Kant called his thesis that our a priori thoughts are independent of sense data and screen what we see a ``Copernican revolution.'' By this he referred
to Copernicus' statement that the earth moves around the sun. Nothing changed as a result of this revolution, and yet everything changed. Or, to put it in Kantian terms, the objective world producing our sense data did not change, but our a priori concept of it was turned inside out. The effect was overwhelming. It was the acceptance of the Copernican revolution that distinguishes modern man from his medieval predecessors.

What Copernicus did was take the existing a priori concept of the world, the notion that it was flat and fixed in space, and pose an alternative a priori
concept of the world, that it's spherical and moves around the sun; and showed that both of the a priori concepts fitted the existing sensory data.