Immanuel Kant’s Critique of Practical Reason (1788) completes the critical examination of reason begun in the Critique of Pure Reason. Where the first Critique established the limits of theoretical knowledge, the second demonstrates how practical reason—reason directed toward action—provides access to truths that theoretical reason cannot reach.
The Problem of Practical Philosophy
Kant begins with a fundamental question: How can reason determine the will? The answer reveals the entire structure of moral philosophy.
The Will and Its Determination
The will is the faculty of desire insofar as it can be determined by reason rather than merely by inclination. A rational being can act according to the representation of laws—not just according to laws themselves, as natural objects do.
This capacity distinguishes moral agents from mere things. We can ask whether an action is right or wrong, good or evil, because we can step back from our inclinations and evaluate them against rational principles.
Empirical vs. Pure Practical Reason
Empirical practical reason determines action based on desires and their satisfaction. It calculates means to ends we already have. This is hypothetical reasoning: if you want X, then do Y.
Pure practical reason, by contrast, determines action through reason alone, independent of any particular desires. It issues categorical commands: do Y, period—regardless of what you happen to want.
The Categorical Imperative
The fundamental law of pure practical reason is the categorical imperative—an unconditional command that applies to all rational beings simply because they are rational.
The Formula of Universal Law
“Act so that the maxim of your will could always hold at the same time as a principle of a universal legislation.”
This formula requires that we act only on principles that we could consistently will to be universal laws. If universalizing your maxim leads to contradiction, the action is morally impermissible.
Examples of Application
False Promises: Can I adopt the maxim “I will make false promises when convenient”? If universalized, promising itself would become meaningless—no one would believe promises. The maxim is self-defeating and therefore impermissible.
Neglecting Talents: Can I universalize “I will let my talents rust”? While not logically contradictory, no rational being could will a world where everyone neglects their capacities. This reveals an imperfect duty to develop oneself.
Refusing to Help: The maxim “I will never help others” could exist as a universal law without contradiction, but we cannot rationally will it—we might need others’ help ourselves. This establishes imperfect duties to others.
The Formula of Humanity
Another formulation illuminates the imperative’s meaning:
“Act so that you treat humanity, whether in your own person or in that of another, always as an end and never as a means only.”
Rational beings have dignity, not merely price. They cannot be used as mere instruments for others’ purposes. This doesn’t prohibit all use of people—we constantly employ each other’s services—but it forbids treating them merely as means, ignoring their own rational agency.
The Autonomy of the Will
The categorical imperative reveals a remarkable fact: in moral action, we are subject only to laws we give ourselves. This is the autonomy of the will.
Self-Legislation
When I recognize that I ought not lie, I am not submitting to an external authority. I am recognizing a law that my own reason legislates. Morality is not imposed on reason from outside but arises from reason’s own nature.
This explains why moral obligation feels binding yet not alien. The moral law is our own law—the law of our rational nature as distinct from our merely empirical desires.
Heteronomy and Its Problems
All previous moral theories, Kant argues, grounded morality in something external to rational will: divine commands, natural desires, social utility, or self-interest. All such theories are heteronomous—they make the will dependent on something outside itself.
Heteronomous theories cannot explain genuine moral obligation. If I should be moral only because God commands it, or because it serves my interests, then morality becomes conditional. But the moral “ought” is unconditional, categorical.
The Fact of Reason
How do we know the moral law? Not through theoretical proof—pure practical reason is not derived from anything more basic. The moral law is a “fact of reason,” immediately certain to any rational being who reflects on their practical situation.
Immediate Certainty
When I contemplate whether to act, I am immediately aware that certain actions are permissible and others are not. This awareness is not a feeling or intuition but reason’s own self-consciousness in its practical employment.
Kant calls this the “sole fact of pure reason”—the one instance where reason shows itself as immediately determining the will.
