Grom wrote:
From "CPR, B344-5":
Understanding [...] limits sensibility, but does not thereby extend its own sphere. In the process of warning the latter that it must not presume to claim applicability to things-in-themselves but only to appearances, it does indeed think for itself an object in itself, but only as transcendental object, which is the cause of appearance and therefore not itself appearance, and which can be thought neither as quantity nor as reality nor as substance etc. (because these concepts always require sensible forms in which they determine an object). We are completely ignorant whether it is to be met within us or outside us, whether it would be at once removed with the cessation of sensibility, or whether in the absence of sensibility it would still remain. If we are pleased to name this object noumenon for the reason that its representation is not sensible, we are free to do so. But since we can apply to it none of the concepts of our understanding, the representation remains for us empty, and is of no service except to mark the limits of our sensible knowledge and to leave open a space which we can fill neither through possible experience nor through pure understanding.