The work begins by dividing philosophy into logic, physics, and ethics, asserting that moral laws must rest entirely on a priori principles of pure reason rather than on empirical observations of human nature. The author argues that an action possesses true moral worth only when performed strictly from duty, independent of inclination, self-interest, or expected outcomes. To act morally is to act out of pure respect for the moral law itself.
Developing this core concept, Kant introduces the categorical imperative as the sole supreme principle of morality. This principle mandates that an individual should act only according to maxims that can simultaneously be willed as universal laws of nature. Through various examples concerning duties to oneself and others, the text demonstrates how fraudulent promises, suicide, neglecting one's talents, and refusing to help those in distress violate this strict formal consistency.
Building upon the categorical imperative, the author formulates the principle of autonomy, wherein rational beings are viewed as ends in themselves rather than merely as means to be used for arbitrary purposes. This leads to the inspiring conception of a kingdom of ends, a systematic union of different rational beings governed by common objective laws where every person possesses intrinsic dignity rather than a mere market value.
The treatise then addresses the apparent conflict between human freedom and physical necessity, a central problem in speculative and practical philosophy. By distinguishing between man as a phenomenon belonging to the sensible world of cause and effect, and man as an intelligence belonging to an intelligible world, the text resolves the antinomy. Freedom is established as a necessary hypothesis for any rational being capable of practical reason and self-legislation.
Finally, the author confronts the inherent limits of human reason in explaining how pure reason can be practical of itself without relying on external empirical springs. While a complete explanation of how the categorical imperative produces a purely moral interest remains beyond human comprehension, the text reaffirms that the autonomy of the will is the sole foundation of all virtue, concluding the inquiry at the absolute boundary of philosophical knowledge.
