Kant actually never shows how mind and body are connected. Rather he shows that they must be connected-but does not show how. [That is not my new idea here. This has been noticed even from the very first review of the Critique by Shultz.] 

So to me it seems that Fries and Leonard Nelson were right in the claim that there is a deeper source of knowledge, non intuitive immediate knowledge, that empirical knowledge and a priori knowledge are just secondary manifestations of. But how do they combine? I think that Hegel was right in this that the way these two origins of knowledge combine together is by a give and take process where each modifies the other --what he terms ''dialectics.'' [Hegel actually also never shows how they are connected. But he does come onto this dialectical process to show how the kind of knowledge that is a part of intuition and the kind of knowledge that is independent of intuition work together.--basing himself on Socrates.]

[And this fact was noticed by Michael Huemer in one of his essays where he shows that there is no such thing as empirical knowledge without some a priori assumptions built into it. [See his list of essays.]

So what you have is the primary source of knowledge that Fries and Nelson call immediate non intuitive. Them the two parts split off into empirical and a priori parts. Then they recombine to create actual knowledge.