In the third section of the Prolegomena, Kant explains in section 40 (at least how I understand it) that ideas are merely the analogues categories of those concepts that cannot be experienced. As I understand it, he suggests that they are merely analogues to what categories were to experience.
However, when he later gives accounts of, for example, the psychological or the cosmological ideas, it seems more like these ideas, precisely, are where reason fails. It seems like he is saying: If we push experience of the cliff of our sensibility, we see the boundaries, and these boundaries are ideas.
So, how do these two views fit together, exactly? Are we saying that reason must be "constructed up" from where understanding fails?