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Primary sourceBook1788 (completed 1787)

Critique of Practical Reason

Immanuel Kant · 1788 (completed 1787)

Kant's 1788 work on moral philosophy, the second of his three great Critiques, exploring how reason can guide action.

Kant's own second Critique, developing his moral philosophy and the idea of the moral law within.

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Title page of the 1788 German edition of Kant's Critique of Practical Reason: 'Critik der practischen Vernunft von Immanuel Kant', with a floral wreath ornament, 'Riga, bey Johann Friedrich Hartknoch 1788', and a purple library stamp at bottom right.
Immanuel Kant, Critik der practischen Vernunft, title page, 1788.

About this source

What it is

Published in 1788 (though completed in late 1787), this book by Immanuel Kant follows his Critique of Pure Reason and develops his theory of the will acting from moral law alone. It is one of his central works on ethics.

Why it matters

Kant had already discussed morality in his Groundwork of the Metaphysics of Morals (1785), but here he places his ethical ideas within his larger philosophical system, discussing concepts like duty, respect for the moral law, and the 'highest good'. The book also connects to his famous idea that contemplating both the vastness of the universe and the moral law within oneself can inspire awe.

Look closer

  • Why might Kant have changed his plans several times about whether to write this book at all?
  • What does it suggest about Kant's working method that this book's structure closely mirrors his earlier Critique of Pure Reason?
  • What is the difference between a law of nature and what Kant calls a law of freedom, and why might that matter for morality?

This book is part of the curated material that grounds the AI version of Immanuel Kant on Humy.ai — when students ask about related events, the AI can quote it and cite it.

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Title page of the 1781 first edition of Kant's Critique of Pure Reason. Blackletter title reads Critik der reinen Vernunft, von Immanuel Kant, Professor in Königsberg. Below: Riga, verlegts Johann Friedrich Hartknoch, 1781, with a floral wreath ornament.Book

Primary source

Critique of Pure Reason

Immanuel Kant · 1781 (revised 1787)

Printed page in German blackletter type headed 'Beantwortung der Frage: Was ist Aufklärung?' with the opening lines of Kant's essay, beginning 'Aufklärung ist der Ausgang des Menschen aus seiner selbst verschuldeten Unmündigkeit.' Page footer reads '2ter Band'.Book

Primary source

Answering the Question: What Is Enlightenment?

Immanuel Kant · 1784

Title page of Kant's Grundlegung zur Metaphysik der Sitten, printed in black Fraktur blackletter: 'Grundlegung zur Metaphysik der Sitten, von Immanuel Kant', with a decorative rococo ornament, a rule, and 'Riga, bey Johann Friedrich Hartknoch 1785' on a cream page.Book

Primary source

Groundwork of the Metaphysic of Morals

Immanuel Kant · 1785

Title page of Kant's 'Zum ewigen Frieden' printed in black Fraktur type. Text reads 'Zum ewigen Frieden. Ein philosophischer Entwurf von Immanuel Kant', with publication line 'Königsberg, bey Friedrich Nicolovius, 1795' at the bottom.Book

Primary source

Perpetual Peace: A Philosophical Sketch

Immanuel Kant · 1795

Title page of the 1790 German edition of Kant's Critik der Urtheilskraft. Large black gothic-style heading 'Critik der Urtheilskraft', 'von Immanuel Kant', a vignette of a small figure and plant engraving, and imprint 'Berlin und Libau, bey Lagarde und Friederich, 1790'. Faint English title 'Critique of Judgment' appears in the background.Book

Primary source

Critique of Judgment

Immanuel Kant · 1790

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