One possible key element about a priori knowledge that would allow a precise understanding of it would raise from the question: is (Kantian) a priori knowledge produced consciously or subconsciously?
Kant would suggest that a priori is mostly produced subconsciously, (e.g. as space or time), so it would allow sense-experience to be gathered and used. That is quite evident. But it would appear that some types of a priori knowledge seem to be a conscious result like mathematics.
Mathematics is clearly a rational product of conscious experience. Its linguistic dimension (math is, on one hand, a language) is evidently a rational development. Its instrumental dimension (math is, on the other hand, a tool), is also evidently a rational development.
What philosophical references address the question of whether a priori knowledge is a conscious or a subconscious development?