Critique of Practical Reason

Looking back at ancient Greek philosophy, Kant argues that the two major philosophical schools, Stoicism and Epicureanism, both in their own ways linked happiness with virtue. However, Kant finds virtue and happiness to be “extremely heterogenous concepts” (90), which means they are very different and even incompatible ideas.

Subsection 1 Summary: “The Antinomy of Practical Reason”

Because of this innate incompatibility, Kant writes that happiness and virtue together form an antinomy—a pair of concepts that appear to contradict each other. Nonetheless, Kant defines the highest good as a combination of both virtue and happiness. Further, the existence of the highest good is proven through the fact it “is an a priori necessary object of our will and inseparably bound up with the moral law” (92).

Subsection 2 Summary: “Critical Resolution of the Antinomy of Practical Reason”

Kant argues that happiness does not make an individual virtuous; likewise, virtue does not necessarily lead to happiness. Even though virtue and happiness are not intrinsically connected, the highest good is “practically possible” and therefore is the goal toward which the will should strive. Still, for virtue to lead to happiness or vice versa, an individual would have to be “conscious of his uprightness in every action” (94). This would require an impossible degree of self-awareness, allowing the individual to perfectly distinguish what they do from what they feel. Kant instead argues that people can easily confuse a “sensible impulse” for a “moral incentive” (95). Instead of happiness in terms of fulfilling one’s desires and pleasure, Kant suggests it is better to think of virtue as linked to “contentment with oneself” (95).

This means for Kant that an individual becomes able to obey the moral law while maintaining “independence from the inclinations” (95)—not being affected by subjective desires. While it may be possible that awareness of one’s morality may bring about happiness, the pursuit of happiness cannot make people moral.

Subsection 3 Summary: “On the Primacy of Pure Practical Reason in Its Connection with Speculative Reason”

Speculative reason determines the understanding of an object a priori, while practical reason decides how a person’s free will shall work toward understanding. This is why, while practical reason can help control the emotions, speculative reason offers no such restraint. For Kant, speculative reason is subordinate to practical reason and never the other way around.

Subsection 4 Summary: “The Immortality of the Soul as a Postulate of Pure Practical Reason”

No human can achieve a state where the free will is in complete conformity with the moral law, a state that Kant describes as “holiness” (99). Still, the pursuit of such a state of holiness is “practically necessary” (99) since it is something that we can theoretically achieve. However, such a state of holiness could not be fulfilled within a limited, moral lifespan, but only through an “endless progress” (99), which would only be possible through the immortality of the human soul. Based on this, Kant proposes that the immortality of the soul is a “postulate of pure practical reason” (99).

Subsection 5 Summary: “The Existence of God as a Postulate of Pure Practical Reason”

Another postulate of pure practical reason, in Kant’s view, is the existence of God. To prove this postulate, he breaks down the concept of the highest good. The “first and principal part” of the highest good is “morality” (100). The second element of the highest good is “happiness,” but only the sort of happiness that comes from moral behavior. At the same time, there is no guarantee that morality will lead to happiness. Nonetheless, Kant still suggests a causal link between morality and happiness, noting that rational beings act in such a way as to demonstrate that morality is the “supreme determining ground of the will” (101). For Kant, the laws of morality and nature compel humans to strive for both virtue and happiness, and only a powerful and just intelligence could have designed nature and humanity in such a way.

For Kant, Christianity itself supports his postulate concerning God. He views Christianity as confirming his own ideas about a moral law that “does not promise any happiness” (103) and about holiness as something that has to constantly be striven for, rather than achieved during one’s mortal lifetime. Kant also views Christianity as emphasizing duty, rather than simply “incentives of fear and hope” (104). He further interprets Christianity as offering the total achievement of the moral law as the purpose and the “ends” (106) of every human being.

Subsection 6 Summary: “On the Postulates of Pure Practical Reason in General”

The three main postulates of pure practical reason proposed by Kant are immortality, freedom, and the existence of God. Immortality is necessary for any being to fulfill the moral law. Humans are able to interact with the world of the senses while making decisions in the realm of pure intellect; this duality, for Kant, shows that free will exists. The existence of God is shown by the moral and natural laws that compel individuals to strive toward what Kant considers the highest good. Kant argues that such concepts as the moral law and the highest good cannot be achieved through speculative reason. Because it lacks the idea of the moral law as a goal and determinant of the will, speculative reason can only address these postulates as “transcendent” (meaning outside our lived experience) rather than “immanent” (meaning part of our lived experience) (107).

Subsection 7 Summary: “How Is It Possible to Think of an Extension of Pure Reason for Practical Purposes Without Thereby Also Extending Its Cognition as Speculative?”

Pure practical reason requires the existence of the three “theoretical concepts” of “freedom, immortality, and God” (108). The fact that there is a highest good, which is understood through a “practical law,” means that these three concepts can be treated as postulates that apply to the material world. In other words, when applied to moral law and the highest good, which Kant considers practical rather than purely theoretical or speculative, free will, immortality, and God are postulates that can form a tangible basis for practical reason.

Following this conclusion, Kant considers whether God is “a concept belonging to physics…or to morals” (111). Kant argues that to consider God a part of physics and prove that the world was created by an all-powerful God, one would have to have an impossibly perfect knowledge of the world and all possible worlds. Nor is it possible to prove God’s existence through pure speculative reason, because to do so would require references to concepts and objects “outside the understanding” (112). Instead, the “concept of the highest good as the object of a pure practical reason” is required to demonstrate not just the existence of God, but God’s attributes as a “supreme being” (112).

Subsection 8 Summary: “On Assent from a Need of Pure Reason”

Pure practical reason is necessary because it “is based on a duty” (114), namely, making the highest good the object of one’s will. Because the highest good is at least theoretically possible—even if it is practically impossible—Kant argues that individuals have a moral duty to strive toward it. This is Kant’s basis for asserting the three postulates. However, Kant admits that, while he views the possibility of the highest good and the need to strive toward it as objectively proven, specific definitions of the highest good are subjective and require the use of reason.

Subsection 9 Summary: “On The Wise Adaptation of the Human Being’s Cognitive Faculties to his Practical Vocation”

Next, Kant asks how an individual can be certain at all they are able to use their cognitive abilities to “strive for the highest good” (117). Speculative reason is limited in its ability to determine what the highest good is. Kant speculates that if we were able to perceive the truth of reality immediately, there would be no need to slowly acquire moral strength or to use reason to resist our inclinations. Instead, obedience to moral law would motivated solely by fear of God, essentially reducing humanity to a “puppet show” (118). Only in a world where God’s existence and nature are ambiguous and uncertain can anyone develop a “truly moral disposition” (118).