Is there one concept that, on its own, could transform everything you think about reality? The simplest ideas of all are often the most difficult to grasp. What could be simpler than that the universe has an inside, and that the inside produces the outside?
The outside is the projection of the inside. Is that not what the Big Bang is all about? The universe begins as pure inside, and then it constructs an outside, a body - a cosmos - all around it. The inside is an immaterial Singularity that's not in spacetime. The outside is material and in spacetime. The inside is noumenon, the outside phenomenon; the inside is metaphysical, the outside physical; the inside mental, the outside material. You cannot observe the inside, only the outside, only the body.