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.
0000000000000000000-----------------------------------------Kant's stated goal is to see if metaphysics can be put on a firm foundation. In scholastic philosophy of the Middle Ages, there are two parts of metaphysics, part one is of concepts applied to individual things. Part two is of the totality of things that we cannot experience directly. In Kant, there are two separate areas of knowledge-(1) sense, direct experience, and (2) concepts. That first area is called "intuition" from the Latin "perception". Intuition is divided into two parts: (1) sense perception and (2) pure intuition. Pure intuition is universals like redness or whiteness, adjectives that apply to many individual things, and the axioms of geometry. [Axioms like the shortest distance between two points is a straight line is pure intuition (not by observation or a priori).] They are known by direct awareness. The area of concepts part one is where you combine universals or axioms of geometry into concepts by means of rules. [But direct perception cannot tell us anything about rules. I can experience directly a sensation of sight, hearing, taste, or feel, but not rules.] Part two of metaphysics is concepts that exist, but that we cannot know directly. [The bridge from intuition to concepts is by apperception the self. The self (a-perception) is the core of the Kantian system that connects sense with concept.] Without the fact that the self is one united whole we could not know anything, and that self shows us that combining universals by rules is true. So synthetic apriori knowledge is possible. (The self is how Kant bridges between mind and body, and by that Kant reaches his main goal of finding out what part of metaphysics is possible to know, i.e. the part that combines concepts with physical things.) But metaphysics part two is closed because when reason is applied to it, it comes up with self contradictions. But to Leonard Nelson and Kelley Ross, there is such a thing as knowledge that is not by intuition, nor by concepts, but is immediate non intuitive. And it forms the basis of all concepts, even metaphysics in the first part. (Thus, by this, one can come to faith.) For Leonard Nelson thought the borrowing of the twelve categories of Aristotle to combine physical things with concepts was a bit too arbitrary. To Nelson, you need to start with axioms. And how do you know you started with the right axioms? According to Kelley Ross, that is by the process defined by Karl Popper of falsification. You can show concepts to be wrong when they do not correspond to reality.--