Just as Copernicus changed the center of our universe from Earth to Sun, Kant relocated the basic principles and categories of reality, as studied by science, from the external world to the mind. Like John Locke (1632–1704), he began with an examination of the powers of the mind and an aim to reject metaphysical claims that could not be rationally justified. He posited a human rational necessity to understand real experience in space and time and a practical need to live with other rational beings, seeking the principles that could fulfill those requirements.

In 1770 Kant argued in On the Form and Principles of the Sensible and Intelligible World that our knowledge of space and time is only about appearances, but that we are still justified in making limited claims about what lies behind those appearances. This was the foundation for what became known as critical philosophy. Kant’s revolutionary claim was that we have a priori knowledge of both space and time because they are the forms of our perception: space is the organization of experience in the outer world, while time is the organization of experience in the inner world. (This was followed by the two editions of his Critique of Pure Reason, with his Prolegomena to any Future Metaphysics published in between to respond to criticism.)