The will is “the faculty of acting” within human beings , and the good will is the will to act merely in accordance with a universal principle of moral goodness. Kant claims that the good will possesses an absolute moral worth in itself. Any form of external metrics, such as actual utility or consequences, are much insufficient to determine the worth of a good will, nor could they drive even the slightest deviation from its intrinsic value.
Reason, according to Kant, is the “highest practical vocation” to the good will . Only through reason alone can one produce a will that is good in itself, because unlike reason, human instincts have a natural tendency to produce a will that serves as a means to some private aims. Therefore, the good will is necessarily and exclusively grounded on reason.
Duty is the realization of the good will under subjective limitations. Only acting from duty itself gives an action its absolute moral worth. This moral worth wholly depends on the moral principle to which the action abides, according to Kant’s second proposition . In contrast, an action motivated by inclinations or self-seeking aims has no true moral worth, even with the conformity of duty. Hence in Kant’s third proposition , he argues that any action out of respect for the law must have duty as its essence. Such respect is the honest recognition within a rational being that acting from duty has the highest goodness over any inclinations, which forms the foundations of a good will, that a rational being acts in accordance with the law, yet without any expectation of its outcome.
The purification of inclination serves to establish a consistent and reliable ground for the universal moral law, which has an absolute and unconditioned validity to every rational being in all circumstances. If inclination is taken into consideration, the moral worth of an action becomes subjective to self-seeking aims such as personal preferences, emotions, or sympathies. Consequently, the same action could be judged as either morally worthy or worthless, solely depending on the perceiving end. In such case, the universality of the moral law is diminished, as it would have lost its generality on a shared rational foundation, but rather varies on individual bases.
On the other side, the exclusion of inclination is essential to preserve Kant’s autonomy formula, which concerns the ability of a rational being to self-legislate the universal moral law . Inclination, as the source of non-rational motives, must be set aside for the legislator of universal law, otherwise subjective emotions or desires can influence the formation of objective moral principle, thereby reducing moral principles to an instance of relativism.
Kant’s moral theory failed to acknowledge the fact that duty and inclination can coexist in human psyche. This defect imposes further difficulties in making moral judgement under Kant’s framework.
Take the example of someone making an honest claim to a border agent. In Kant’s moral framework, two possibilities can account for this action. In the first case, the person is telling the truth because of the duty to not lie, hence this action conforms a good will and has moral worth. In the second case, the person acts under the fear of legal consequences, which according to Kant, is an instance of inclination, lacks moral worth even though the action itself aligns with moral principles. Under this setup, the only approach to examine the true will is to find a country border that has no punishment for lying, which is ultimately unrealistic.
Furthermore, it is entirely possible that the action is motivated by mainly the duty to not lie, along with some fear of punishment. Hence, no external observer or even the subject of action is capable of making a sharp distinction between duty and inclination from the motivation. As a result, making moral judgements under Kant’s framework is difficult, which further raises the challenge that the recognition of a pure motive could never be feasible in practice.