The second of Immanuel Kant's three critiques, the Critique of Practical Reason

It covers Kant's moral philosophy and comes after his first critique, the Critique of Pure Reason. The Groundwork of the Metaphysic of Morals (1785), one of Kant's major works on moral philosophy, had already been published, but the Critique of Practical Reason was meant to cover a wider range and to situate his ethical beliefs within the overall framework of his system of critical philosophy. Beginning with Johann Gottlieb Fichte's Doctrine of Science and evolving into the primary source for deontological moral philosophy in the 20th century, the second Critique had a significant impact on how the study of ethics and moral philosophy developed in the years that followed.

Here, Kant outlines what will happen next. These two chapters compare the circumstances of theoretical and practical reason for the most part, and in doing so, explore how the Critique of Practical Reason contrasts with the Critique of Pure Reason. The first critique, titled "of Pure Reason," criticised individuals who utilise pure theoretical reason and assert that they may reach metaphysical truths that are outside the scope of applied reasoning. The conclusion was that pure theoretical reason must be regulated since, when utilised outside of its proper context, it generates muddled arguments. The Critique of Practical Reason, on the other hand, defends pure practical reason as having the capacity to underpin behaviour that is superior to that underpinned by desire-based, rather than criticising it.