Critique of Pure Reason

This book is one of the most important works ever written in philosophy. It marked a turning point in the modern and contemporary philosophical approach, making it possible to distinguish what came before Kant from what came after him. This turning point was particularly evident in its resolution of the problem necessitated by the decline of metaphysics in the face of the advances in the positive sciences. His era inherited from the Greeks the notion that the like can only perceive the like, and from Descartes the notion that the mind is a substance distinct from the substance of extended matter. Therefore, it is philosophically astonishing that the mind, with a subject distinct from itself, could establish a science that finds application in reality.
The solution was to view the mind as an active agent in the formation of knowledge and to view our knowledge as a human, not absolute, science. In what is called the Copernican Revolution, Copernicus assumed, in order to explain the movements of the heavens, that the observer (on Earth) revolves around the sun, in contrast to the ancient assumption that the stars and the sun revolve around the observer. Kant placed the mind in the position of the sun, and the object in the position of the observer.