Shankara was the first to state it clearly, about a thousand years before Kant:
this world is a phenomenal appearance. Of what, in what and for what is the subject matter of nested ontology. In treating these subjects nested ontology broaches topics which historically have been considered under names such as philosophical idealism, noumenalism, and representationalism, yet without being committed to all they have stood for or been taken to stand for. Hence the name nested ontology, which might also be called entheticism, from the Greek enthetos, enthetikos (nested) and entithenai (to put in).