Kant was a transcendental idealist. He believed in the primacy of existence but thought that real noumenal existence is unknown in principle. We only could know its phenomenal appearance, which is perception of noumenal world.
"If by ‘noumenon’ we mean a thing so far as it is not an object of our sensible intuition, and so abstract from our mode of intuiting it, this is a noumenon in the
negative
sense of the term".
"But if we understand by it an object of a non-sensible intuition, we thereby presuppose a special mode of intuition, namely, the intellectual, which is not that which we possess, and of which we cannot comprehend even the possibility. This would be ‘noumenon’ in the
positive
sense of the term."
Since, however, such a type of intuition, intellectual intuition, forms no part whatsoever of our faculty of knowledge, it follows that the employment of the categories can never extend further than to the objects of experience. Doubtless, indeed, there are intelligible entities corresponding to the sensible entities; there may also be intelligible entities to which our sensible faculty of intuition has no relation whatsoever; but our concepts of understanding, being mere forms of thought for our sensible intuition, could not in the least apply to them. That, therefore, which we entitle ‘noumenon’ must be understood as being such only in a negative sense.
“Further, the concept of a noumenon is necessary, to prevent sensible intuition from being extended to things in themselves, and thus to limit the objective validity of sensible knowledge”.
“What our understanding acquires through this concept of a noumenon, is a negative extension; that is to say, understanding is not limited through sensibility; on the contrary, it itself limits sensibility by applying the term noumena to things in themselves (things not regarded as appearances). But in so doing it at the same time sets limits to itself, recognising that it cannot know these noumena through any of the categories, and that it must therefore think them only under the title of an unknown something”.
Critique of Pure Reason
A250/B307,P267(NKS)
A250/B30,P2677(NKS)
B309,P270(NKS)
A253/B310
A256/B312,P273