Critique of Practical Reason

Central Problem

The Critique of Practical Reason (1788) addresses the fundamental question of how moral action is possible and what constitutes the foundation of ethics. Kant investigates how pure reason, independent of experience and sensibility, can determine the will and guide human conduct. The central tension lies between reason and sensibility: humans are beings torn between rational moral demands and sensible inclinations.

The work examines whether morality requires a pure practical reason that operates independently from empirical conditions (pathological determinations), or whether all practical reason is merely instrumental, serving our sensible desires. Kant argues that if morality exists at all, it must be unconditional (unbedingt) — that is, it must presuppose a practical reason capable of freeing itself from sensible inclinations and guiding conduct in a stable, universal manner.

Unlike the Critique of Pure Reason, which criticized theoretical reason’s illegitimate pretensions to transcend experience, the Critique of Practical Reason criticizes empirical practical reason’s pretension to be the sole motive of action. Pure practical reason operates legitimately a priori and does not need criticism in its pure part because it obeys a universal law.

Main Thesis

Kant’s central thesis is that pure reason can be practical — that is, reason alone, without any empirical motives, can determine the will. The moral law is given as a “fact of pure reason” (Factum der Vernunft) of which we are conscious a priori and apodictically certain. This law commands unconditionally through the categorical imperative.

The categorical imperative differs fundamentally from hypothetical imperatives. While hypothetical imperatives have the form “if… then you ought…” and prescribe means to contingent ends, the categorical imperative commands with a pure “you ought” (Sollen) regardless of any particular purpose. Its formula states:

“Act so that the maxim of your will could always hold at the same time as a principle of a universal legislation.”

The equation morality = unconditionality = freedom = universality and necessity forms the core of Kant’s ethical analysis. From this derive the essential attributes of the moral law:

1. Categoricity: The moral law commands absolutely, not conditionally
2. Formality: The law prescribes not what to do but how to act — according to universalizable maxims
3. Autonomy: The will gives the law to itself; morality is not imposed externally

Kant presents three formulations of the categorical imperative:

• First formula (Universal Law): Act only according to that maxim which you can will to become a universal law
• Second formula (Humanity): Act so as to treat humanity, in yourself and others, always as an end and never merely as a means
• Third formula (Autonomy): Act so that your will can regard itself as giving universal law through its maxims

The moral agent’s intention matters fundamentally: an action is truly moral only when performed for the sake of duty (aus Pflicht), not merely in conformity with duty (pflichtmäßig). This is the good will — the only thing unconditionally good in the world.

Historical Context

The Critique of Practical Reason was published in 1788, following the Groundwork of the Metaphysics of Morals (1785) and the Critique of Pure Reason (1781). Kant was responding to multiple philosophical traditions that he found inadequate for grounding morality.

The rationalist tradition (represented by Wolff) had based morality on metaphysics, founding it on the order of the world or on God. The empiricist tradition (represented by Hume) had grounded morality in sentiment, particularly sympathy. Kant found both approaches insufficient: rationalism failed to preserve moral autonomy, while empiricism made morality too subjective and unstable.

Rousseau’s influence is evident in Kant’s emphasis on human dignity and the idea that in the “kingdom of ends” each person is simultaneously subject and legislator. The English moral sense theorists like Hutcheson had emphasized moral feeling, which Kant transformed into his concept of “respect for the law” as the only legitimate moral sentiment.

Kant explicitly criticizes all “heteronomous” moral systems — those that ground duty in forces external to human reason itself, whether in education (Montaigne), civil government (Mandeville), physical sentiment (Epicurus), moral sentiment (Hutcheson), perfection (Wolff and the Stoics), or divine will (Crusius and theological moralists).

Philosophical Lineage

flowchart TD
    Plato --> Kant
    Stoics --> Kant
    Leibniz --> Wolff
    Wolff --> Kant
    Hume --> Kant
    Rousseau --> Kant
    Hutcheson --> Kant
    Kant --> Fichte
    Kant --> Hegel
    Kant --> Schopenhauer

    class Plato,Stoics,Leibniz,Wolff,Hume,Rousseau,Hutcheson,Kant,Fichte,Hegel,Schopenhauer internal-link;

