 This passage begins with the familiar point that the very concept of appearance requires that there be something that is not appearance that appears. Usually Kant makes this point using the concept “things in themselves” (e.g., in the Prolegomena (Ak. 4:314–5); cf. Bxxvi–xxvii, B306, and B307). However, here he claims that this idea—that it cannot be “appearances all the way down”—brings with it the idea of noumena. This is puzzling. Why must whatever it is that appear to us as phenomena be conceived of as an objects of intellectual intuition?