My own understanding of Kant’s project is that he was trying to determine how it is possible that we can reason about phenomena about which we have no direct experience; such as the question of what happens to us after we die (Kant, 2001, pp.25, 35).
The first thought that came to me was that a priori phenomena could be explained from the perspective of evolutionary psychology; our minds are filled with concepts like space and time because they conferred a survival value to our ancestors. Even the concept of God can be explained using this framework as it could be argued that our idea of God is a product of an ‘agency detection module’ in the human brain.
However, my thoughts then turned back on themselves in defence of Kant. It occurred to me that from Kant’s point of view scientific theories are still abstract a priori lenses through which we group together and interpret certain phenomena. For example, we cannot see evolution ‘in itself’ because we don’t live long enough to see it take place. Likewise, we can see the effects of gravity but we cannot see gravity ‘in itself’.
Kant, I. (2001) Introduction. In: Wood, A. W. .ed. Basic Writings of Kant. New York: The Modern Library, pp.25, 35.