Act only according to that maxim whereby you can, at the same time, will that it should become a universal law. — Groundwork of the Metaphysics of Morals (1785)
Immanuel Kant was a German philosopher of the Enlightenment and one of the most influential thinkers in Western history. His work transformed philosophy by establishing that human knowledge is shaped by the structure of the mind itself—not merely by passive reception of the world. Kant spent most of his life in Königsberg (now Kaliningrad), where he taught at the university and maintained a famously punctual, regimented existence. His systematic approach to metaphysics, ethics, and aesthetics laid the groundwork for much of subsequent philosophy and continues to shape debates today.
Kant's central moral principle: act only on maxims that you could will to be universal laws. Morality is not conditional on desires but is unconditionally binding.
We experience reality only as it appears to us (phenomena), shaped by our minds. Things-in-themselves (noumena) exist but are beyond our direct cognition.
Kant argued that some knowledge is both synthetic (expanding our understanding) and a priori (independent of experience)—e.g., mathematics and certain metaphysical principles.
Phenomena are objects as we experience them; noumena are things as they are in themselves. We can never know noumena; we can only reason about them.
Rational beings are autonomous: they give themselves the moral law. Freedom is not lawlessness but self-legislation in accordance with reason.
Kant's foundational work examining the limits of knowledge: what can we know? Synthetic a priori knowledge is possible because the mind structures experience.
Kant compared his own philosophical turn to Copernicus's revolution in astronomy. Just as Copernicus had the observer revolve around the sun rather than the sun around the observer, Kant proposed that objects conform to our cognition rather than our cognition conforming to objects. Previous philosophy assumed that knowledge must match the world; Kant argued that the world must conform to the structure of our minds. This "transcendental" approach—grounded in the conditions of possible experience—explained how synthetic a priori knowledge could exist and why traditional metaphysics had failed.
Kant's influence extends across philosophy, ethics, political theory, aesthetics, and epistemology. German Idealism (Fichte, Schelling, Hegel) arose from engagement with his work; analytic philosophy was partly shaped by reactions to his ideas. His notion of human dignity as an end in itself informs modern human rights discourse. The separation of phenomena and noumena, the emphasis on autonomy, and the critique of dogmatic metaphysics remain central to philosophical debates. Kant is often regarded as the philosopher who brought the Enlightenment to its fullest expression while also laying the groundwork for its critique.
Two things fill the mind with ever new and increasing admiration and reverence: the starry heavens above me and the moral law within me. — Critique of Practical Reason (1788)