The Critique of Pure Reason is one of the most important philosophical texts ever written. Like Copernicus, Kant dared to question the ordinary perspective from which we habitually view the world.
Kant's moderate form of skepticism is known as "transcendental idealism," and its primary tenet is that we cannot know things as they are in themselves because we only know things as they appear to us. His thesis had a monumental influence on the culture of the last two centuries, giving rise to cultural movements and theoretical approaches including: German Idealism, Romanticism, Modernism, Marxism, Existentialism, Psychoanalysis, Structuralism, Post-Structuralism, and even Quantum Physics.