Or is it simply a reiteration of his sceptical contrast between phenomena and noumena, and of his confinement of (valid) knowledge to the former?
But it is not maintained, on the side either of phenomena or of noumena; and hence its tendency to materialism.
C. As to existence, Kant's position is the wholly illogical one that, though all known things are phenomena, there are things in themselves, or noumena; things which are said to cause sensations of outer sense and to receive sensations of inner sense, though they are beyond the category of causality which is defined as one of the notions uniting phenomena; and things which are assumed to exist and have these causal attributes, though declared unknowable by any logical use of reason, because logical reason is limited by the mental matter and form of experience to phenomena; and all this according to Kant himself.
As to the use of reason beyond knowledge, Kant's position is that, in spite of its logical inability to transcend phenomena, reason in its pure, or a priori use, contains necessary a priori " ideals " (Ideen), and practical reason, in order to account for moral responsibility, frames postulates of the existence of things in themselves, or noumena, corresponding to these " ideals "; postulates of a real free-will to practise morality, of a real immortality of soul to perfect it, and of a real God to crown it with happiness.
But they disagreed with Kant, and agreed with Fichte about things in themselves or noumena, and contended that the mental things we know are not mere phenomena of sense, but noumena, precisely because noumena are as mental as phenomena, and therefore can be known from similar data: this was the central point of their noumenal idealism.
How would you define noumena? Add your definition here.