Kant's moral idealism: The nature and role of the highest good in Kant's theory of practical reason
The highest good is the synthesis of the natural and moral goods: happiness in perfect proportion to virtue. Kant's justification for including this idea in his theory of practical reason arises from the need to overcome the dualism that emerges between these two goods. Happiness and virtue are both justified objects for the will of a finite rational being. The pursuit of happiness is an empirically conditioned good for finite rational beings; virtue is the unconditioned good for finite rational beings. But the requirements of virtue often run counter to the pursuit of happiness; indeed, the moral law requires the subordination of the pursuit of happiness to the exercise of virtue. Reason seeks to overcome the opposition between virtue and happiness by synthesizing them in a single object—the highest good. The realization of the highest good is possible only if there is a synthetic a priori connection between virtue and happiness, but such a connection is absent in experience. Yet the highest good is a morally necessary object of the will deduced from the moral law. The realization of the highest good is, therefore, morally necessary and empirically impossible. Kant resolves this antinomy by taking recourse to transcendental idealism: the highest good is noumenally possible. However, finite rational beings can only promote, but not realize the highest good, so the moral necessity of the highest good justifies the postulation of the conditions that allow for its fulfillment—the immortality of the soul and the existence of God. Belief in the highest good is the object, but not the determining grounds of the will, and is therefore consistent with pure moral motivation. As the promotion of the highest good requires moral agents to adopt the happiness of others as an end, the practical role of this idea is to guide the fulfillment of the imperfect duties.
O'Connell, Eoin, "Kant's moral idealism: The nature and role of the highest good in Kant's theory of practical reason" (2009). ETD Collection for Fordham University. AAI3377052.