The Critique of Practical Reason is the second of Immanuel Kant's three critiques. It follows on from Kant's Critique of Pure Reason and deals with his moral philosophy. The second Critique exercised a decisive influence over the subsequent development of the field of ethics and moral philosophy, beginning with Johann Gottlieb Fichte's Doctrine of Science and becoming, during the 20th century, the principal reference point for deontological moral philosophy.

Immanuel Kant (1724-1804) was a German philosopher, who, according to the Stanford Encyclopedia of Philosophy is "the central figure of modern philosophy." Kant argued that fundamental concepts of the human mind structure human experience, that reason is the source of morality, that aesthetics arises from a faculty of disinterested judgment, that space and time are forms of our understanding, and that the world as it is "in-itself" is unknowable. Kant took himself to have effected a Copernican revolution in philosophy, akin to Copernicus' reversal of the age-old belief that the sun revolved around the earth.