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.
Kant felt he had done the same thing in metaphysics. If you presume that the a priori concepts in our heads are independent of what we see and actually screen what we see, this means that you are taking the old Aristotelian concept of scientific man as a passive observer, a "blank tablet," and truly turning this concept inside out. Kant and his millions of followers have maintained that as a result of this inversion you get a much more satisfying understanding of how we know things.
—Robert Pirsig, Zen and the Art of Motorcycle Maintenance, p. 135