Planes of existence in my story....help?

I think 'Plane of Mind' as suggested above would be a good one. It could be a world where advanced technology is nonexistent or at least not widespread, but 'pure' disciplines like mathematics, philosophy, etc have been advanced far beyond our own. I think the perspective of a character from a world like that would provide an interesting addition to the existing three, it would have its own set of strengths and weaknesses that would help round out the whole thing.