Not just medieval - Hindu philosophy has definitely had that concept for the few thousand years it's been in existence. The concept of "koti koti brahmanda" literally millions of universes is a pretty central concept.
Another one is the concept of 'Omkaaram' which is supposed to be a primordal "sound" or sensation from which all the universe was created, which can be thought of as saying that all reality emerges from consciousness..[1]
The stuff in Vedas and how it got written are really really intriguing.
That our social constructs somehow have a bearing on what is actually real?