Kant points out that we can have no experience of the thing-in-itself: the phenomenon tells us nothing about the noumenon. Physical manifestation as the ground of phenomenon is itself a noumenal hypothesis, and is unwarranted by the very position being cited. This is all bad Fichte (which is the smell you get when you inadvertantly leave one of your catch of fichte at the bottom of your boat for two or three days).
yours, Peter T.