Key Thinkers

Thinker	Dates	Movement	Main Work	Core Concept
Kant	1724-1804	German Idealism	Critique of Practical Reason	Categorical imperative, autonomy
Hume	1711-1776	Empiricism	Treatise of Human Nature	Moral sentiment, sympathy
Rousseau	1712-1778	Enlightenment	Social Contract	General will, human dignity
Wolff	1679-1754	Rationalism	German Metaphysics	Perfection as moral principle
Hutcheson	1694-1746	Moral Sense Theory	Inquiry into Beauty and Virtue	Moral sense, benevolence

Key Concepts

Concept	Definition	Related to
Pure practical reason	Reason that determines the will independently of experience and sensibility	Kant, categorical imperative
Categorical imperative	Command that prescribes duty unconditionally, with the form “you ought” pure and simple	Kant, moral law
Hypothetical imperative	Command prescribing means to particular ends, with the form “if… then you ought…”	Kant, prudence
Maxim	Subjective principle of action, rule valid only for the individual’s own will	Kant, practical principles
Autonomy	Self-legislation of the will; the will giving law to itself	Kant, freedom
Heteronomy	Dependence of the will on external laws or material principles	Kant, critique of morals
Good will	Intention of the will to conform to the moral law; the only unconditionally good thing	Kant, duty
Respect for the law	The only legitimate moral sentiment, produced by reason, disposing one to obedience to duty	Kant, moral feeling
Postulates	Propositions that cannot be demonstrated but must be assumed for morality to be possible	Kant, practical reason
Kingdom of ends	Ideal community of rational beings living according to moral laws, recognizing each other’s dignity	Kant, human dignity

Authors Comparison

Theme	Kant	Hume	Rousseau
Foundation of morality	Pure practical reason	Moral sentiment, sympathy	General will, natural goodness
Role of reason	Determines will autonomously	Slave of the passions	Guides but does not dominate
Role of feeling	Only respect for law is legitimate	Primary source of morality	Natural compassion (pitié)
Freedom	Noumenal autonomy	Compatibilist	Civil freedom through social contract
Universal validity	A priori, necessary	Empirically derived	Through general will
Human nature	Finite rational being	Creature of habit and passion	Naturally good, corrupted by society

Influences & Connections

• Predecessors: Kant ← influenced by ← Hume (awakened from dogmatic slumber), Rousseau (dignity of humanity), Wolff (rational ethics), Leibniz (rationalist tradition)
• Contemporaries: Kant ↔ dialogue with ↔ Mendelssohn, Jacobi, Herder
• Followers: Kant → influenced → Fichte (ethical idealism), Hegel (critique and dialectical transformation), Schopenhauer (pessimistic ethics)
• Opposing views: Kant ← criticized by ← Hegel (empty formalism), Schopenhauer (rigorism), later Utilitarianism

Summary Formulas

• Kant: Pure reason is practical in itself; the moral law commands categorically through the imperative “act only according to that maxim which you can at the same time will to become a universal law.”

• Kant on autonomy: The will is not merely subject to the law but must be considered self-legislating, and only thus subject to the law of which it is itself the author.

• Kant on freedom: “You ought, therefore you can” — the moral law reveals our freedom; freedom is the ratio essendi of the moral law, while the moral law is the ratio cognoscendi of freedom.

• Kant on postulates: Immortality of the soul and the existence of God are “necessary presuppositions from a practical point of view” that make possible the highest good (union of virtue and happiness).

Timeline

Year	Event
1724	Kant born in Königsberg
1781	Kant publishes Critique of Pure Reason
1785	Kant publishes Groundwork of the Metaphysics of Morals
1788	Kant publishes Critique of Practical Reason
1790	Kant publishes Critique of Judgment
1793	Kant publishes Religion within the Limits of Reason Alone
1797	Kant publishes Metaphysics of Morals
1804	Kant dies in Königsberg

Notable Quotes

“Duty! Sublime and mighty name, that embraces nothing charming or insinuating, but requires submission; yet seeks not to move the will by threatening anything that would arouse natural aversion or terror, but merely holds forth a law that of itself finds entrance into the mind.” — Kant

“Act so that you treat humanity, whether in your own person or in that of any other, always as an end and never merely as a means.” — Kant

“Two things fill the mind with ever new and increasing admiration and reverence: the starry heavens above me and the moral law within me.” — Kant

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NOTE

This summary has been created to present the key points from the source text, which was automatically extracted using LLM. Please note that the summary may contain errors. It serves as an essential starting point for study and reference purposes.