Kant's synthesis between empirical knowledge and rational knowledge has been a problem as soon as the ink was dry on the first Critique as was immediately noted by Schulze  and Maimon. The answer to their objections I have thought was best answered by Jacob Fries until I noticed what I think is the similar answer given by Reinhold. [I saw this answer in a paper by   Peter Sperber]. Schulze had objected to a sort of circularity in Kant  [that  sense perception works by way of cause and effect. The object outside of oneself causes the perception. But if causality is the only thing that makes perception possible, then it can't be part of perception.][The problem is that without causality, perception is pure delusion] and Salomon Maimon had objected to any possible contact between a priori concepts and empirical senses. The answer is there are concepts that are known immediately without have to go through  reasoning process--they are the categories of where, when, how, why, etc.