Kant reference again to The Critique of Pure Reason. Bely had in his possession in Dornach during the original writing of this work his father's copy of Kant. "forewarning of perception" may refer to what in English is translated as "apperception." "the categories are only capable of empirical use, inasmuch as they serve merely to subject phenomena to the universal rules of synthesis, by means of an a priori necessary unity (on account of the necessary union of all consciousness in one original apperception); and so to render them susceptible of a complete connection in one experience. But within this whole of possible experience lie all our cognitions, and in the universal relation to this experience consists transcendental truth, which antecedes all empirical truth, and renders the latter possible."
supercomprehensible German "übersinnlichen." I have found this word translated as "supersensible" and "suprasensible." It describes both the "supersensible" world, i.e. the one beyond our physical - sensory consciousness, and the ability to perceive such a world through "supersensible consciousness."