Tom Greaves considers the relation between the knowable and the real.
[Kant's] idea of categorial intuition does not clear up the problem of whether it is possible to think of receptive sensibility and active understanding as originating together in our way of encountering things, a problem that was to preoccupy Heidegger.
In his book Kant and the Problem of Metaphysics, published in 1929, Heidegger argues that this problem can be addressed by returning to Kant himself. We can, he argues, return to the root of the problem of how things appear to us by looking again at what Kant says about the two faculties that are the sources of that appearance. He finds that Kant actually suggests that these faculties, although quite distinct, might have a common root. If we can understand that source then, instead of simply positing two faculties and trying to see how they work together, we might achieve a more basic characterization of how experience comes about. For Heidegger, Kant actually gives us a clue as to what this common root might be in his own account of how the two faculties are brought together in experience. The key is what Kant calls the 'transcendental power of imagination'. This should not be under stood as what produces the 'merely imaginary' or unreal, but as what allows for the formation of all appearances, including the real and the imaginary. According to Heidegger's interpretation, it is not simply a third faculty alongside the other two, but the root power from which the other two spring and which allows for their conjoining in experience. The transcendental power of imagination that is in us opens up a field in which things can be encountered and thus is not itself a purely passive reception of things or an active intellectual grasping of them. It is the source of both of those faculties together. It is with the characterization of this root power that opens up and cultivates thc range of possible appearing as such that Heidegger thought we must concern ourselves. Kant, however, having once recognized this power at the root of receptive sensibility and active sense-making understanding, later shrank back from it and assimilated it to the spontaneous activity of the intellect.