For Kant, the noumena is cloaked in mystery- and his cognitive model forces him, against his own judgmental capacities, into a contradictory conceptual position. He assumes that there are universals inhering in human consciousness, beyond his own consciousness, while also stipulating implicitly that assuming universality is both necessary for discourse and contradictory to his own premises about the inaccessibility of the noumena, behind phenomenal appearances. What the systems of organized religion, and some systems of philosophy, claim- principles of pure reason/pure conceptions of understanding about/around the noumena- is an issue which Kant invests textual time in debunking, but without placing his own conjectures in their stead. Thus, the central mystery inhering in "Critique of Pure Reason," the noumena- is one that Kant appears to respect enough not to address. That cognition, at all points and on all levels of his three-tiered cognitive model (sensibility-understanding-reason), ends with phenomena, and that the noumena can go so far as to be named and nothing else- renders the textual situation around Kant and the Subject rather shrouded- especially because the Subject, we see, must have some inherent relationship to substance, causality, an interior past the merely phenomenal. This is why the book's central premise/conceit- that very little in human consciousness can, in a rigorous and properly grounded way, pass from the conceptions of the understanding into the solid principles of pure reason- makes the sideways acknowledgment that the Subject, for Kant (or raw subjectivity) is one that is troublesome to manage, even for the most disciplined, and orderly, forms of understanding.
On the mystery of the noumena- and to bring Kant's inquiry, somewhat ironically, back to Deconstructionism- Kant evinces a kind of impressive textual modesty, against the grain of the authority with which he presents his Transcendental Aesthetic. Specifically as a text, "Critique of Pure Reason" has a premise as much negative as positive- to demonstrate the lack of grounded rigor in the vast majority of posited principles and premises, and the cognitive structures capable of generating principles to begin with. The balance, in "Critique," between textual modesty (around the noumena) and iron-willed ambition is a unique one; veering to the left or right of arrogance (as Schopenhauer does not), while never eschewing the imperative to command.