Respect for the Law
The moral law produces a distinctive feeling: respect. Unlike pleasure or fear, respect is a feeling produced by reason itself. It combines humiliation of our self-conceit (the moral law shows us we are not the center of value) with elevation of our rational nature (we recognize ourselves as moral legislators).
The Postulates of Practical Reason
Though theoretical reason cannot prove God’s existence, freedom, or immortality, practical reason requires us to postulate them as conditions of moral life.
Freedom
Morality presupposes freedom. “Ought” implies “can”—if I am bound by the moral law, I must be able to obey it. Since the moral law is certain, freedom must be real, even if theoretical reason cannot comprehend how it is possible.
Immortality
The moral law commands us to achieve the highest good—complete virtue matched with complete happiness. But this perfect correspondence is never achieved in finite life. We must therefore postulate an infinite progress toward holiness, which requires immortality.
God’s Existence
The highest good also requires that virtue be rewarded with proportionate happiness. But nature does not guarantee this connection. We must therefore postulate a moral author of nature—God—who can ensure that the virtuous ultimately achieve the happiness they deserve.
The Highest Good
Morality aims at the highest good: the union of virtue and happiness. This is not a conditional pursuit of happiness through virtue but the unconditioned pursuit of virtue with the hope that happiness will follow.
Virtue as Supreme
Virtue—worthiness to be happy—is the supreme good, conditioning all other goods. Happiness is good only when deserved. The wicked person’s happiness is not truly good; it offends our moral sense.
The Unity of Virtue and Happiness
Yet virtue alone is not the complete good. Kant rejects the Stoic view that virtue is sufficient for happiness. A moral world requires that the virtuous flourish—that the moral order align with the natural order.
This unity cannot be guaranteed by nature alone or achieved by finite will. It requires God’s existence and our immortality. Thus practical reason, unable to prove these theoretically, must postulate them practically.
Practical Reason’s Primacy
In the conflict between theoretical and practical reason, practical reason has primacy. This doesn’t mean practical interests can make us believe falsehoods, but that where theoretical reason leaves questions undecided, practical reason may determine our commitment.
The Limits of Theory
Theoretical reason shows that freedom, God, and immortality are possible—they don’t contradict what we can know. But it cannot prove them actual. The questions remain open.
Practical Necessity
Practical reason demonstrates that we must believe in freedom (as a condition of morality), immortality (as a condition of moral progress), and God (as a condition of the highest good). These practical necessities close the theoretical gaps.
Influence and Legacy
The Critique of Practical Reason transformed moral philosophy:
Deontological Ethics
Kant established deontology—the view that morality concerns the intrinsic rightness of actions, not their consequences. An action’s moral worth lies in its motive, not its results.
Rights and Dignity
The concept of human dignity as grounded in rational autonomy became foundational for human rights discourse. We have rights because we are ends in ourselves, not merely means.
Moral Psychology
Kant’s analysis of practical reason, the will, and moral motivation continues to inform debates about how reason relates to action and emotion.
Critics and Developments
Hegel criticized Kant’s formalism—the categorical imperative, he argued, provides no concrete guidance. Utilitarians reject the separation of virtue from consequences. Virtue ethicists question the focus on duty over character.
Yet even critics engage with Kant’s framework. His questions define the field: What makes an action right? What gives moral agents dignity? How does reason relate to will?
Reading the Second Critique
The work rewards careful study:
- Master the key distinctions: theoretical/practical, categorical/hypothetical, autonomy/heteronomy
- Work through the examples of the categorical imperative
- Understand why the postulates are “practical” not “theoretical”
- Compare with the Groundwork for a more accessible introduction
- Consider how the moral philosophy connects to the Critique of Pure Reason
Conclusion
The Critique of Practical Reason establishes that pure reason can be practical—that we can act from duty alone, independent of desire. This capacity defines us as moral beings and grounds human dignity.
Though Kant’s formalism has faced criticism, his insight remains powerful: morality cannot be reduced to feeling, convention, or calculation. It expresses the autonomy of rational beings—our capacity to give laws to ourselves and to live by them.