How should I behave?
We seek the answer from Kant, the reservoir of Western philosophy.

Kant is often likened to the largest reservoir at the center of Western philosophy.
This means that all philosophy before Kant flowed into Kant, and all philosophy after Kant flowed from Kant.
The most famous of Kant's works are the Critique of Pure Reason, the Critique of Practical Reason, and the Critique of Judgment, commonly referred to as the "Three Critiques."
Professor Park Jeong-ha, who has specialized in Kantian philosophy and has run a program for many years in which he directly reads and teaches the three Critiques, attempted to provide a faithful and thorough reading of the core concepts of the Critique of Practical Reason, which is the culmination of Kant's ethics, while encompassing them as much as possible.

Among Kant's books, 『Critique of Practical Reason』 is a book that clearly displays the characteristics of Kant's philosophy, which completed the Enlightenment and matured philosophical modernity.
It also marks the starting point for Kant to fully develop his new philosophy.
This is because Kant used the content obtained here as a stepping stone to expand practical philosophy in a more serious and concrete way.
And in terms of its subsequent philosophical influence and its ongoing influence, the Critique of Practical Reason provides a solid foundation for the universally recognized human dignity of today, and presents the principle of deontology, one of the major ethical theories of modern times.

In the Critique of Practical Reason, the author states that Kant's question is directed towards the 'practical use of reason.'
That is, Kant's task in the Critique of Practical Reason is to explain how practical reason can determine the will and enable us to fulfill our duties; in other words, this means elucidating how practical reason makes us moral beings.

Among Kant's books, 『Critique of Practical Reason』 is a book that clearly displays the characteristics of Kant's philosophy, which completed the Enlightenment and matured philosophical modernity.
Specifically, it is a book that condenses the contents of Kant's ethics.
Kant's ethics is also presented in the Groundwork of the Metaphysics of Morals, but the Critique of Practical Reason can be said to be the book that truly advanced the philosophical justification of moral philosophy.
(……) The Critique of Practical Reason provides a solid foundation for human dignity, which is universally recognized today, and presents the principle of deontology, one of the major ethical theories of modern times.

--- p.5-6, from the “Preface”

Kant's critical philosophy criticized continental rationalism and British empiricism, thereby synthesizing the long-standing debates and conflicts of modern philosophy and thereby elucidating the philosophical foundations of modern natural science.
And with the emergence of Kant, the European intellectual world entered a period of great revolution, giving birth to German idealism through Fichte, Schelling, and Hegel, and subsequently through the Neo-Kantian school, exerting a tremendous influence on the history of philosophy up to the present day.
--- p.20, from “Chapter 1: Kant, the Philosopher Who Perfected Modern Reason”

Kant viewed the core function of modern reason as the critical function.
The early modern period, when the Enlightenment emerged, was, to borrow Kant's expression, the 'age of criticism.'
(……) Critical reason, on the one hand, appears as a theoretical activity that examines one by one what is proclaimed as truth by existing authorities in an era dominated by authority and power, and on the other hand, appears as a practical activity that seeks to create a society in which human dignity is absolutely recognized.
Because he established and justified this reason, Kant is recognized as the perfecter of modern reason.
--- p.26-27, from “Chapter 1: Kant, the Philosopher Who Perfected Modern Reason”

Kant's ethics presented in the Critique of Practical Reason is not only of historical value today, but is also considered an important model in discussions of mainstream ethics.
Kant is considered an important interlocutor on the justification of moral norms.
That's also a very attractive opponent.
Why? First, Kantian ethics satisfies the minimum conditions that modern normative ethics aspires to have.
In ethics, Kant takes a position opposing relativism, skepticism, and dogmatism.
Kant also makes it clear that moral judgment and moral action are not matters of personal feelings or arbitrary decisions, nor are they matters of social culture, heritage, lifestyle, or custom.
Kant sets out the principles of morality and approaches ethical issues based on these.
--- p.46-47, from “Chapter 1: Kant, the Philosopher Who Perfected Modern Reason”

Kant sees moral laws as based on human reason.
On the one hand, humans are animals governed by nature, but on the other hand, they are beings with the freedom to transcend the laws of nature.
Freedom gives humans the power to overcome their natural instincts and desires and to fulfill their duties.
Kant placed such importance on the concept of freedom that he described it as the cornerstone of his philosophy.
Through the power of this freedom, humans can pursue a world of morality.
So the source of moral law is human reason.
Human reason is practical in itself and has the power to give moral laws.
Kant called this practical reason.
--- p.79-80, from “Chapter 1: Kant, the Philosopher Who Perfected Modern Reason”

Kant's conclusion is different.
Because moral laws are based on human reason.
On the one hand, humans are animals governed by nature, but on the other hand, they are beings with the freedom to transcend the laws of nature.
Freedom gives humans the power to transcend their natural instincts and desires.
Through this power, humans can pursue a world of morality.
So the source of moral law is human reason.
Human reason is practical in itself and has the power to give moral laws.
Therefore, the moral law is autonomous.
Because human reason gave it to itself.
--- p.105-106, from “Reading Chapter 2, Critique of Practical Reason”

In order for a man to feel satisfaction in the thought that he has acted in accordance with the moral law, and to feel bitter rebuke in reproach for having broken it, he must first realize how important duty is, how sacred the moral law is, and how valuable it is to him to keep it.
Kant's analysis is that satisfaction or anxiety cannot be felt before the awareness of this moral responsibility, and that these feelings cannot be used as the basis for responsibility.
--- p.128-129, from “Reading Chapter 2, Critique of Practical Reason”

The whole of creation can be used as a mere means to satisfy human desires.
But only man, and with him all rational creatures, is an end in itself.
Man soon becomes a subject of the divine moral law by the power of his freedom and autonomy.
A rational being can refuse to submit to any intention contrary to the moral law, and is therefore not used merely as a means, but rather is at the same time recognized as an end in itself.
--- p.145, from “Reading Chapter 2, Critique of Practical Reason”

What is the goal of practical reason, or, to put it another way, what is the object of practical reason? According to Kant, it is the highest good.
Practical reason also pursues its own object and objectivity.
Of course, this object is completely different in nature from what our desires or natural needs are.
And the important thing is that, as we have said before, in the case of desire or need, it is entirely determined by the object, but in the case of practical reason, it is not the goal and object that determine practical reason.
The only thing that determines our pure will and practical reason is the moral law.
Practical reason can be determined by moral laws on the one hand, while on the other hand it can have its own object, object, or goal.
At this time, the object of practical reason is the highest good.

--- p.146-147, from “Reading Chapter 2, Critique of Practical Reason”