THE RELATIVITY OF ONTOLOGICAL GROUNDING
This has a wordy title but is actually easy to get and I think both neat and, in an abstract way, important.
People sometimes talk about reality as if it has different levels. So for example we have an experience of emotion but it is explained by neurobiology. Or in more grand terms that there are physical laws but they are really explained by events in another universe, and our universe is a simulation being run on a computer in that universe.
The details vary, but all these pictures have the same structure — a level of reality A is grounded by a “deeper” level of reality B.
If we look at the development of the concept of up and down though I think it can help us see these issues more clearly. To pre-scientific peoples “higher than” and “lower than” are primitive relationships, abstracted from their bodily experience. A mountain just is higher than the ground. But now that we have developed a scientific worldview we understand that “up” and “down” are relative concepts. To say that a mountain is higher than the sea just means the sea is closer to a given center of gravity — in this case the Earth. If you took the Earth and put it on the sun, you could say that the sea was higher than the mountain (depending upon the orientation of the whole earth).
China is not higher than America or lower — they are just points on a sphere.
I think the same thing is the case with ontological grounding. If there are multiple universes there is no fact of the matter as to which grounds which. In some contexts one grounds the other, but going from another direction the second grounds the first.