Critique of Pure Reason by Immanuel Kant — Summary, Key Ideas & Quotes

Kant argues that human reason has boundaries. While we can use reason to organize and interpret experience, we cannot use it to access knowledge about things beyond possible experience—what he calls 'things-in-themselves.' This means that metaphysical speculation about the soul, God, or the universe as a whole cannot yield genuine knowledge, since these topics lie beyond the scope of what our faculties can grasp.

Kant famously claims to have initiated a 'Copernican revolution' in philosophy. Instead of assuming our knowledge must conform to the external world, he suggests that the world as we experience it conforms to the structures of our mind. Space and time, for example, are not features of things-in-themselves but forms of human intuition—ways our mind organizes sensory data.