WorkImmanuel Kant

The Critique of Practical Reason

The 1788 sequel that rescues Kant's ethics: it stops trying to prove human freedom and grounds it instead in the plain fact that the moral law commands us.

by Immanuel Kant508 passages held

German, the original language
English, a translation held here, and your language

Original title: Kritik der praktischen Vernunft
First published: 1788(aged 64)
Original language: German

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Overview

Three years after the Groundwork, Kant came back to answer the objection its critics had gone straight for. His ethics stood or fell on human freedom, and he had conceded that freedom could never be proved. This book, published in 1788, stops trying. Its move is to reverse the order of discovery: we know the moral law directly — the ordinary consciousness that one ought to do a thing regardless of what one wants is a bare given, a "fact of reason" that no argument installs and none can remove — and because that law commands, the freedom it presupposes must be real. Morality becomes the evidence of freedom, rather than the reverse. From this Kant draws the three great postulates: to make sense of a moral life aimed at a good it can never complete in a single lifetime, practical reason is entitled to assume freedom, immortality and God — not as knowledge, which the first Critique had ruled out, but as commitments reason must live by. Where that first Critique had shut speculative theology down, the second lets the moral agent believe, on practical grounds, what the theorist may not claim to know. The book closes on the image Kant is quoted for above all others: the starry sky overhead set beside the moral law within, the two things he said fill the mind with ever-new awe. It sealed his standing as the century's moralist, and set the terms every later Kantian has had to work through.

Key concepts

What is the 'fact of reason'?

The fact of reason is the moral law's immediate hold on us — the plain consciousness that I ought to do a thing regardless of what I want. Kant calls it a fact because no argument installs it and none can argue it away; it is simply given to ordinary moral awareness, and it is the new starting point of the whole book.

What is the 'highest good' Kant says reason commands?

The highest good is the complete object of a moral will: virtue and the happiness exactly proportioned to it, joined together. Reason commands us to promote it, yet we can neither complete it within one lifetime nor guarantee that virtue and happiness will ever match. That gap between the command and our power is what forces the postulates.

What are the three postulates of practical reason?

Freedom, immortality and God: not objects reason can know — the first Critique ruled that out — but assumptions the moral life cannot do without. Freedom is presupposed by the law that commands; immortality gives room for endless moral progress; God guarantees that virtue and happiness can finally meet in the highest good. They are commitments, held on practical grounds.

What is the 'primacy of practical reason'?

Where theoretical reason must stay silent about God, freedom and immortality, practical reason is entitled to affirm them on moral grounds. When the two interests of reason meet, the practical takes the lead. Reason as agent may commit to what reason as knower can never prove — this is how the second Critique lets the moral agent believe what the theorist may not claim.

Themes of the book

What this book returns to, gathered into themes and ordered by how much of the text each one occupies.

The Fact of Reason

Kant's whole ethics rested on a freedom he admitted could never be proved — so what if morality is not built on freedom at all, but is the evidence for it?

The order of discovery reverses. We know the moral law directly — the plain consciousness that one ought to act regardless of what one wants is a bare given, a fact of reason that no argument installs and none removes — and because that law commands, the freedom it presupposes must be real. Morality becomes the proof of freedom rather than its consequence. It is Kant's answer to the critics who went straight for the Groundwork's weakest point, and it lets him stop trying to demonstrate the one thing his system could not do without.

practical reason
morality
duty
moral agency
will
freedom
free will
reason
autonomy
causality

Enter a dialogue

God, Freedom, Immortality

A moral life aims at a good it can never complete in a single lifetime — so what may reason legitimately assume to keep that striving from being absurd?

To make sense of a will bound to a highest good it cannot finish, practical reason is entitled to three postulates — freedom, immortality and God — held not as knowledge, which the first Critique had ruled out, but as commitments reason must live by. Where speculative theology was shut down, the moral agent is licensed to believe on practical grounds what the theorist may not claim to know. The book closes on the image Kant is quoted for above all others: the starry sky overhead set beside the moral law within.

theology
happiness
virtue
moral philosophy
teleology
reason
autonomy

Enter a dialogue

Within this work

passages held: 508
distinct concepts: 482
by passages held, corpus-wide: #63

What this book thinks together

Every arc around the wheel is one concept in this book. A ribbon joins two the text reaches for together, and its width is how many of its passages hold both. Strongest: causality with teleology.

Concept co-occurrence, strongest first 8 concepts, joined by 13 pairings. 191 shared passages in all. 87 weaker pairings reach beyond these concepts and are not drawn.
Concept	Paired concept	Shared passages
causality	teleology	35
ethics	practical reason	27
duty	ethics	23
ethics	rationality	22
duty	rationality	16
ethics	morality	15
duty	morality	11
morality	practical reason	9
morality	teleology	9
aesthetics	morality	7
aesthetics	teleology	6
morality	rationality	6
duty	practical reason	5

Works held

The author's work in order, this book marked in place. Your-language title first, the original beneath it where the two differ; side chips show which full texts are held.

