are prescriptive statements which tell us what ought to be the case. Moral laws govern all rational agents and as such must necessarily obtain for all rational agents. Thus, when one acts out of duty for duty's sake one acts upon the formal maxim which applies to all rational agents. It follows that acting out of reverence for the law is acting from a universal ground of obligation which binds all rational agents. Acting from duty is thus acting out of reverence for a universal moral law. Such a law is a command or imperative for all rational agents.
An imperative is any command of reason that necessitates the will. Recall that the function of reason is to make the will good in itself. Reason guides the will through the use of imperatives so that we may act in accord with the moral law. Kant argues that there is but one imperative which is free from contingency, and that is the categorical imperative. Kant's categorical imperative states: "Act only on that maxim through which you can at the same time will that it should become a universal law"(88). Reason, reflecting upon choice, uses the categorical imperative in order to decide between alternatives. From an act reason abstracts a principle of action, and judges it against the categorical imperative; reason then asks 'ought I to wish this principle of action become a universal law of nature?'. Since it is the ultimate measure by which all acts are judged morally good or bad, the categorical imperative is thus Kant's supreme principle of morality.