Kant not only describes systemp generally as the outworking of an immanent metaphysics, but also states specifically that, with the move from the speculative perspective in systemt to the practical standpoint of systemp, reason's 'transcendent use is changed into an immanent use'. Something which 'for understanding' is 'transcendent' can be 'immanent' for reason because of the different standpoint it adopts.
That the phenomenon-noumenon distinction as employed in systemp is not intended to transcend, or 'undo', the theories of systemt is also made evident by Kant [see e.g., Kt7:482-5]. For each of his systems 'is grounded in and limited to human experience'. Therefore, a proper understanding of the perspectival relationship between systemt and systemp can resolve 'the enigma of the critical philosophy' [Kt4:5], viz., the problem of how the categories (especially the principle of causality) can properly refer to a supersensible object: 'The inconsistency vanishes because the use which is now made of these concepts [i.e., the standpoint from which they are now viewed] is different from that required by speculative reason. The chief difference is that the 'practical standpoint', as Kant declares in Kt4:5, requires 'no theoretical determination of the categories and no extension of our knowledge to the supersensible.' Because it is concerned only with our action and its moral basis, not with our knowledge, 'the application of the categories to the supersensuous, which occurs only from a practical standpoint, gives to pure theoretical reason not the least encouragement to run riot into the transcendent'.
Why so many interpreters and critics have ignored Kant's firm and precise warnings about the proper interpretation of the phenomenon-noumenon distinction as it applies to the relation between systemt and systemp is difficult to say. It usually has to do, at least in part, with an insufficient appreciation of how the principle of perspective operates in Kant's ...