1764 Inquiry Concerning the Distinctness of the Principles of Natural Theology and Morality (Untersuchung über die Deutlichkeit der Grundsätze der natürlichen Theologie und der Moral)
Kant's runner-up entry in the Berlin Academy's prize contest, and the sharpest of the pre-critical essays: it already sets philosophy's method against mathematics', the problem the first Critique would finally resolve.
DE
1764 Observations on the Feeling of the Beautiful and Sublime (Beobachtungen über das Gefühl des Schönen und Erhabenen)
The popular essay of Kant's pre-critical years, twenty-six years before the Critique of Judgement gave taste a rigorous grounding; its remarks on race and sex now drive much of the scholarship on him.
DE
1766 Dreams of a Spirit-Seer, Elucidated by Dreams of Metaphysics (Träume eines Geistersehers, erläutert durch Träume der Metaphysik)
The anonymous, ironic book of 1766 in which Kant's faith in speculative metaphysics visibly collapses — the clearing of the ground that the first Critique, fifteen years later, would build on.
DE
1781 The Critique of Pure Reason (Kritik der reinen Vernunft)
The book at the centre of everything after it: the first of the three Critiques, revised almost past recognition for its 1787 second edition, and the work the Prolegomena was written to explain.
DEEN
1784 An Answer to the Question: What Is Enlightenment? (Beantwortung der Frage: Was ist Aufklärung?)
The most quoted thing Kant wrote, and among the shortest: a 1784 magazine answer to a question a Berlin pastor had left hanging, and the defining slogan of the German Enlightenment.
DE
1784 Idea for a Universal History with a Cosmopolitan Purpose (Idee zu einer allgemeinen Geschichte in weltbürgerlicher Absicht)
The essay where Kant first treats history as more than chronicle — nine propositions arguing it has a hidden rational shape; the seed of Perpetual Peace, and a device Hegel later took over.
DE
1785 Fundamental Principles of the Metaphysic of Morals (Grundlegung zur Metaphysik der Sitten)
The short, deliberately preliminary book that isolates the categorical imperative before building anything on it; three years later the Critique of Practical Reason would give the same moral law its fuller grounding.
DEEN
1786 What Does It Mean to Orient Oneself in Thinking? (Was heißt: sich im Denken orientieren?)
An intervention in the pantheism controversy that split the German Enlightenment: written months after Mendelssohn's death, it stakes out a rational faith between dogmatic proof and Jacobi's leap.
DE
1788 The Critique of Practical Reason (Kritik der praktischen Vernunft) you are here DEEN
1793 Religion within the Boundaries of Mere Reason (Die Religion innerhalb der Grenzen der bloßen Vernunft)
The book that got Kant ordered to stop writing on religion: four essays, from 1793, rebuilding Christian doctrine on reason and consigning revelation and clergy to what Kant called counterfeit service.
DE
1795 Perpetual Peace: A Philosophical Sketch (Zum ewigen Frieden: Ein philosophischer Entwurf)
The late essay whose plan for a league of free states reached the twentieth century intact; Kant, past seventy, wrote it as Prussia pulled out of the war against revolutionary France.
DE
1797 Einleitung in die Rechtslehre
The framing section of the Doctrine of Right, the legal half of Kant's 1797 Metaphysics of Morals; it defines a right through freedom alone and ties every right to a power to coerce.
DE
1798 Von der Macht des Gemüts, durch den bloßen Vorsatz seiner krankhaften Gefühle Meister zu sein
The third part of Kant's late Conflict of the Faculties (1798), written as a reply to the physician Hufeland — a philosopher's dietetics, and his most personal page on governing the body through the mind.
DE
1803 Lectures on Pedagogy (Über Pädagogik)
One of the last books to appear under Kant's name (1803), though his editor Rink built it from lecture notes; the fullest record of Kant on education, and of his debt to Rousseau.
DE
Critique of Judgement
The book the Romantics and the German Idealists built on: Kant's 1790 account of beauty and of purpose in nature, written to bridge the gulf that had opened between his first two Critiques.
DEEN
Prolegomena to Any Future Metaphysics
Written to salvage the Critique of Pure Reason after its baffled reception, this shorter 1783 restatement runs the same argument in reverse, and became most readers' first encounter with Kant.
DEEN
The Metaphysical Elements of Ethics (Die Metaphysik der Sitten)
Long read as a mere appendix to the Groundwork, this ethical half of the 1797 Metaphysics of Morals is where Kant spells out the duties of virtue; later Kantians recovered it as his theory of character.
DEEN

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