Critique of Practical Reason

Immanuel Kant 1788

This philosophical study asks how reason can guide moral choices without relying on rewards or personal desires. Its closing reflection pairs the starry sky above with the moral law within.

227 pages 4h 25m reading time

The second of Kant's three great "Critiques," and one of the key works of modern moral philosophy.
Argues that freedom is real because we feel bound by duty: if we know we ought to do something, we must be able to choose to do it.
Argues that belief in God and in a life after death, though they cannot be proven, makes sense as hope that doing right and being happy will in the end go together.
Together with Kant's earlier "Groundwork of the Metaphysics of Morals," it laid the base for duty-based ethics, a main way of thinking about right and wrong that is still taught and debated today.
The closing lines about the starry sky and the moral law are among the most quoted in philosophy, and a version of them is carved on the memorial plaque at Kant's tomb in Kaliningrad (once Königsberg).

Too big for an appendix

Kant first planned it as part of the revised "Critique of Pure Reason," but it grew into its own book.
Answers a critic

In the preface, Kant replies to a reviewer who scoffed that he offered no new moral rule, only a "new formula."
Printed far from home

Like his first "Critique," it came out through Johann Friedrich Hartknoch, a publisher based in Riga.
Mocked in verse

The poet Friedrich Schiller joked that by Kant's strict view, helping friends gladly made the help less moral.