In all commentaries on Kant's philosophy and his Critique of Pure Reason, it is stated that noumenon is completely unknowable. For example in the entry of 'Appearance' in Encyclopaedia Britannica we see:
... Immanuel Kant created the term noumenon to signify unknowable reality, which he distinguished from phenomenon, the appearance of reality.
But at the same time we see in the same commentaries that space, time and causality are ascribed to phenomenon AND NOT the noumenon. Then, is it not the case that although Kant claims the noumenon to be unknowable, he is in a sense describing it indirectly? I mean how the noumenon is unknowable reality and at the same time we can say for example:
Time can characterize only phenomena; it could not characterize the noumenal.
(The Philosophy of Schopenhauer by Bryan Magee p.128)