Immanuel Kant was a philosopher who sought to synthesize the competing schools of rationalism and empiricism. In his 'Critique of Pure Reason,' he argued that while our knowledge begins with experience, the human mind provides the structural framework (such as time and space) through which we perceive that experience. His genius lay in 'transcendental idealism,' the idea that we can never know the world as it is 'in itself,' but only as it appears to us. He also formulated the 'Categorical Imperative,' a moral rule stating that one should only act according to maxims that they would want to become universal laws. Kant's work fundamentally changed modern ethics, metaphysics, and aesthetics.