칸트는 이론적 영역을 다루는 순수이성과 도덕적 영역을 다루는 실천이성을 구별하였던 바, 우리의 순수이성으로서는 영혼불멸 및 신의 존재에 대해 그 진위를 알 수 없다고 주장하였다. 실천이성에 들어와 이 이념들이 다시 불러들여지는데, 여기에서 열쇠 역할을 감당하는 것이 최상선과 최고선의 개념이다. 우리의 실천이성은 도덕적 주체가 끝까지 도덕적 행위를 수행한 다음 행복까지 보장받는 최고선을 추구한다. 최고선의 전제조건인 최상선을 성립시키기 위해 우리의 실천이성은 영혼불멸을 요청한다. 또한 덕과 행복을 완전히 일치시킬 수 있도록 하기 위해 신의 존재를 요청한다. 칸트는 형이상학적 이념들이 부정되려는 위기의 시대를 살아가면서 인간의 존엄성과 숭고함을 지켜내려 하였다. 그는 지식의 세계와 도덕의 세계가 따로 있음을 주장함으로써 그 이념들을 살려내고, 그리하여 숭고한 도덕법칙을 창출해낼 수 있었다.
Kant thought that all metaphysics must undergo a rigorous test of human reason, and that the role of the tribunal should be handled by Critique of Pure Reason. As a result of self-examination of pure reason, namely our theoretical reason, it did not know its authenticity as to the immortality of the soul and the existence of God. Because, we humans can not experience them. Kant, however, focuses on the concept of the highest good and the best good as a key to draw such ideologies. What is the highest good? It refers to the moral act that human beings as a moral subject conduct to the end without wavering. And the concept that goes a step further is the best good. The best good is to say that a certain moral subject is morally committed to the end, and then is guaranteed happiness in exchange for it. In this respect, our practical reason constantly pursues the best good most of all, in which virtue and happiness are in full agreement. Therefore, in order to realize the best good, the highest good must be realized first of all. In order to establish the highest good where human perfectly coincides with morality, we must assume the infinite progression of the will toward it. However, human beings as a moral subject must someday die, and in this regard, a request is made that the soul should survive even if the body disappears. In other words, our practical reason is asking for the immortality of the soul for the realization of the best good. Let’s suppose that the highest good is accomplished in this way. But subsequent happiness can not be realized by human power. Here, an omnipotent being is required to fully match virtue and happiness, and we call it God. In retrospect, our pure reason (speculative reason, theoretical reason) could not confirm the objective reality of the ideologies of soul immortality and the existence of God. However, as it came into pure practical reason, their reality could be recognized, and there is the very advantage of practical reason for theoretical reason. Kant lived in the era of crisis in which the conventional ideologies of God, the immortality of the soul and the freedom of the will in Western philosophy tended to be denied. In the crisis of fatefulness, in which materialistic thought swept across Europe and human beings also fell to the level of machines and mere animals, Kant attempted to preserve human dignity and sublime. Something different from animals had to be in human beings, and Kant found it in a world of morality. By insisting that there were a world of knowledge, a world of morals, a world of science and a world of faith separately, he tried to harmonize the materialistic, scientific, moral and religious positions. And finally, by proving that the two worlds can co-exist without rejecting each other, he could revive metaphysical ideas (the immortality of the soul, the existence of God, the freedom of the will, etc.) and finally create the noblest moral law. In this process, it is considered to be the concept of the highest good and the best good that has played a key role in this process.