In Critique of Pure Reason Kant describes in detail what aspects of our knowledge are a priori, and how they function, but on the empirical aspects he is sketchy and cryptic. Very briefly, our sensations/intuitions are synthesized into perceptions via schemes of productive imagination, including a priori forms of space and time. These schemes are somehow coordinated with concepts of understanding, which allows perceptions to be "brought" under them. The highest concepts, categories, are a priori, and come not even from understanding, but from reason itself. But what about empirical concepts and schemes coordinated with them that are not spatiotemporal, where do they come from? Size and shape are a priori, how about color, weight, firmness... universal gravity?
Kant admits that empirical concepts are "acquired". But how? Do productive imagination and understanding have them on hand when they synthesize perceptions and make judgments, or do they synthesize them by grasping regularities of the "sensory manifold" itself? If it is the former, are they "acquired" over time from what is "in the senses", or is the "sensory manifold" completely undifferentiated, and productive imagination "invents" them (with assistance from understanding) to frame it?
Both answers seem to undermine Kant's construction. If empirical concepts ultimately originate in our mind then the dichotomy between empirical and a priori collapses, they would be as a priori as space, time and the categories. If on the other hand, they are extracted from senses (somehow), then what collapses is Kant's impenetrable wall between appearances and things in themselves. Indeed, the sensory comes from the senses, which are impressed upon by things in themselves without any a priori interference. An alternative to both is to deny that we can isolate the sensory in perception (contrary to Husserl's phenomenology with its "bracketing out" of presumptions, and reaching "to things themselves"), but it is also problematic. Kant has no trouble isolating the a priori, and the rest has to come from the senses. Did Kant indicate somewhere which option he chose and how he dealt with complications?