The German philosopher Immanuel Kant published his Critique of Practical Reason (in German, Kritik der praktischen Vernunft) in 1788. It is considered a cornerstone not only of Kant’s own moral philosophy, but of modern philosophy in general and of the study of ethics. Because it is the second part of a trilogy of philosophical critiques on fundamental concepts of reason along with Critique of Pure Reason (1781) and Critique of Judgment (1790), the book is sometimes called The Second Critique today. Kant builds on his argument in Critique of Pure Reason that no one can prove the existence of God or anything metaphysical or transcendent through pure abstract reasoning alone. Here, Kant argues that instead it is practical reason—­the understanding of how and why we act morally in response to specific and real situations, hence “practical”—that provides evidence for transcendent ideas like universal moral laws, the existence of God, and the highest good.

This guide is based on Immanuel Kant, Critique of Practical Reason, trans. Mary Gregor, rev. ed. (Cambridge: Cambridge University Press, 2015).

Summary

In his prior critique, Critique of Pure Reason, Kant argued that there is no way to prove metaphysical ideas like the existence of God or the immortality of the human soul through abstract or “speculative reason” alone. However, Kant also questions the legitimacy of relying solely on empirical reasoning—or reasoning based on direct sensory experience. If, like the philosopher David Hume, we question even whether there is even such a thing as cause and effect, then we simply head into an “abyss of skepticism” in which nothing can be definitively known (3). Instead, Kant argues that by examining the way humans think and make moral decisions, one can logically assume three key theoretical concepts or postulates: the existence of freedom or free will, the existence of God, and the immortality of the human soul (106).

Instead of being guided by the need for pleasure and happiness or by an innate “special moral sense” (35), as past philosophers and the philosophers of Kant’s time believed, Kant argues that every person has a free will that intuitively seeks out the highest good and is guided by an intuitive understanding of universal moral law and a feeling of duty toward it. Rather than being purely unambiguous, though, people need reason to understand the moral law and how to fulfill their duty toward it. In contrast to our ideas of pleasure, pain, and happiness, which depend on circumstances and subjective factors, this moral law is objective. However, even for those who strive toward the moral law and the greatest good, determining how to fulfill its demands and understand it is an unending struggle constantly requiring the use of reason. Kant defines this act of constant reasoning toward the fulfillment of the moral law as virtue.

Kant believes the best way to instruct students on morality is not through examples of highly virtuous people, but by looking at how people act morally despite their emotions and highly demanding circumstances. Kant even anticipates that it is possible to have a science of morality. In practice, this science would involve “repeated experiments on common human understanding” (130) that can be used to precisely understand ethical thinking and the formation of morality in the minds of individuals.