  Immanuel Kant made us aware that while we learn about nature from experience, in order to do this we have to know something before experiencing anything. According to Kant, a priori synthetic knowledge is unchangeable and plays an active role in cognition: our picture of the world is formed as much by things in themselves as by our forms of perception and categories of thought, and we will never know whether and how that picture is similar to what exists independently of our cognitive acts. Fleck adopted Kant’s thesis on the active role of cognitive a priori: an empty mind would neither perceive nor think. So, before a mind starts to experience, and on the basis of experience starts to think, it has to be filled with some initial knowledge. But the quantum revolution in physics has shown that there is nothing necessary or unchangeable in a priori forms and categories. A non-Kantian explanation of the origins of cognitive a priori was badly needed.