There has recently been some interesting discussion about whether one should hold a 'two-world' interpretation of Kant or a 'two-aspect' interpretation at Kant Blog, Transcendental Idealism and DuckRabbit. I really don't have much to say on the matter, but I always wonder whether this divide in the commentators might be the result of failing to take Kant seriously enough. That is, while the actual positions that are sometimes put forward under these labels are usually more sophsticated than the labels themselves, in the end either of these ways of putting the matter makes the mistake of starting with the world(s) as if that were an unproblematic place to clarify the matter, when part of Kant's whole point is that it is not. Of course, Kant does explicitly on occasion talk of two worlds (mundus sensibilis et intelligibilis); and sometimes within the same footnote or paragraph goes on to say something that sounds more like a two-aspect account -- e.g., that if the senses represent something as it appears, this something must also be in itself a thing. But the reason there is this divide is precisely that Kant is tricky on this point, because the point itself is tricky. For a noumenon is a thing insofar as it is not an object of sensible intuition; but it is not self-contradictory that it could be an object of a non-sensible intuition. We just don't have such an intuition, or even a notion of what such an intuition would be like. We know the noumena not because we experience them, but because experiencing the objects we do experience requires that we conceive of these unknown-somethings required for the possibility of experience. Precisely because they are unknown-somethings, we can't say how they are related to appearances except insofar as they are required for their possibility; and even then we are not saying anything about the unknown-something as such. It's like negative theology generalized: we know nothing about the noumena except what they are not and those of our relations to them that lead us to posit them in the first place. Our capacity for receiving impressions is limited; it cannot give us things except in the mode in which they appear to us. But these appearances by their very nature require us to say that there is something in itself correlated with the appearance. The immediate representation of the thing is its appearance; but there is a thing in itself, independent of sensibility, of which the appearance is a representation. But without some intelligible intuition, we have no way of saying more than this: there are intelligible entities, of some sort, that correspond, in some way or another, to the sensible entities that we experience. These intelligible entities are distinguishable from sensible entities -- indeed, the two cannot possibly be the same, since the former must be independent of all sensation and are never sensed. We cannot even conclude that any noumenon actually exists; all we can conclude is that it is not self-contradictory that it do so, and that the accurate characterization of our experience requires the concept of it. Thus the concept of a noumenon has no function in our reasoning except to tell us what sensibility cannot do; whether it is another world, or another aspect, or what have you, is simply unavailable to us. (And more than this: while we cannot eliminate the possibility that there might be a vantage point from which an answer might be available, and so cannot eliminate the possibility that there might be an answer about whether noumena and phenomena are 'two worlds' or 'two aspects', we cannot even conceive of what such a vantage point would be.) As possible (or at least not self-contradictory), the noumenon is admissible as a concept; as a limiting concept, the noumenon is utterly indispensable, since without it we fall into illusion and contradiction; but this is all we are able to say of the matter, and neither of these gives us enough to favor either a 'two worlds' account or a 'two aspects' account. That we are inclined to prefer either is just a sign that we have missed the whole point. The real point we should be taking away is that, whether there's one world, or two, or even whether there's any way for the noumena to be objects of some sort of intuition we don't have (and thus to be experienced as existing), we're kidding ourselves if we think we can do without them, and equally kidding ourselves if we think we can know anything about them at all beyond their bare possibility and the particular rational needs that make it so that we can't do without them. As I said: it's like negative theology generalized. Just as negative theology says that we can't know what God is but only what God is not and how things depend on Him, so transcendental philosophy says that we can't know what things in themselves are, but only what they are not and how we depend on them. And that is the point, the one which (apparently) makes the argument about whether there are two worlds or only two aspects completely otiose, and (perhaps) not even quite intelligible. I think this applies to all the little variations of this type of dispute as well.
I cannot say for certain whether this is all exactly right, particularly given that my interaction with Kant is relatively limited; it seems to me to fit much of what Kant says, but I freely admit that there's a lot about Kant's view that I simply do not know.
