Critique of Practical Reason

Summary

Immanuel Kant's Critique of Practical Reason, published at Riga in 1788, is the second of his three Critiques. Where the first Critique limited what reason can know about God, freedom and the soul, this book argues that pure reason is practical by itself: it can determine the will without any help from desire, and in doing so it gives freedom, God and immortality a secure footing as matters of rational belief. The Preface calls freedom the keystone of the whole system of reason. Freedom cannot be proved by theory, but we become aware of it through the moral law, which we cannot escape knowing. Book I, the Analytic, begins with definitions and theorems. All practical principles that rest on an object of desire are material, belong to the principle of self-love or one's own happiness, and can never give a law valid for every rational being. Only the form of a maxim, its fitness to be a universal law, can do that. Kant states the fundamental law of pure practical reason: act so that the maxim of your will could always hold as a principle of universal legislation. Consciousness of this law is a fact of reason. A man who claims his lust is irresistible would master it if a gallows stood at the door; asked whether he could refuse, on pain of death, to bear false witness against an innocent man, he must admit that he could, and so learns that he is free. Autonomy of the will is the sole principle of morality; heteronomy, taking the law from an outside object, cannot ground duty. The second chapter defines good and evil as objects of a will so determined and offers a typic, the natural law used as a model, to apply the law in experience. The third chapter treats respect for the law as the one moral feeling, humbling self-conceit, and closes with the famous address to Duty. Book II, the Dialectic, asks about the highest good, virtue joined with happiness in proportion to it. The Epicureans made virtue a means to happiness, the Stoics made virtue sufficient for it; both erred by treating the two as identical. Since the moral law commands us to promote the highest good, it must be possible. Holiness, the perfect fit of the will to the law, requires endless progress, and so the immortality of the soul is postulated; the proportion of happiness to worth requires a moral author of nature, and so the existence of God is postulated. Morality, Kant writes, is not the doctrine of how to make ourselves happy but of how to become worthy of happiness. The Methodology describes moral education, imagining a ten-year-old who hears of an honest man who will not join the slander of Anne Boleyn. The Conclusion sets side by side the starry heavens above and the moral law within.

Top Lessons

• Ought implies can

  Kant's gallows example in Book I argues that our sense of what we ought to do reveals what we are able to do. A person who doubts his own freedom learns it the moment he recognizes that he could refuse to betray an innocent man even at the cost of life. Freedom is discovered through duty, not proven before it.

• Worthiness before happiness

  Kant does not despise happiness; he includes it in the highest good. But he insists that morality teaches how to become worthy of happiness, not how to get it, and that only virtue makes happiness deserved. The order matters more than the parts.

• Respect as a moral feeling

  In Chapter III Kant argues that the moral law produces a distinctive feeling, respect, which humbles self-conceit and raises the person who obeys. Respect is owed first to the law and then to persons who exemplify it. This gives a precise account of why moral examples move us.

• Faith grounded in practice

  The postulates of God and immortality are not proofs of theory but conditions without which the command to seek the highest good would be empty. Kant thereby places religious belief on moral ground. Readers can weigh both what this secures and what it leaves out.

Verified Quotes

Medical Lens

Kant's ethics is not medical, but two points matter to a psychiatric reader. First, Kant calls a will determined by inclination or feeling pathologically determined; the word means affected by sensibility, from the Greek pathos, not diseased, so the term carries no clinical sense here. Second, his account of respect as a feeling produced by reason, humbling self-conceit, is an early, precise moral psychology of an emotion. Kant later wrote on the mind's power to master morbid feelings by resolution, in The Conflict of the Faculties (1798), a separate work in which he describes managing his own hypochondriac tendencies.

• Book I, Chapter I, section 1 and Remark

  Kant defines practical principles and contrasts a will pathologically affected by desire with one determined by reason alone.

• Book I, Chapter III

  The analysis of respect as a feeling produced by the moral law, which strikes down self-conceit and limits self-love.